Fish are
aquatic,
craniate
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...
,
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
-bearing animals that lack
limbs with
digits. Included in this definition are the living
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
,
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s, and
cartilaginous
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck a ...
and
bony fish
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...
as well as various extinct related groups. Approximately 95% of living fish species are ray-finned fish, belonging to the class
Actinopterygii
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
, with around 99% of those being
teleost
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...
s.
The earliest organisms that can be classified as fish were soft-bodied
chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s that first appeared during the
Cambrian
The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...
period. Although they lacked a
true spine, they possessed
notochord
In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consi ...
s which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ...
era, diversifying into a wide variety of forms. Many fish of the Paleozoic developed
external armor that protected them from predators. The first fish with
jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...
s appeared in the
Silurian
The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...
period, after which many (such as
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s) became formidable marine predators rather than just the prey of
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
s.
Most fish are
ectotherm
An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
ic ("cold-blooded"), allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like
white shark
The great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large mackerel shark which can be found in the coastal surface waters of all the major oceans. It is nota ...
and
tuna
A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
can hold a higher
core temperature
Normal human body-temperature (normothermia, euthermia) is the typical temperature range found in humans. The normal human body temperature range is typically stated as .
Human body temperature varies. It depends on sex, age, time of day, exer ...
. Fish can acoustically communicate with each other, most often in the context of feeding, aggression or courtship.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g.,
char
Char may refer to:
People
* Char Fontane, American actress
* Char Margolis, American spiritualist
* René Char (1907–1988), French poet
*The Char family of Colombia:
** Fuad Char, Colombian senator
** Alejandro Char Chaljub, mayor of Barranquill ...
and
gudgeon
A gudgeon is a socket-like, cylindrical (i.e., ''female'') fitting attached to one component to enable a pivoting or hinging connection to a second component. The second component carries a pintle fitting, the male counterpart to the gudgeon, e ...
) to the
abyssal and even
hadal depths of the deepest oceans (e.g.,
cusk-eels
The cusk-eel family, Ophidiidae, is a group of marine bony fishes in the Ophidiiformes order. The scientific name is from the Greek ''ophis'' meaning "snake", and refers to their eel-like appearance. True eels, however, diverged from other ray- ...
and
snailfish), although no species has yet been documented in the deepest 25% of the ocean. With 34,300 described species, fish exhibit greater species diversity than any other group of vertebrates.
Fish are an important resource for humans worldwide, especially
as food. Commercial and subsistence fishers hunt fish in
wild fisheries
A wild fishery is a natural body of water with a sizeable free-ranging fish or other aquatic animal (crustaceans and molluscs) population that can be harvested for its commercial value. Wild fisheries can be marine ( saltwater) or lacustrine/r ...
or
farm
A farm (also called an agricultural holding) is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is use ...
them in ponds or in cages in the ocean (in
aquaculture
Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
). They are also caught by
recreational fishers, kept as pets, raised by
fishkeepers, and exhibited in public
aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.
Tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s (
amphibian
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s,
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...
s,
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s and
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s) emerged within
lobe-finned fishes, so
cladistically
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
they are fish as well. However, traditionally fish (pisces or ichthyes) are rendered
paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
by excluding the tetrapods, and are therefore not considered a formal taxonomic grouping in
systematic biology
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tr ...
, unless it is used in the
cladistic
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
sense, including tetrapods, although usually "
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
" is preferred and used for this purpose (fish plus tetrapods) instead. Furthermore,
cetaceans
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
, although mammals, have often been considered fish by various cultures and time periods.
Etymology
The word for ''fish'' in English and the other
Germanic languages
The Germanic languages are a branch of the Indo-European language family spoken natively by a population of about 515 million people mainly in Europe, North America, Oceania and Southern Africa. The most widely spoken Germanic language, ...
(
German
German(s) may refer to:
* Germany (of or related to)
**Germania (historical use)
* Germans, citizens of Germany, people of German ancestry, or native speakers of the German language
** For citizens of Germany, see also German nationality law
**Ge ...
;
Gothic
Gothic or Gothics may refer to:
People and languages
*Goths or Gothic people, the ethnonym of a group of East Germanic tribes
**Gothic language, an extinct East Germanic language spoken by the Goths
**Crimean Gothic, the Gothic language spoken b ...
) is inherited from
Proto-Germanic
Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the reconstructed proto-language of the Germanic branch of the Indo-European languages.
Proto-Germanic eventually developed from pre-Proto-Germanic into three Germanic br ...
, and is related to the
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
and
Old Irish
Old Irish, also called Old Gaelic ( sga, Goídelc, Ogham script: ᚌᚑᚔᚇᚓᚂᚉ; ga, Sean-Ghaeilge; gd, Seann-Ghàidhlig; gv, Shenn Yernish or ), is the oldest form of the Goidelic/Gaelic language for which there are extensive writte ...
, though the exact root is unknown; some authorities reconstruct a
Proto-Indo-European
Proto-Indo-European (PIE) is the reconstructed common ancestor of the Indo-European language family. Its proposed features have been derived by linguistic reconstruction from documented Indo-European languages. No direct record of Proto-Indo ...
root , attested only in
Italic,
Celtic
Celtic, Celtics or Keltic may refer to:
Language and ethnicity
*pertaining to Celts, a collection of Indo-European peoples in Europe and Anatolia
**Celts (modern)
*Celtic languages
**Proto-Celtic language
*Celtic music
*Celtic nations
Sports Foo ...
, and
Germanic.
The English word once had a much broader usage than its current biological meaning. Names such as
starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
,
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...
,
shellfish
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
and
cuttlefish
Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
attest to almost any fully aquatic animal (including whales) once being ''fish''. "Correcting" such names (e.g. to ''sea star'') is an attempt to retroactively apply the current meaning of ''fish'' to words that were coined when it had a different meaning.
Evolution
Fish, as vertebrata, developed as sister of the tunicata. As the tetrapods emerged deep within the fishes group, as sister of the lungfish, characteristics of fish are typically shared by tetrapods, including having vertebrae and a cranium.
Early fish from the fossil record are represented by a group of small, jawless, armored fish known as
ostracoderm
Ostracoderms () are the armored jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are closer to cyclostomes) and ...
s. Jawless fish lineages are mostly extinct. An extant clade, the
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s may approximate ancient pre-jawed fish. The first jaws are found in
Placodermi
Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally ' plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were co ...
fossils. They lacked distinct teeth, having instead the oral surfaces of their jaw plates modified to serve the various purposes of teeth. The diversity of jawed vertebrates may indicate the evolutionary advantage of a
jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.
Fish may have evolved from a creature similar to a coral-like
sea squirt
Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" m ...
, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have
kept the larval form into adulthood (as some sea squirts do today).
Phylogeny
Fishes are a
paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
group: that is, any
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
containing all fish also contains the
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s, which are not fish (though they include fish-shaped forms, such as
Whales and Dolphins or the
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
ichthyosaurs
Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alth ...
, which acquired a fish-like body shape due to
secondary aquatic adaptation
Several groups of tetrapods have undergone secondary aquatic adaptation, an evolutionary transition from being purely terrestrial to living at least part of the time in water. These animals are called "secondarily aquatic" because although their a ...
, see
evolution of cetaceans
The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals bel ...
).
The following
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
shows
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
s - some with, some without
extant
Extant is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Extant taxon, a taxon which is not extinct, ...
relatives - that are traditionally considered as "fishes" (
cyan
Cyan () is the color between green and blue on the visible spectrum of light. It is evoked by light with a predominant wavelength between 490 and 520 nm, between the wavelengths of green and blue.
In the subtractive color system, or CMYK col ...
line) and the tetrapods (four-limbed vertebrates), which are mostly terrestrial.
Extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
groups are marked with a
dagger
A dagger is a fighting knife with a very sharp point and usually two sharp edges, typically designed or capable of being used as a thrusting or stabbing weapon.State v. Martin, 633 S.W.2d 80 (Mo. 1982): This is the dictionary or popular-use def ...
(†).
Taxonomy
Fishes are a
paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
group and for this reason, groups such as the class ''Pisces'' seen in older reference works are no longer used in formal classifications. Traditional classification divides fish into three
extant
Extant is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Extant taxon, a taxon which is not extinct, ...
classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes:
[ Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985)]
* Class
Agnatha
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...
(jawless fish)
** Subclass
Cyclostomata
Cyclostomi, often referred to as Cyclostomata , is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have jawless mouths with horny epidermal structures that function as teeth called cer ...
(
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
and
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s)
** Subclass
Ostracodermi (armoured jawless fish) †
* Class
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
(cartilaginous fish)
** Subclass
Elasmobranchii
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including sharks (superorder Selachii), rays, skates, and sawfish (superorder Batoidea). Members of this subclass are characterised by having five to seven pairs of ...
(
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s and
rays
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gra ...
)
** Subclass
Holocephali
Holocephali ("complete heads"), sometimes given the term Euchondrocephali, is a subclass of cartilaginous fish in the class Chondrichthyes. The earliest fossils are of teeth and come from the Devonian period. Little is known about these primitiv ...
(
chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At ...
s and extinct relatives)
* Class
Placodermi
Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally ' plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were co ...
(armoured fish) †
* Class
Acanthodii
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, ...
("spiny sharks", sometimes classified under Actinopterygii)
* Class
Osteichthyes
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...
(bony fish)
** Subclass
Actinopterygii
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
(ray finned fishes)
** Subclass
Sarcopterygii
Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...
(lobe finned fishes, ancestors of tetrapods)
The above scheme is the one most commonly encountered in non-specialist and general works. Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of
phylogenetic nomenclature
Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...
, the fishes has been split up into a more detailed scheme, with the following major groups:
* Class Myxini (
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
)
* Class
Pteraspidomorphi
Pteraspidomorphi is an extinct class of early jawless fish. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as basal traits for all ...
† (early jawless fish)
* Class
Thelodonti
Thelodonti (from Greek: "feeble teeth")Maisey, John G., Craig Chesek, and David Miller. Discovering fossil fishes. New York: Holt, 1996. is a class of extinct jawless fishes with distinctive scales instead of large plates of armor.
There is much ...
†
* Class
Anaspida
Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived primarily during the Silurian period, and became extinct soon after the start of the Devonian. They were classically regarded as the ancestors of lampre ...
†
* Class
Petromyzontida or Hyperoartia
** Petromyzontidae (
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s)
* Class
Conodont
Conodonts ( Greek ''kōnos'', " cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, whi ...
a (conodonts) †
* Class
Cephalaspidomorphi † (early jawless fish)
** (unranked)
Galeaspida
Galeaspida (from Latin, 'Helmet shields') is an extinct taxon of jawless marine and freshwater fish. The name is derived from ''galea'', the Latin word for ''helmet'', and refers to their massive bone shield on the head. Galeaspida lived in shallo ...
†
** (unranked)
Pituriaspida
The Pituriaspida ('Pituri-shield' or 'hallucinogen-shield') are a small group of extinct armored jawless fishes with tremendous nose-like rostrums, which lived in the marine, deltaic environments of Middle Devonian Australia (about 390 Ma). The ...
†
** (unranked)
Osteostraci
The class Osteostraci (meaning "bony shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Middle Silurian to Late Devonian.
Anatomically speaking, t ...
†
* Infraphylum
Gnathostomata
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, livi ...
(jawed vertebrates)
** Class
Placodermi
Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally ' plate-skinned') is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were co ...
† (armoured fish)
** Class
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
(cartilaginous fish)
** Class
Acanthodii
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, ...
† (spiny sharks)
** Superclass
Osteichthyes
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...
(bony fish)
*** Class
Actinopterygii
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
(ray-finned fish)
**** Subclass
Chondrostei
Chondrostei is a group of non-neopterygian ray-finned fish, while the term originally referred to a paraphyletic group of all non-neopterygian ray-finned fish, it was redefined by Patterson in 1982 to be a clade comprising the Acipenseriformes (w ...
***** Order
Acipenseriformes
Acipenseriformes is an order of basal ray-finned fishes that includes living and fossil sturgeons and paddlefishes (Acipenseroidei), as well as the extinct families Chondrosteidae and Peipiaosteidae. They are the second earliest diver ...
(
sturgeon
Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous, and are descended from other, earlier acipenseriform fish, which date back to the Early ...
s and
paddlefish
Paddlefish (family Polyodontidae) are a family of ray-finned fish belonging to order Acipenseriformes, and one of two living groups of the order alongside sturgeons (Acipenseridae). They are distinguished from other fish by their titular elong ...
es)
***** Order
Polypteriformes
Bichirs and the reedfish comprise Polypteridae , a family of archaic ray-finned fishes and the only family in the order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The Diversity of Fishes. West Sussex, UK: Blackwell Pu ...
(
reedfishes and
bichir
Bichirs and the reedfish comprise Polypteridae , a family of archaic ray-finned fishes and the only family in the order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The Diversity of Fishes. West Sussex, UK: Blackwell Pu ...
s).
**** Subclass
Neopterygii
Neopterygii (from Greek νέος ''neos'' 'new' and πτέρυξ ''pteryx'' 'fin') is a subclass of ray-finned fish (Actinopterygii). Neopterygii includes the Holostei and the Teleostei, of which the latter comprise the vast majority of extant ...
***** Infraclass
Holostei
Holostei is a group of ray-finned bony fish. It is divided into two major clades, the Halecomorphi, represented by a single living species, the bowfin ('' Amia calva''), as well as the Ginglymodi, the sole living representatives being the gars ...
(
gars and
bowfin
The bowfin (''Amia calva'') is a bony fish, native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being the sole surviving species of the Halecomorp ...
s)
***** Infraclass
Teleostei
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Te ...
(many orders of common fish)
*** Class
Sarcopterygii
Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includi ...
(lobe-finned fish)
**** Subclass
Actinistia
The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
(
coelacanth
The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s)
**** Subclass Dipnoi (
lungfish
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
, sister group to the
tetrapods
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...
)
† – indicates extinct taxon
Some palaeontologists contend that because
Conodonta are
chordate
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s, they are primitive fish. For a fuller treatment of this taxonomy, see the
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
article.
The position of
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
in the phylum Chordata is not settled. Phylogenetic research in 1998 and 1999 supported the idea that the hagfish and the lampreys form a natural group, the
Cyclostomata
Cyclostomi, often referred to as Cyclostomata , is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have jawless mouths with horny epidermal structures that function as teeth called cer ...
, that is a sister group of the Gnathostomata.
The various fish groups account for more than half of vertebrate species. As of 2006, there are almost 28,000 known
extant
Extant is the opposite of the word extinct. It may refer to:
* Extant hereditary titles
* Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English
* Extant taxon, a taxon which is not extinct, ...
species, of which almost 27,000 are bony fish, with 970
sharks, rays, and chimeras and about 108 hagfish and lampreys. A third of these species fall within the nine largest families; from largest to smallest, these families are
Cyprinidae
Cyprinidae is a family of freshwater fish commonly called the carp or minnow family. It includes the carps, the true minnows, and relatives like the barbs and barbels. Cyprinidae is the largest and most diverse fish family and the largest ve ...
,
Gobiidae
Gobiidae or gobies is a family of bony fish in the order Gobiiformes, one of the largest fish families comprising more than 2,000 species in more than 200 genera. Most of gobiid fish are relatively small, typically less than in length, and the ...
,
Cichlidae
Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted thi ...
,
Characidae
Characidae, the characids or characins is a family of freshwater subtropical and tropical fish, belonging to the order Characiformes. The name "characins" is the historical one, but scientists today tend to prefer "characids" to reflect their ...
,
Loricariidae
The Loricariidae is the largest family of catfish (order Siluriformes), with 92 genera and just over 680 species. Loricariids originate from freshwater habitats of Costa Rica, Panama, and tropical and subtropical South America. These fish are not ...
,
Balitoridae,
Serranidae
The Serranidae are a large family of fishes belonging to the order Perciformes. The family contains about 450 species in 65 genera, including the sea basses and the groupers (subfamily Epinephelinae). Although many species are small, in some ca ...
,
Labridae, and
Scorpaenidae
The Scorpaenidae (also known as scorpionfish) are a family of mostly marine fish that includes many of the world's most venomous species. As their name suggests, scorpionfish have a type of "sting" in the form of sharp spines coated with venom ...
. About 64 families are
monotypic
In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispe ...
, containing only one species. The final total of extant species may grow to exceed 32,500. Each year, new
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
are discovered and
scientifically described
A species description is a formal description of a newly discovered species, usually in the form of a scientific paper. Its purpose is to give a clear description of a new species of organism and explain how it differs from species that have be ...
. As of 2016,
there are over 32,000 documented species of bony fish and over 1,100 species of cartilaginous fish. Species are lost through
extinction
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...
(see
biodiversity crisis
Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...
). Recent examples are the
Chinese paddlefish
The Chinese paddlefish (''Psephurus gladius''; : literal translation: "white sturgeon"), also known as the Chinese swordfish, is an extinct species of fish that was formerly native to the Yangtze and Yellow River basins in China. With records of ...
or the
smooth handfish
The smooth handfish (''Sympterichthys unipennis'') is a possibly extinct species of handfish in the genus ''Sympterichthys''. It was endemic to waters off the coast of Tasmania, mainly the D’Entrecasteaux Channel. It was declared extinct b ...
.
Diversity
Pacific_hagfish_Myxine.jpg, Agnatha
( Pacific hagfish)
Hornhai (Heterodontus francisci).JPG, Chondrichthyes
(Horn shark
The horn shark (''Heterodontus francisci'') is a species of bullhead shark, in the family Heterodontidae. It is endemic to the coastal waters off the western coast of North America, from California to the Gulf of California. Young sharks are seg ...
)
Salmo trutta.jpg, Actinopterygii
(Brown trout
The brown trout (''Salmo trutta'') is a European species of salmonid fish that has been widely introduced into suitable environments globally. It includes purely freshwater populations, referred to as the riverine ecotype, ''Salmo trutta'' morp ...
)
Latimeria chalumnae01.jpg, Sarcopterygii
(Coelacanth
The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
)
The term "fish" most precisely describes any non-
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
craniate
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...
(i.e. an animal with a skull and in most cases a backbone) that has
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s throughout life and whose limbs, if any, are in the shape of fins. Unlike groupings such as birds or
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s, fish are not a single
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
but a
paraphyletic
In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In ...
collection of
taxa
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
, including
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
es,
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s,
sharks and rays,
ray-finned fish
Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species.
The ray-finned fishes are so called because their fins are webs of skin supported by bony or h ...
,
coelacanth
The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s, and
lungfish
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
. Indeed, lungfish and coelacanths are closer relatives of
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s (such as mammals, birds,
amphibians
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
, etc.) than of other fish such as ray-finned fish or sharks, so the
last common ancestor
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
of all fish is also an ancestor to tetrapods. As paraphyletic groups are no longer recognised in modern
systematic biology
Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tr ...
, the use of the term "fish" as a biological group must be avoided.
Many types of
aquatic animal
An aquatic animal is any animal, whether invertebrate or vertebrate, that lives in water for most or all of its lifetime. Many insects such as mosquitoes, mayflies, dragonflies and caddisflies have aquatic larvae, with winged adults. Aquatic ...
s commonly referred to as "fish" are not fish in the sense given above; examples include
shellfish
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
,
cuttlefish
Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
,
starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
,
crayfish
Crayfish are freshwater crustaceans belonging to the clade Astacidea, which also contains lobsters. In some locations, they are also known as crawfish, craydids, crawdaddies, crawdads, freshwater lobsters, mountain lobsters, rock lobsters, m ...
and
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...
. In earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also
seals
Seals may refer to:
* Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly:
** Earless seal, or "true seal"
** Fur seal
* Seal (emblem), a device to impress an emblem, used as a means of a ...
, whales,
amphibian
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s,
crocodile
Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant me ...
s, even
hippopotamus
The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two exta ...
es, as well as a host of aquatic invertebrates, as fish.
However, according to the definition above, all mammals, including
cetacean
Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel th ...
s like whales and dolphins, are not fish. In some contexts, especially in
aquaculture
Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. lot ...
, the true fish are referred to as finfish (or fin fish) to distinguish them from these other animals.
A typical fish is
ectothermic
An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
, has a
streamlined
Streamlines, streaklines and pathlines are field lines in a fluid flow.
They differ only when the flow changes with time, that is, when the flow is not steady.
Considering a velocity vector field in three-dimensional space in the framework of ...
body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with
scales
Scale or scales may refer to:
Mathematics
* Scale (descriptive set theory), an object defined on a set of points
* Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original
* Scale factor, a number w ...
, and lays eggs.
Each criterion has exceptions.
Tuna
A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
,
swordfish
Swordfish (''Xiphias gladius''), also known as broadbills in some countries, are large, highly migratory predatory fish characterized by a long, flat, pointed bill. They are a popular sport fish of the billfish category, though elusive. Swordf ...
, and some species of
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s show
some warm-blooded adaptations – they can heat their bodies significantly above ambient water temperature. Streamlining and swimming performance varies from fish such as tuna,
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
, and
jacks that can cover 10–20 body-lengths per second to species such as
eel
Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
s and
rays
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gra ...
that swim no more than 0.5 body-lengths per second. Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures.
Lungfish
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
have paired lungs similar to those of tetrapods,
gourami
Gouramis, or gouramies , are a group of freshwater anabantiform fishes that comprise the family Osphronemidae. The fish are native to Asia—from the Indian Subcontinent to Southeast Asia and northeasterly towards Korea. The name "gourami", of ...
s have a structure called the
labyrinth organ
The Anabantoidei are a suborder of anabantiform ray-finned freshwater fish distinguished by their possession of a lung-like labyrinth organ, which enables them to breathe air. The fish in the Anabantoidei suborder are known as anabantoids or lab ...
that performs a similar function, while many catfish, such as ''
Corydoras
''Corydoras'' is a genus of freshwater catfish in the family Callichthyidae and subfamily Corydoradinae. The species usually have more restricted areas of endemism than other callichthyids, but the area of distribution of the entire genus almost ...
'' extract oxygen via the intestine or stomach. Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as
seahorse
A seahorse (also written ''sea-horse'' and ''sea horse'') is any of 46 species of small marine fish in the genus ''Hippocampus''. "Hippocampus" comes from the Ancient Greek (), itself from () meaning "horse" and () meaning "sea monster" or ...
s,
pufferfish
Tetraodontidae is a family of primarily marine and estuarine fish of the order Tetraodontiformes. The family includes many familiar species variously called pufferfish, puffers, balloonfish, blowfish, blowies, bubblefish, globefish, swellfis ...
,
anglerfish
The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
, and
gulpers. Similarly, the surface of the skin may be naked (as in
moray eel
Moray eels, or Muraenidae (), are a family of eels whose members are found worldwide. There are approximately 200 species in 15 genera which are almost exclusively marine, but several species are regularly seen in brackish water, and a few are f ...
s), or covered with scales of a variety of different types usually defined as
placoid
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as w ...
(typical of sharks and rays),
cosmoid (fossil lungfish and coelacanths),
ganoid (various fossil fish but also living
gars and
bichir
Bichirs and the reedfish comprise Polypteridae , a family of archaic ray-finned fishes and the only family in the order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The Diversity of Fishes. West Sussex, UK: Blackwell Pu ...
s),
cycloid
In geometry, a cycloid is the curve traced by a point on a circle as it rolls along a straight line without slipping. A cycloid is a specific form of trochoid and is an example of a roulette, a curve generated by a curve rolling on another cu ...
, and
ctenoid
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...
(these last two are found on most
bony fish
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...
). There are even fish that live mostly on land or lay their eggs on land near water.
Mudskipper
Mudskippers are any of the 23 extant species of amphibious fish from the subfamily Oxudercinae of the goby family Oxudercidae. They are known for their unusual body shapes, preferences for semiaquatic habitats, limited terrestrial locomotion ...
s feed and interact with one another on mudflats and go underwater to hide in their burrows. A single
undescribed species
In taxonomy, an undescribed taxon is a taxon (for example, a species) that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. U ...
of ''
Phreatobius
''Phreatobius'' is a genus of very small catfishes ( order Siluriformes) from tropical South America.
Taxonomy
For quite some time ''Phreatobius cisternarum'' was the only species in the genus. However, two new species were described relatively ...
'' has been called a true "land fish" as this worm-like catfish strictly lives among waterlogged
leaf litter
Plant litter (also leaf litter, tree litter, soil litter, litterfall or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that have fallen to the ground. This detritus or dead organic material and its constituent ...
. Many species live in
underground lake
An underground lake or subterranean lake is a lake underneath the surface of the Earth. Most naturally occurring underground lakes are found in areas of Karst topography, where limestone or other soluble rock has been weathered away, leaving a cav ...
s,
underground rivers or
aquifer
An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characte ...
s and are popularly known as
cavefish
Cavefish or cave fish is a generic term for fresh and brackish water fish adapted to life in caves and other underground habitats. Related terms are subterranean fish, troglomorphic fish, troglobitic fish, stygobitic fish, phreatic fish and ...
.
Fish range in size from the huge
whale shark
The whale shark (''Rhincodon typus'') is a slow-moving, filter feeder, filter-feeding carpet shark and the largest known Extant taxon, extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Bran ...
to the tiny
stout infantfish
''Schindleria brevipinguis'' is a species of marine fish in family Gobiidae of Perciformes. Known as the stout infantfish, it is native to Australia's Great Barrier Reef and to Osprey Reef in the Coral Sea.
Anatomy
''S. brevipinguis'' is among ...
.
Fish
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
diversity is roughly divided equally between marine (oceanic) and
freshwater
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
ecosystems.
Coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s in the
Indo-Pacific
The Indo-Pacific is a vast biogeographic region of Earth.
In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
constitute the center of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large
river basin
A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, ...
s of
tropical rainforest
Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equator ...
s, especially the
Amazon
Amazon most often refers to:
* Amazons, a tribe of female warriors in Greek mythology
* Amazon rainforest, a rainforest covering most of the Amazon basin
* Amazon River, in South America
* Amazon (company), an American multinational technolog ...
,
Congo, and
Mekong
The Mekong or Mekong River is a trans-boundary river in East Asia and Southeast Asia. It is the world's twelfth longest river and the third longest in Asia. Its estimated length is , and it drains an area of , discharging of water annual ...
basins. More than 5,600 fish species inhabit
Neotropic
The Neotropical realm is one of the eight biogeographic realms constituting Earth's land surface. Physically, it includes the tropical terrestrial ecoregions of the Americas and the entire South American temperate zone.
Definition
In ...
al freshwaters alone, such that
Neotropical fish The freshwater fish of tropical South and Central America represent one of the most diverse and extreme aquatic ecosystems on Earth, with more than 5,600 species, representing about 10% all living vertebrate species. The exceptional diversity of sp ...
es represent about 10% of all
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
species on the Earth. Exceptionally rich sites in the Amazon basin, such as
Cantão State Park
The Cantão State Park ( pt, Parque Estadual do Cantão) is a state park in the state of Tocantins, Brazil.
It protects the Cantão wetlands, an area of igapó flooded forest in the ecotone where the Amazon rainforest merges into the cerrado.
Loc ...
, can contain more freshwater fish species than occur in all of Europe.
The deepest living fish in the ocean so far found is the Mariana snailfish (''
Pseudoliparis swirei
''Pseudoliparis swirei'', the Mariana snailfish or Mariana hadal snailfish, is a species of snailfish found at hadal depths in the Mariana Trench in the western Pacific Ocean. It is known from a depth range of , including a capture at the deepest ...
'') which lives at deeps of 8,000 meters (26,200 feet) along the Mariana Trench near Guam.
The
diversity
Diversity, diversify, or diverse may refer to:
Business
*Diversity (business), the inclusion of people of different identities (ethnicity, gender, age) in the workforce
*Diversity marketing, marketing communication targeting diverse customers
* ...
of living fish (finfish) is unevenly distributed among the various groups, with
teleosts
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Teleo ...
making up the bulk of living fishes (96%), and over 50% of all
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
.
The following
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
shows the
evolutionary relationships of all groups of living fishes (with their respective diversity
) and the four-limbed vertebrates (
tetrapods
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...
).
Anatomy and physiology
Respiration
Gills
Most fish exchange gases using
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s on either side of the
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
. Gills consist of threadlike structures called
filaments. Each filament contains a
capillary
A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the bod ...
network that provides a large
surface area
The surface area of a solid object is a measure of the total area that the surface of the object occupies. The mathematical definition of surface area in the presence of curved surfaces is considerably more involved than the definition of ...
for exchanging
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
and
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing
countercurrent exchange
Countercurrent exchange is a mechanism occurring in nature and mimicked in industry and engineering, in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each ...
. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s and
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s, possess multiple gill openings. However,
bony fish
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...
have a single gill opening on each side. This opening is hidden beneath a protective bony cover called an
operculum.
Juvenile
bichir
Bichirs and the reedfish comprise Polypteridae , a family of archaic ray-finned fishes and the only family in the order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The Diversity of Fishes. West Sussex, UK: Blackwell Pu ...
s have external gills, a very primitive feature that they share with larval
amphibian
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s.
Air breathing
Fish from multiple groups can live out of the water for extended periods.
Amphibious fish
Amphibious fish are fish that are able to leave water for extended periods of time. About 11 distantly related genera of fish are considered amphibious. This suggests that many fish genera independently evolved amphibious traits, a process known ...
such as the
mudskipper
Mudskippers are any of the 23 extant species of amphibious fish from the subfamily Oxudercinae of the goby family Oxudercidae. They are known for their unusual body shapes, preferences for semiaquatic habitats, limited terrestrial locomotion ...
can live and move about on land for up to several days, or live in stagnant or otherwise oxygen-depleted water. Many such fish can breathe air via a variety of mechanisms. The skin of
anguillid eels may absorb oxygen directly. The
buccal cavity
The buccal space (also termed the buccinator space) is a fascial space of the head and neck (sometimes also termed fascial tissue spaces or tissue spaces). It is a potential space in the cheek, and is paired on each side. The buccal space is super ...
of the
electric eel
The electric eels are a genus, ''Electrophorus'', of neotropical freshwater fish from South America in the family Gymnotidae. They are known for their ability to stun their prey by generating electricity, delivering shocks at up to 860 volt ...
may breathe air. Catfish of the families
Loricariidae
The Loricariidae is the largest family of catfish (order Siluriformes), with 92 genera and just over 680 species. Loricariids originate from freshwater habitats of Costa Rica, Panama, and tropical and subtropical South America. These fish are not ...
,
Callichthyidae
Callichthyidae is a family of catfishes (order Siluriformes), called armored catfishes due to the two rows of bony plates (or scutes) along the lengths of their bodies. It contains some of the most popular freshwater aquarium fish, such as many ...
, and
Scoloplacidae
''Scoloplax'' is the only genus in the catfish ( order Siluriformes) family Scoloplacidae, the spiny dwarf catfishes.
Species
The six currently recognized species in this genus are:
* '' Scoloplax baileyi'' Rocha, Lazzarotto & Rapp Py-Daniel ...
absorb air through their digestive tracts.
Lungfish
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
, with the exception of the
Australian lungfish
The Australian lungfish (''Neoceratodus forsteri''), also known as the Queensland lungfish, Burnett salmon and barramunda, is the only surviving member of the family Neoceratodontidae. It is one of only six extant lungfish species in the world. ...
, and
bichir
Bichirs and the reedfish comprise Polypteridae , a family of archaic ray-finned fishes and the only family in the order Polypteriformes .Helfman GS, Collette BB, Facey DE, Bowen BW. 2009. The Diversity of Fishes. West Sussex, UK: Blackwell Pu ...
s have paired lungs similar to those of
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
s and must surface to gulp fresh air through the mouth and pass spent air out through the gills.
Gar and
bowfin
The bowfin (''Amia calva'') is a bony fish, native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being the sole surviving species of the Halecomorp ...
have a vascularized swim bladder that functions in the same way.
Loaches
Loaches are fish of the superfamily Cobitoidea. They are freshwater, benthic (bottom-dwelling) fish found in rivers and creeks throughout Eurasia and northern Africa. Loaches are among the most diverse groups of fish; the 1249 known species of C ...
,
trahiras, and many
catfish
Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive, ...
breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to frogs). A number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. Labyrinth fish (such as
gourami
Gouramis, or gouramies , are a group of freshwater anabantiform fishes that comprise the family Osphronemidae. The fish are native to Asia—from the Indian Subcontinent to Southeast Asia and northeasterly towards Korea. The name "gourami", of ...
s and
betta
''Betta'' is a large genus of small, active, often colorful, freshwater ray-finned fishes, in the gourami family (biology), family (Osphronemidae). The best known ''Betta'' species is ''B. splendens,'' commonly known as the Siamese fighting fi ...
s) have a
labyrinth organ
The Anabantoidei are a suborder of anabantiform ray-finned freshwater fish distinguished by their possession of a lung-like labyrinth organ, which enables them to breathe air. The fish in the Anabantoidei suborder are known as anabantoids or lab ...
above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably
snakeheads,
pikeheads, and the
Clariidae catfish family.
Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and
cichlid
Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted th ...
s, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of
aestivation
Aestivation ( la, aestas (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a ...
(summertime hibernation) until water returns.
Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the
African lungfish
''Protopterus'' is the genus of four species of lungfish found in Africa. ''Protopterus'' was formerly thought to be the sole genus in the family Protopteridae, but more recent studies have classified it with ''Lepidosiren'' in the family Lepi ...
, ''must'' breathe air periodically or they suffocate. Facultative air breathers, such as the catfish ''
Hypostomus plecostomus
''Hypostomus plecostomus'', also known as the suckermouth catfish or the common pleco, is a tropical freshwater fish belonging to the armored catfish family (Loricariidae), named for the longitudinal rows of armor-like scutes that cover the upp ...
'', only breathe air if they need to and will otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators.
Circulation
Fish have a
closed-loop circulatory system. The
heart
The heart is a muscular Organ (biology), organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as ca ...
pumps the blood in a single loop throughout the body. In most fish, the heart consists of four parts, including two chambers and an entrance and exit.
The first part is the
sinus venosus
The sinus venosus is a large quadrangular cavity which precedes the atrium on the venous side of the chordate heart. In mammals, it exists distinctly only in the embryonic heart, where it is found between the two venae cavae. However, the sinus v ...
, a thin-walled sac that collects blood from the fish's
vein
Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated ...
s before allowing it to flow to the second part, the
atrium
Atrium may refer to:
Anatomy
* Atrium (heart), an anatomical structure of the heart
* Atrium, the genital structure next to the genital aperture in the reproductive system of gastropods
* Atrium of the ventricular system of the brain
* Pulmona ...
, which is a large muscular chamber. The atrium serves as a one-way antechamber, sends blood to the third part,
ventricle. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part,
bulbus arteriosus
In the circulatory system of fish, the bulbus arteriosus is a pear shaped chamber that functions as a capacitor, maintaining continuous blood flow into the gill arch
Branchial arches, or gill arches, are a series of bony "loops" present in fish ...
, a large tube, and then out of the heart. The bulbus arteriosus connects to the
aorta
The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes o ...
, through which blood flows to the gills for oxygenation.
Digestion
Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the
esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called
pyloric caeca
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
, which secrete digestive
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s and absorb nutrients. Organs such as the
liver
The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
and
pancreas
The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.
Excretion
As with many aquatic animals, most fish release their nitrogenous wastes as
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
. Some of the wastes
diffuse
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
through the gills. Blood wastes are
filtered
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...
by the
kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...
s.
Saltwater fish tend to lose water because of
osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...
. Their kidneys return water to the body. The reverse happens in
freshwater fish
Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%. These environments differ from marine conditions in many ways, especially the difference in levels o ...
: they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.
Scales
The scales of fish originate from the
mesoderm
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Emb ...
(skin); they may be similar in structure to teeth.
Sensory and nervous system
Central nervous system
Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, most notably
mormyrids and
shark
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s, which have brains about as massive relative to body weight as birds and
marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in ...
s.
Fish brains are divided into several regions. At the front are the
olfactory lobes
The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) ...
, a pair of structures that receive and process signals from the
nostril
A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...
s via the two
olfactory nerve
The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell.
The afferent nerve fibers of the olfactory receptor neurons t ...
s. The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed
telencephalon
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. ...
, the structural equivalent to the
cerebrum
The cerebrum, telencephalon or endbrain is the largest part of the brain containing the cerebral cortex (of the two cerebral hemispheres), as well as several subcortical structures, including the hippocampus, basal ganglia, and olfactory bulb. ...
in
higher vertebrates
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distingu ...
. In fish the telencephalon is concerned mostly with
olfaction
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, ...
. Together these structures form the forebrain.
Connecting the forebrain to the midbrain is the
diencephalon
The diencephalon (or interbrain) is a division of the forebrain (embryonic ''prosencephalon''). It is situated between the telencephalon and the midbrain (embryonic ''mesencephalon''). The diencephalon has also been known as the 'tweenbrain in o ...
(in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with
hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
s and
homeostasis
In biology, homeostasis ( British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
. The
pineal body lies just above the diencephalon. This structure detects light, maintains
circadian
A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., Endogeny (biology), endogeno ...
rhythms, and controls color changes.
The
midbrain
The midbrain or mesencephalon is the forward-most portion of the brainstem and is associated with vision, hearing, motor control, sleep and wakefulness, arousal ( alertness), and temperature regulation. The name comes from the Greek ''mesos'', ...
(or mesencephalon) contains the two
optic lobes. These are very large in species that hunt by sight, such as
rainbow trout
The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coast ...
and
cichlid
Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted th ...
s.
The hindbrain (or
metencephalon
The metencephalon is the embryonic part of the hindbrain that differentiates into the pons and the cerebellum. It contains a portion of the fourth ventricle and the trigeminal nerve (CN V), abducens nerve (CN VI), facial nerve (CN VII), and a port ...
) is particularly involved in swimming and balance. The cerebellum is a single-lobed structure that is typically the biggest part of the brain. Hagfish and
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
s have relatively small cerebellae, while the
mormyrid
The Mormyridae, sometimes called "elephantfish" (more properly freshwater elephantfish), are a family of weakly electric freshwater fish in the order Osteoglossiformes native to Africa. It is by far the largest family in the order with around 20 ...
cerebellum is massive and apparently involved in their
electrical sense.
The brain stem (or
myelencephalon) is the brain's posterior. As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
and
osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration o ...
.
Sense organs
Most fish possess highly developed sense organs. Nearly all daylight fish have color vision that is at least as good as a human's (see
vision in fishes
Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals (including humans) normally adjust focus ...
). Many fish also have chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears, many fish may not hear very well. Most fish have sensitive receptors that form the
lateral line system, which detects gentle currents and vibrations, and senses the motion of nearby fish and prey.
The sense information obtained from the lateral line system can be considered both a sense of
touch
In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch ( haptic perception), as well as temperature ( thermoception), body position (proprioception), and pain. It ...
and
hearing
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audit ...
.
Blind cave fish
The Mexican tetra (''Astyanax mexicanus''), also known as the blind cave fish, blind cave characin, and blind cave tetra, is a freshwater fish of the family Characidae of the order Characiformes. The type species of its genus, it is native to ...
navigate almost entirely through the sensations from their lateral line system. Some fish, such as catfish and sharks, have the
ampullae of Lorenzini
Ampullae of Lorenzini (singular ''Ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of cartilaginous fish (sharks, rays, and chimaeras) and of basal actinopterygi ...
,
electroreceptor
Electroreception and electrogenesis are the closely-related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes to stu ...
s that detect weak electric currents on the order of millivolt. Other fish, like the South American electric fishes
Gymnotiformes
The Gymnotiformes are an order of teleost bony fishes commonly known as Neotropical knifefish or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found almost exclusively in fresh water (t ...
, can produce weak electric currents, which they use in navigation and social communication.
Fish orient themselves using landmarks and may use mental maps based on multiple landmarks or symbols. Fish behavior in mazes reveals that they possess spatial memory and visual discrimination.
Vision
Vision
Vision, Visions, or The Vision may refer to:
Perception Optical perception
* Visual perception, the sense of sight
* Visual system, the physical mechanism of eyesight
* Computer vision, a field dealing with how computers can be made to gain und ...
is an important
sensory system
The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved i ...
for most species of fish. Fish eyes are similar to those of
terrestrial
Terrestrial refers to things related to land or the planet Earth.
Terrestrial may also refer to:
* Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to ...
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s like
birds
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
and mammals, but have a more
spherical
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
. Their
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
s generally have both
rods and
cones
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines conn ...
(for
scotopic
In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek ''skotos'', meaning "darkness", and ''-opia'', meaning "a condition of sight". In the human eye, ...
and
photopic vision
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visu ...
), and most species have
colour vision
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
. Some fish can see
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
and some can see
polarized light
Polarization ( also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of t ...
. Amongst
jawless fish
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present ( cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...
, the
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
has well-developed eyes, while the
hagfish
Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, ...
has only primitive
eyespots. Fish vision shows
adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
to their visual environment, for example
deep sea fish
Deep-sea fish are fish that live in the darkness below the sunlit surface waters, that is below the epipelagic or photic zone of the sea. The lanternfish is, by far, the most common deep-sea fish. Other deep sea fishes include the flashligh ...
es have eyes suited to the dark environment.
Hearing
Hearing
Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is audit ...
is an important sensory system for most species of fish. Fish sense sound using their
lateral line
The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
s and their
ears
An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists o ...
.
Cognition
New research has expanded preconceptions about the cognitive capacities of fish. For example,
manta rays
Manta rays are large rays belonging to the genus ''Mobula'' (formerly its own genus ''Manta''). The larger species, '' M. birostris'', reaches in width, while the smaller, '' M. alfredi'', reaches . Both have triangular pectoral fins, horn-sh ...
have exhibited behavior linked to
self-awareness
In philosophy of self, self-awareness is the experience of one's own personality or individuality. It is not to be confused with consciousness in the sense of qualia. While consciousness is being aware of one's environment and body and lifest ...
in
mirror test
The mirror test—sometimes called the mark test, mirror self-recognition (MSR) test, red spot technique, or rouge test—is a behavioral technique developed in 1970 by American psychologist Gordon Gallup Jr. as an attempt to determine whether an ...
cases. Placed in front of a mirror, individual rays engaged in contingency testing, that is, repetitive behavior aiming to check whether their reflection's behavior mimics their body movement.
Wrasses
The wrasses are a family, Labridae, of marine fish, many of which are brightly colored. The family is large and diverse, with over 600 species in 81 genera, which are divided into 9 subgroups or tribes.
They are typically small, most of them ...
have also passed the mirror test in a 2018 scientific study.
Cases of tool use have also been noticed, notably in the
Choerodon
''Choerodon'' is a genus of wrasses native to the Indian Ocean and the western Pacific Ocean. They are commonly referred to as tuskfish, because most species have sharp tusk-like teeth.
Species
The 24 currently recognized species in this gen ...
family, in
archerfish
The archerfish (spinner fish or archer fish) form a monotypic family, Toxotidae, of fish known for their habit of preying on land-based insects and other small animals by shooting them down with water droplets from their specialized mouths. ...
and
Atlantic cod
The Atlantic cod (''Gadus morhua'') is a benthopelagic fish of the family Gadidae, widely consumed by humans. It is also commercially known as cod or codling.[pain
Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...]
and fear responses. For instance, in Tavolga's experiments,
toadfish
Toadfish is the common name for a variety of species from several different families of fish, usually because of their toad-like appearance. "Dogfish" is a name for certain species along the gulf coast.
Dolphin-Toadfish relationship
Toadfish mak ...
grunted when electrically shocked and over time they came to grunt at the mere sight of an electrode.
In 2003, Scottish scientists at the
University of Edinburgh
The University of Edinburgh ( sco, University o Edinburgh, gd, Oilthigh Dhùn Èideann; abbreviated as ''Edin.'' in post-nominals) is a public research university based in Edinburgh, Scotland. Granted a royal charter by King James VI in 1 ...
and the Roslin Institute concluded that rainbow trout exhibit behaviors often associated with
pain
Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
in other animals.
Bee
Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfamil ...
venom
Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a st ...
and
acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
injected into the lips resulted in fish rocking their bodies and rubbing their lips along the sides and floors of their tanks, which the researchers concluded were attempts to relieve pain, similar to what mammals would do.
Neurons fired in a pattern resembling human neuronal patterns.
Professor James D. Rose of the
University of Wyoming
The University of Wyoming (UW) is a public land-grant research university in Laramie, Wyoming. It was founded in March 1886, four years before the territory was admitted as the 44th state, and opened in September 1887. The University of Wyoming ...
claimed the study was flawed since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours". Rose argues that since fish brains are so different from human brains, fish are probably not conscious in the manner humans are, so that reactions similar to human reactions to pain instead have other causes. Rose had published a study a year earlier arguing that fish cannot feel pain because their brains lack a
neocortex
The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...
. However, animal behaviorist
Temple Grandin
Mary Temple Grandin (born August 29, 1947) is an American academic and animal behaviorist. She is a prominent proponent for the humane treatment of livestock for slaughter and the author of more than 60 scientific papers on animal behavior. Gra ...
argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions."
Animal welfare advocates raise concerns about the possible
suffering
Suffering, or pain in a broad sense, may be an experience of unpleasantness or aversion, possibly associated with the perception of harm or threat of harm in an individual. Suffering is the basic element that makes up the negative valence of a ...
of fish caused by angling. Some countries, such as Germany, have banned specific types of fishing, and the British
RSPCA
The Royal Society for the Prevention of Cruelty to Animals (RSPCA) is a charity operating in England and Wales that promotes animal welfare. The RSPCA is funded primarily by voluntary donations. Founded in 1824, it is the oldest and largest a ...
now formally prosecutes individuals who are cruel to fish.
Emotion
In 2019, scientists have shown that members of the
monogamous
Monogamy ( ) is a form of dyadic relationship in which an individual has only one partner during their lifetime. Alternately, only one partner at any one time ( serial monogamy) — as compared to the various forms of non-monogamy (e.g., pol ...
species ''
Amatitlania siquia
''Amatitlania siquia'', the Honduran red point cichlid, is a species of cichlid native to Central America. These cichlid can be identified by black melanin-based stripes in males and an additional bright orange spot on the abdomen for females.
...
'' exhibit pessimistic behavior when they are prevented from being with their partner.
Muscular system
Most fish move by alternately contracting paired sets of muscles on either side of the backbone. These contractions form S-shaped curves that move down the body. As each curve reaches the back fin, backward force is applied to the water, and in conjunction with the fins, moves the fish forward. The fish's fins function like an airplane's flaps. Fins also increase the tail's surface area, increasing speed. The streamlined body of the fish decreases the amount of friction from the water. Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fish have an internal organ called a
swim bladder
The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their current water depth wit ...
that adjusts their buoyancy through manipulation of gases.
Endothermy
Although most fish are exclusively
ectothermic
An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life ...
, there are exceptions. The only known bony fishes (infraclass
Teleostei
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Te ...
) that exhibit
endothermy
An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inste ...
are in the suborder
Scombroidei – which includes the
billfish
The term billfish refers to a group of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than . Extant billfish include sailfish and marlin, which make up the family Istioph ...
es, tunas, and the
butterfly kingfish
The butterfly kingfish (''Gasterochisma melampus'') is an ocean-dwelling ray-finned bony fish in the mackerel family, Scombridae – a family which it shares with the tunas, mackerels, Spanish mackerels, and bonitos. Unlike the 50 species from th ...
, a
basal species of mackerel
– and also the
opah
Opahs, also commonly known as moonfish, sunfish (not to be confused with Molidae), kingfish, redfin ocean pan are large, colorful, deep-bodied pelagic lampriform fishes comprising the small family Lampridae (also spelled Lamprididae).
The f ...
. The opah, a
lampriform, was demonstrated in 2015 to use "whole-body endothermy", generating heat with its swimming muscles to warm its body while countercurrent exchange (as in respiration) minimizes heat loss. It is able to actively hunt prey such as squid and swim for long distances due to the ability to warm its entire body, including its heart, which is a trait typically found in only mammals and birds (in the form of
homeothermy). In the cartilaginous fishes (class
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
), sharks of the families
Lamnidae
The Lamnidae are the family of mackerel sharks known as white sharks. They are large, fast-swimming predatory fish found in oceans worldwide, though prefer environments with colder water. The name of the family is formed from the Greek word ''l ...
(porbeagle, mackerel, salmon, and great white sharks) and
Alopiidae
Thresher sharks are large lamniform sharks of the family Alopiidae found in all temperate and tropical oceans of the world; the family contains three extant species, all within the genus ''Alopias''.
All three thresher shark species have been l ...
(thresher sharks) exhibit endothermy. The degree of endothermy varies from the billfishes, which warm only their eyes and brain, to the
bluefin tuna Bluefin tuna is a common name used to refer to several species of tuna
A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, th ...
and the
porbeagle shark
The porbeagle (''Lamna nasus'') is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecological equivalent is ...
, which maintain body temperatures in excess of above ambient water temperatures.
Endothermy, though metabolically costly, is thought to provide advantages such as increased muscle strength, higher rates of central
nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
processing, and higher rates of
digestion
Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...
.
Reproductive system
Fish reproductive organs include
testicle
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...
s and
ovaries
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
. In most species, gonads are paired organs of similar size, which can be partially or totally fused.
There may also be a range of secondary organs that increase reproductive fitness.
In terms of
spermatogonia distribution, the structure of
teleost
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Tele ...
s testes has two types: in the most common, spermatogonia occur all along the
seminiferous tubule
Seminiferous tubules are located within the testes, and are the specific location of meiosis, and the subsequent creation of male gametes, namely spermatozoa. Structure
The epithelium of the tubule consists of a type of sustentacular cells known ...
s, while in
atherinomorph fish they are confined to the
distal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
portion of these structures. Fish can present cystic or semi-cystic
spermatogenesis
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubu ...
in relation to the release phase of germ cells in cysts to the seminiferous tubules
lumen.
Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the
oocyte
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
s are released directly into the
coelom
The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, ...
ic cavity and then enter the
ostium
An ostium (plural ostia) in anatomy is a small opening or orifice.
Ostium or ostia may refer to: Human anatomy
* Ostium of fallopian tube
* Ostium of the uterus (disambiguation)
* Ostium primum of the developing heart
* Ostium secundum ( foramen ...
, then through the
oviduct
The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
and are eliminated. Secondary gymnovarian ovaries shed
ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.
Gymnovaries are the primitive condition found in
lungfish
Lungfish are freshwater vertebrates belonging to the order Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, i ...
,
sturgeon
Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous, and are descended from other, earlier acipenseriform fish, which date back to the Early ...
, and
bowfin
The bowfin (''Amia calva'') is a bony fish, native to North America. Common names include mudfish, mud pike, dogfish, grindle, grinnel, swamp trout, and choupique. It is regarded as a relict, being the sole surviving species of the Halecomorp ...
. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.
Secondary gymnovaries are found in
salmonid
Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefi ...
s and a few other teleosts.
Oogonia
An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes.
In the mammalian fetus
Oogonia are formed in l ...
development in teleosts fish varies according to the group, and the determination of oogenesis dynamics allows the understanding of maturation and fertilization processes. Changes in the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
* Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
, ooplasm, and the surrounding layers characterize the oocyte maturation process.
Postovulatory
follicles are structures formed after oocyte release; they do not have
endocrine
The endocrine system is a messenger system comprising feedback loops of the hormones released by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neur ...
function, present a wide irregular lumen, and are rapidly reabsorbed in a process involving the
apoptosis
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes ( morphology) and death. These changes in ...
of follicular cells. A degenerative process called
follicular atresia
Follicular atresia refers to the process in which a follicle fails to develop, thus preventing it from ovulating and releasing an egg. It is a normal, naturally occurring progression that occurs as mammalian ovaries age. Approximately 1% of mamma ...
reabsorbs vitellogenic oocytes not spawned. This process can also occur, but less frequently, in oocytes in other development stages.
Some fish, like the
California sheephead
The California sheephead (''Semicossyphus pulcher'') is a species of wrasse native to the eastern Pacific Ocean. Its range is from Monterey Bay, California, to the Gulf of California, Mexico. It can live for up to 20 years in favorable condition ...
, are
hermaphrodite
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have ...
s, having both testes and ovaries either at different phases in their life cycle or, as in
hamlets
A hamlet is a human settlement that is smaller than a town or village. Its size relative to a parish can depend on the administration and region. A hamlet may be considered to be a smaller settlement or subdivision or satellite entity to a lar ...
, have them simultaneously.
Over 97% of all known fish are
oviparous
Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...
,
[Peter Scott: ''Livebearing Fishes'', p. 13. Tetra Press 1997. ] that is, the eggs develop outside the mother's body. Examples of oviparous fish include
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
,
goldfish
The goldfish (''Carassius auratus'') is a freshwater fish in the family Cyprinidae of order Cypriniformes. It is commonly kept as a pet in indoor aquariums, and is one of the most popular aquarium fish. Goldfish released into the wild have bec ...
,
cichlid
Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted th ...
s,
tuna
A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
, and
eel
Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
s. In the majority of these species, fertilisation takes place outside the mother's body, with the male and female fish shedding their
gamete
A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s into the surrounding water. However, a few oviparous fish practice internal fertilization, with the male using some sort of
intromittent organ
An intromittent organ is any external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most non-mammalian aquatic species fertilize their eggs e ...
to deliver sperm into the genital opening of the female, most notably the oviparous sharks, such as the
horn shark
The horn shark (''Heterodontus francisci'') is a species of bullhead shark, in the family Heterodontidae. It is endemic to the coastal waters off the western coast of North America, from California to the Gulf of California. Young sharks are seg ...
, and oviparous rays, such as
skates. In these cases, the male is equipped with a pair of modified
pelvic fin
Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods.
Structure and function Structure
In actinopterygians, the pelvic fin consists of two ...
s known as
clasper
In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating.
Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's c ...
s.
Marine fish can produce high numbers of eggs which are often released into the open water column. The eggs have an average diameter of .
File:Oeufs002b,57.png, Egg of lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
File:Oeufs002b,54.png, Egg of catshark
Catsharks are ground sharks of the family Scyliorhinidae. They are the largest family of sharks with around 160 species placed in 17 genera. Although they are generally known as catsharks, some species can also be called dogfish due to previous ...
(mermaids' purse
An egg case or egg capsule, often colloquially called a mermaid's purse, is the casing that surrounds the eggs of oviparous sharks, skates and chimaeras. Egg cases typically contain one embryo, except for big skate and mottled skate egg case ...
)
File:Oeufs002b,55.png, Egg of bullhead shark
The bullhead sharks are a small order (Heterodontiformes ) of modern sharks (Neoselachii). The nine living species are placed in a single genus, ''Heterodontus'', in the family Heterodontidae. All are relatively small, with the largest species ...
File:Oeufs002b,56.png, Egg of chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes , known informally as ghost sharks, rat fish, spookfish, or rabbit fish; the last three names are not to be confused with rattails, Opisthoproctidae, or Siganidae, respectively.
At ...
The newly hatched young of oviparous fish are called
larvae
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
T ...
. They are usually poorly formed, carry a large
yolk sac
The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...
(for nourishment), and are very different in appearance from juvenile and adult specimens. The larval period in oviparous fish is relatively short (usually only several weeks), and larvae rapidly grow and change appearance and structure (a process termed
metamorphosis
Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...
) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on
zooplankton
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
prey, a process which depends on typically inadequate zooplankton density, starving many larvae.
In
ovoviviparous fish the eggs develop inside the mother's body after internal fertilization but receive little or no nourishment directly from the mother, depending instead on the
yolk
Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example ...
. Each embryo develops in its own egg. Familiar examples of ovoviviparous fish include
guppies
The guppy (), also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish and one of the most popular freshwater aquarium fish species. It is a member of the family Poeciliidae and, like almost all ...
,
angel shark
The angelsharks are a group of sharks in the genus ''Squatina'' of the family Squatinidae. They commonly inhabit sandy seabeds close to in depth. Many species are now classified as critically endangered by the International Union for Conservat ...
s, and
coelacanth
The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s.
Some species of fish are
viviparous
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...
. In such species the mother retains the eggs and nourishes the embryos. Typically, viviparous fish have a structure analogous to the
placenta
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...
seen in
mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
connecting the mother's blood supply with that of the embryo. Examples of viviparous fish include the
surf-perches,
splitfins, and
lemon shark
The lemon shark (''Negaprion brevirostris'') is a species of shark from the family Carcharhinidae and is classified as a Vulnerable species by the International Union for the Conservation of Nature. Lemon sharks can grow to in length. They are ...
. Some viviparous fish exhibit
oophagy, in which the developing embryos eat other eggs produced by the mother. This has been observed primarily among sharks, such as the
shortfin mako
The shortfin mako shark (; ; ''Isurus oxyrinchus''), also known as the blue pointer or bonito shark, is a large Lamniformes, mackerel shark. It is commonly referred to as the mako shark, as is the longfin mako shark (''Isurus paucus''). The shor ...
and
porbeagle
The porbeagle (''Lamna nasus'') is a species of mackerel shark in the family Lamnidae, distributed widely in the cold and temperate marine waters of the North Atlantic and Southern Hemisphere. In the North Pacific, its ecological equivalent i ...
, but is known for a few bony fish as well, such as the
halfbeak
Hemiramphidae is a family of fishes that are commonly called halfbeaks, spipe fish or spipefish. They are a geographically widespread and numerically abundant family of epipelagic fish inhabiting warm waters around the world. The halfbeaks ar ...
''Nomorhamphus ebrardtii''.
[Meisner, A & Burns, J: Viviparity in the Halfbeak Genera ''Dermogenys'' and ''Nomorhamphus'' (Teleostei: Hemiramphidae)" ''Journal of Morphology'' 234, pp. 295–317, 1997] Intrauterine cannibalism is an even more unusual mode of vivipary, in which the largest embryos eat weaker and smaller siblings. This behavior is also most commonly found among sharks, such as the
grey nurse shark
The sand tiger shark (''Carcharias taurus''), gray nurse shark, spotted ragged-tooth shark or blue-nurse sand tiger, is a species of shark that inhabits subtropical and temperate waters worldwide. It inhabits the continental shelf, from sandy sho ...
, but has also been reported for ''Nomorhamphus ebrardtii''.
Aquarists
Fishkeeping is a popular hobby, practiced by aquarists, concerned with keeping fish in a home aquarium or garden pond. There is also a piscicultural fishkeeping industry, serving as a branch of agriculture.
Origins of fishkeeping
Fish have ...
commonly refer to ovoviviparous and viviparous fish as
livebearers.
Acoustic communication
Acoustic communication in fish involves the transmission of acoustic signals from one individual of a species to another. The production of sounds as a means of communication among fish is most often used in the context of feeding, aggression or courtship behaviour.
The sounds emitted can vary depending on the species and stimulus involved. Fish can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.
Stridulatory
There are some species of fish that can produce sounds by rubbing or grinding their bones together. These noises produced by bone-on-bone interactions are known as 'stridulatory sounds'.
An example of this is seen in ''
Haemulon flavolineatum
''Haemulon flavolineatum'', the French grunt, banana grunt, gold laced grunt, open-mouthed grunt, redmouth grunt, or yellow grunt, is a species of marine ray-finned fish, a grunt belonging to the family Haemulidae. It is native to the western A ...
'', a species commonly referred to as the 'French grunt fish', as it produces a grunting noise by grinding its teeth together.
This behaviour is most pronounced when the ''H. flavolineatum'' is in distress situations.
The grunts produced by this species of fishes generate a frequency of approximately 700 Hz, and last approximately 47 milliseconds.
The ''H. flavolineatum'' does not emit sounds with frequencies greater than 1000 Hz, and does not detect sounds that have frequencies greater than 1050 Hz.
In a study conducted by Oliveira et al. (2014), the longsnout seahorse, ''
Hippocampus reidi
The slender seahorse or longsnout seahorse (''Hippocampus reidi'') is a species of fish in the family Syngnathidae that usually inhabits subtropical regions.
Description
The slender seahorses have a gestation period of around two weeks and typi ...
'', was recorded producing two different categories of sounds; 'clicks' and 'growls'. The sounds emitted by the ''H. reidi'' are accomplished by rubbing their coronet bone across the grooved section of their neurocranium. 'Clicking' sounds were found to be primarily produced during courtship and feeding, and the frequencies of clicks were within the range of 50 Hz-800 Hz.
The frequencies were noted to be on the higher end of the range during spawning periods, when the female and male fishes were less than fifteen centimeters apart.
Growl sounds were produced when the ''H. reidi'' encountered stressful situations, such as handling by researchers.
The 'growl' sounds consist of a series of sound pulses and are emitted simultaneously with body vibrations.
Non-stridulatory
Some fish species create noise by engaging specialized muscles that contract and cause swimbladder vibrations.
Oyster toadfish
The oyster toadfish, ''Opsanus tau'', also known as the oyster toad, ugly toad, oyster cracker, oyster catcher, and bar dog, is a Northeast Atlantic species of fish of the family Batrachoididae. The maximum length of this toadfish is , but they i ...
produce loud grunting sounds by contracting muscles located along the sides of their swim bladder, known as sonic muscles
Female and male toadfishes emit short-duration grunts, often as a fright response. In addition to short-duration grunts, male toadfishes produce "boat whistle calls".
These calls are longer in duration, lower in frequency, and are primarily used to attract mates.
The sounds emitted by the ''O. tao'' have frequency range of 140 Hz to 260 Hz.
The frequencies of the calls depend on the rate at which the sonic muscles contract.
The red drum, ''
Sciaenops ocellatus'', produces drumming sounds by vibrating its swimbladder.
Vibrations are caused by the rapid contraction of sonic muscles that surround the dorsal aspect of the swimbladder.
These vibrations result in repeated sounds with frequencies that range from 100 to >200 Hz.
The ''S. ocellatus'' can produce different calls depending on the stimuli involved.
The sounds created in courtship situations are different from those made during distressing events such as predatorial attacks.
Unlike the males of the ''S. ocellatus'' species, the females of this species do not produce sounds and lack sound-producing (sonic) muscles.
Diseases
Like other animals, fish suffer from diseases and parasites. To prevent disease they have a variety of defenses. ''Non-specific'' defenses include the skin and scales, as well as the mucus layer secreted by the
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rel ...
that traps and inhibits the growth of
microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...
s. If
pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a g ...
s breach these defenses, fish can develop an
inflammatory response that increases blood flow to the infected region and delivers
white blood cells
White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
that attempt to destroy pathogens. Specific defenses respond to particular pathogens recognised by the fish's body, i.e., an
immune response
An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which coul ...
. In recent years,
vaccine
A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified. s have become widely used in aquaculture and also with ornamental fish, for example
furunculosis vaccines in farmed
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
and
koi herpes virus
''Cyprinid herpesvirus 3'' (also CyHV-3, koi herpes virus or KHV) is a species of virus causing a viral disease that is very contagious to the common carp ''Cyprinus carpio''.
Pathology
It is most commonly found in ornamental koi, which are of ...
in
koi.
Some species use
cleaner fish
Cleaner fish are fish that show a specialist feeding strategy by providing a service to other species, referred to as clients, by removing dead skin, ectoparasites, and infected tissue from the surface or gill chambers. This example of cleaning ...
to remove external parasites. The best known of these are the
bluestreak cleaner wrasses of the genus ''Labroides'' found on
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of Colony (biology), colonies of coral polyp (zoology), polyps held together by calcium carbonate. Most coral reefs are built from stony corals, wh ...
s in the
Indian
Indian or Indians may refer to:
Peoples South Asia
* Indian people, people of Indian nationality, or people who have an Indian ancestor
** Non-resident Indian, a citizen of India who has temporarily emigrated to another country
* South Asia ...
and
Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...
oceans. These small fish maintain so-called "cleaning stations" where other fish congregate and perform specific movements to attract the attention of the cleaners. Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus, ''
Etroplus maculatus
The orange chromide (''Pseudetroplus maculatus'') is a species of cichlid fish that is endemic to freshwater and brackish streams, lagoons and estuaries in southern India and Sri Lanka.Pethiyagoda, R., Maduwage, K. & Manamendra-Arachchi, K. (2014 ...
'', the cleaner, and the much larger ''Etroplus suratensis''.
Immune system
Immune organs vary by type of fish. In the
jawless fish
Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present ( cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...
(lampreys and hagfish), true
lymph
Lymph (from Latin, , meaning "water") is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues ...
oid organs are absent. These fish rely on regions of
lymphoid tissue
The lymphatic system, or lymphoid system, is an organ system in vertebrates that is part of the immune system, and complementary to the circulatory system. It consists of a large network of lymphatic vessels, lymph nodes, lymphatic or lymphoid o ...
within other organs to produce immune cells. For example,
erythrocytes
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
,
macrophages
Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...
and
plasma cells
Plasma cells, also called plasma B cells or effector B cells, are white blood cells that originate in the lymphoid organs as B lymphocytes and secrete large quantities of proteins called antibodies in response to being presented specific substan ...
are produced in the anterior kidney (or
pronephros
Pronephros is the most basic of the three excretory organs that develop in vertebrates, corresponding to the first stage of kidney development. It is succeeded by the mesonephros, which in fish and amphibians remains as the adult kidney. In amnio ...
) and some areas of the gut (where
granulocytes
Granulocytes are
cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. Such granules distinguish them from the various agranulocytes. All myeloblastic granulocytes are polymorphonuclear. They ha ...
mature.) They resemble primitive
bone marrow
Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...
in hagfish.
Cartilaginous fish
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
(sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to
Chondrichthyes
Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
; the epigonal organs (lymphoid tissue similar to mammalian bone) that surround the gonads, the
Leydig's organ Leydig's organ (named after the German histologist Franz Leydig who first described it in 1857) is a unique structure found only in some, but not all, elasmobranchs (sharks and rays). Nestled along the top and bottom of the esophagus, it produces ...
within the walls of their esophagus, and a
spiral valve in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable
thymus
The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or '' T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders ...
and a well-developed
spleen
The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes . (their most important immune organ) where various
lymphocyte
A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic a ...
s, plasma cells and macrophages develop and are stored.
Chondrostean
Chondrostei is a group of non-neopterygian ray-finned fish, while the term originally referred to a paraphyletic group of all non-neopterygian ray-finned fish, it was redefined by Patterson in 1982 to be a clade comprising the Acipenseriformes (w ...
fish (sturgeons, paddlefish, and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the
meninges
In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in ...
(membranes surrounding the central nervous system.) Their heart is frequently covered with tissue that contains lymphocytes,
reticular cell
A reticular cell is a type of fibroblast that synthesizes collagen alpha-1(III) and uses it to produce reticular fibers. The cell surrounds the fibers with its cytoplasm, isolating them from other tissue components and cells. Reticular cells pro ...
s and a small number of
macrophage
Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...
s. The chondrostean kidney is an important
hemopoietic
Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells ...
organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.
Like chondrostean fish, the major immune tissues of bony fish (or
teleostei
Teleostei (; Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of all extant species of fish. Te ...
) include the kidney (especially the anterior kidney), which houses many different immune cells. In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (e.g. in the skin, gills, gut and gonads). Much like the mammalian immune system, teleost erythrocytes, neutrophils and granulocytes are believed to reside in the spleen whereas lymphocytes are the major cell type found in the thymus. In 2006, a lymphatic system similar to that in mammals was described in one species of teleost fish, the
zebrafish
The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...
. Although not confirmed as yet, this system presumably will be where naive (unstimulated)
T cell
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...
s accumulate while waiting to encounter an
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respon ...
.
B and T lymphocytes bearing
immunoglobulin
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of th ...
s and
T cell receptor
The T-cell receptor (TCR) is a protein complex found on the surface of T cells, or T lymphocytes, that is responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding ...
s, respectively, are found in all jawed fishes. Indeed, the
adaptive immune system
The adaptive immune system, also known as the acquired immune system, is a subsystem of the immune system that is composed of specialized, systemic cells and processes that eliminate pathogens or prevent their growth. The acquired immune system ...
as a whole
evolved
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
in an ancestor of all jawed vertebrates.
Conservation
The 2006
IUCN
The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...
Red List
The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biologi ...
names 1,173 fish species that are threatened with extinction. Included are species such as
Atlantic cod
The Atlantic cod (''Gadus morhua'') is a benthopelagic fish of the family Gadidae, widely consumed by humans. It is also commercially known as cod or codling.[Devil's Hole pupfish
The Devils Hole pupfish (''Cyprinodon diabolis'') is a critically endangered species of the family Cyprinodontidae (pupfishes) found only in Devils Hole, a water-filled cavern in the US state of Nevada. It was first described as a species in ...]
,
coelacanth
The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
s, and
great white shark
The great white shark (''Carcharodon carcharias''), also known as the white shark, white pointer, or simply great white, is a species of large mackerel shark which can be found in the coastal surface waters of all the major oceans. It is nota ...
s. Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the
Devil's Hole pupfish
The Devils Hole pupfish (''Cyprinodon diabolis'') is a critically endangered species of the family Cyprinodontidae (pupfishes) found only in Devils Hole, a water-filled cavern in the US state of Nevada. It was first described as a species in ...
occupies only a single pool.
Overfishing
Overfishing is a major threat to edible fish such as cod and
tuna
A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
. Overfishing eventually causes
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
(known as
stock
In finance, stock (also capital stock) consists of all the shares by which ownership of a corporation or company is divided.Longman Business English Dictionary: "stock - ''especially AmE'' one of the shares into which ownership of a company ...
) collapse because the survivors cannot produce enough young to replace those removed. Such commercial extinction does not mean that the species is extinct, merely that it can no longer sustain a fishery.
One well-studied example of fishery collapse is the
Pacific sardine
''Sardinops'' is a monotypic genus of sardines of the family Clupeidae. The only member of the genus is ''Sardinops sagax''. It is found in the Indo-Pacific and East Pacific oceans. Its length is up to . It has numerous common or vernacular names ...
''Sadinops sagax caerulues'' fishery off the California coast. From a 1937 peak of the catch steadily declined to only in 1968, after which the fishery was no longer economically viable.
The main tension between
fisheries science
Fisheries science is the academic discipline of managing and understanding fisheries. It is a multidisciplinary science, which draws on the disciplines of limnology, oceanography, freshwater biology, marine biology, meteorology, conservation, eco ...
and the
fishing industry
The fishing industry includes any industry or activity concerned with taking, culturing, processing, preserving, storing, transporting, marketing or selling fish or fish products. It is defined by the Food and Agriculture Organization as including ...
is that the two groups have different views on the resiliency of fisheries to intensive fishing. In places such as Scotland, Newfoundland, and Alaska the fishing industry is a major employer, so governments are predisposed to support it. On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be wiped out within fifty years.
Habitat destruction
A key stress on both freshwater and marine ecosystems is habitat degradation including
water pollution
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water ...
, the building of dams, removal of water for use by humans, and the introduction of
exotic
Exotic may refer to:
Mathematics and physics
* Exotic R4, a differentiable 4-manifold, homeomorphic but not diffeomorphic to the Euclidean space R4
* Exotic sphere, a differentiable ''n''-manifold, homeomorphic but not diffeomorphic to the ordina ...
species. An example of a fish that has become endangered because of habitat change is the
pallid sturgeon
The pallid sturgeon (''Scaphirhynchus albus'') is an endangered species of ray-finned fish, endemic to the waters of the Missouri and lower Mississippi river basins of the United States. It may have even reached the St. Croix River before colo ...
, a North American freshwater fish that lives in rivers damaged by human activity.
Exotic species
Introduction of
non-native species occurs in many habitats. A notable case in point is the
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...
which has become a major ‘hotspot’ of exotic invaders since the opening of the Suez Canal in 1869. Since that time a thousand marine species of all sorts - fishes, seaweeds, invertebrates - originating from the Red Sea and more broadly from the Indo-Pacific have crossed the Canal from south to north to settle in the eastern Mediterranean Basin. Nowadays many of these tropical migrants, also called Lessepsian species, have extended their range towards the west, obviously favoured by the general warming of the Mediterranean. The resulting change in biodiversity is without precedent in human memory and is accelerating: a long-term cross-Basin survey engaged by the
Mediterranean Science Commission
The Mediterranean Science Commission, or CIESM, (French: ''Commission Internationale pour l'Exploration Scientifique de la Méditerranée'') unites 24 Member States, hundreds of marine Institutes, and thousands of marine researchers from all sho ...
recently documented that in just twenty years, from 2001 till 2021, no less than 107 alien fish species have reached the Mediterranean from both the tropical Atlantic and the Red Sea, which is more than the total recorded during the whole 130 preceding years.
Another mode of introduction for marine species is transport across thousands of kms on ship hulls or in ballast waters. Examples abound of marine organisms being transported in
ballast water
Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to vo ...
, among them the invasive
comb jelly
Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), a ...
''Mnemiopsis leidyi'', the dangerous bacterium ''Vibrio cholerae'', or the fouling
zebra mussel
The zebra mussel (''Dreissena polymorpha'') is a small freshwater mussel. The species originates from the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in ma ...
. The Mediterranean and Black Seas, with their high volume shipping from exotic harbors, are particularly impacted by this problem.
Deliberate introductions of species with market potential are another frequent vector: one of the best studied examples is the introduction of the
Nile perch
The Nile perch (''Lates niloticus''), also known as the African snook, Goliath perch, African barramundi , Goliath barramundi, Giant lates or the Victoria perch, is a species of freshwater fish in family Latidae of order Perciformes. It is wid ...
into
Lake Victoria
Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...
in the 1960s. Nile perch gradually exterminated the lake's 500
endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
cichlid
Cichlids are fish from the family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses ( Labridae), in the order Perciformes, but molecular studies have contradicted th ...
species. Some of them now survive in captive breeding programmes, but others are probably extinct.
Carp
Carp are various species of oily freshwater fish from the family Cyprinidae, a very large group of fish native to Europe and Asia. While carp is consumed in many parts of the world, they are generally considered an invasive species in parts of ...
,
snakeheads,
tilapia
Tilapia ( ) is the common name for nearly a hundred species of cichlid fish from the coelotilapine, coptodonine, heterotilapine, oreochromine, pelmatolapiine, and tilapiine tribes (formerly all were "Tilapiini"), with the economically most ...
,
European perch
The European perch (''Perca fluviatilis''), also known as the common perch, redfin perch, big-scaled redfin, English perch, Euro perch, Eurasian perch, Eurasian river perch, Hatch, poor man’s rockfish or in Anglophone parts of Europe, simply th ...
,
brown trout
The brown trout (''Salmo trutta'') is a European species of salmonid fish that has been widely introduced into suitable environments globally. It includes purely freshwater populations, referred to as the riverine ecotype, ''Salmo trutta'' morph ...
,
rainbow trout
The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributaries of the Pacific Ocean in Asia and North America. The steelhead (sometimes called "steelhead trout") is an anadromous (sea-run) form of the coast ...
, and
sea lamprey
The sea lamprey (''Petromyzon marinus'') is a parasitic lamprey native to the Northern Hemisphere. It is sometimes referred to as the "vampire fish".
Description
The sea lamprey has an eel-like body without paired fins. Its mouth is jawless, ro ...
s are other examples of fish that have caused problems by being introduced into alien environments.
Importance to humans
Economic
Throughout history, humans have used
fish as a food source for
dietary protein
Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue and can also serve as a fuel source. As a fuel, proteins provide as much energy density as carbohydrates: 4 kcal (17 kJ) per gram; in c ...
. Historically and today, most fish harvested for human consumption has come by means of catching wild fish. However, fish farming, which has been practiced since about 3,500 BCE in ancient China, is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish.
That proportion is considerably elevated in some
developing nation
A developing country is a sovereign state with a lesser developed industrial base and a lower Human Development Index (HDI) relative to other countries. However, this definition is not universally agreed upon. There is also no clear agreem ...
s and regions heavily dependent on
seafood
Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish. Shellfish include various species of molluscs (e.g. bivalve molluscs such as clams, oysters and mussels, and cephalopods such as octopus an ...
. In a similar manner, fish have been tied to
primary industry
The primary sector of the economy includes any industry involved in the extraction and production of raw materials, such as farming, logging, fishing, forestry and mining.
The primary sector tends to make up a larger portion of the economy in ...
and associated
food
Food is any substance consumed by an organism for nutritional support. Food is usually of plant, animal, or fungal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. The substance is inge ...
,
feed,
pharmaceutical
A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field and re ...
production
Production may refer to:
Economics and business
* Production (economics)
* Production, the act of manufacturing goods
* Production, in the outline of industrial organization, the act of making products (goods and services)
* Production as a stati ...
and
service industries
Service industries are those not directly concerned with the production of physical goods (such as agriculture and manufacturing).
Some service industries, including transportation, wholesale trade and retail trade are part of the supply chain del ...
.
Catching fish for the purpose of food or sport is known as
fishing
Fishing is the activity of trying to catch fish. Fish are often caught as wildlife from the natural environment, but may also be caught from stocked bodies of water such as ponds, canals, park wetlands and reservoirs. Fishing techniques inclu ...
, while the organized effort by humans to catch fish is called a
fishery
Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, both ...
(which also describes the
area where such enterprise operates). Fisheries are a huge global business and provide income for millions of people.
The annual yield from all fisheries worldwide is about 154 million tons, with popular species including
herring
Herring are forage fish, mostly belonging to the family of Clupeidae.
Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Oceans, i ...
,
cod
Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...
,
anchovy
An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water.
More than 140 species are placed in 1 ...
,
tuna
A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
,
flounder
Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries.
Taxonomy
The name "flounder" is used for several only distantly related species, thou ...
, and
salmon
Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
. However, the term fishery is broadly applied, and includes more organisms than just fish, such as
mollusks
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
and crustaceans, which are often collectively called "
shellfish
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
" when used as food.
Recreation
Fishkeeping
Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in aquariums in homes, offices, or public settings.
Recreational fishing
''Recreational fishing'' is fishing primarily for pleasure or competition; it can be contrasted with commercial fishing, which is fishing for profit, or artisanal fishing, which is fishing primarily for food. The most common form of recreational fishing is done with a fishing rod, rod, fishing reel, reel, fishing line, line, fish hook, hooks, and any one of a wide range of bait (luring substance), baits. Recreational fishing is particularly popular in North America and Europe and state, provincial, and federal government agencies actively management target fish species. Angling is a method of fishing, specifically the practice of catching fish by means of an "angle" (hook). Anglers must select the right hook, cast net, cast accurately, and retrieve at the right speed while considering water and weather conditions, species, fish response, time of the day, and other factors.
Culture
Fish themes have symbolic significance in many religions. In ancient Mesopotamia, fish offerings were made to the gods from the very earliest times.
Fish were also a major symbol of Enki, the god of water.
Fish frequently appear as filling motifs in cylinder seals from the First Babylonian dynasty, Old Babylonian ( 1830 BC – 1531 BC) and Neo-Assyrian Empire, Neo-Assyrian (911–609 BC) periods.
Starting during the Kassites, Kassite Period ( 1600 BC – 1155 BC) and lasting until the early Achaemenid Empire, Persian Period (550–30 BC), healers and exorcists dressed in ritual garb resembling the bodies of fish.
During the Seleucid Empire, Seleucid Period (312–63 BC), the legendary Babylonian culture hero Oannes (mythology), Oannes, described by Berossus, was said to have dressed in the skin of a fish.
Fish were sacred to the Syrian goddess Atargatis
and, during her festivals, only her priests were permitted to eat them.
In the Book of Jonah, a work of Jewish literature probably written in the fourth century BC, the central figure, a prophet named Jonah, is swallowed by a giant fish after being thrown overboard by the crew of the ship he is travelling on.
The fish later vomits Jonah out on shore after three days.
This book was later included as part of the Hebrew Bible, or Christian Old Testament, and a version of the story it contains is summarized in Surah 37:139-148 of the Quran. Early Christianity, Early Christians used the ''ichthys'', a symbol of a fish, to represent Jesus,
because the Greek word for fish, ΙΧΘΥΣ Ichthys, could be used as an acronym for "Ίησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ" (Iesous Christos, Theou Huios, Soter), meaning "Jesus Christ, Son of God, Saviour".
The gospels also refer to "fishers of men" and feeding the multitude. In the Dharma (Buddhism), dhamma of Buddhism, the fish symbolize happiness as they have complete freedom of movement in the water. Often drawn in the form of Asian carp, carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span.
Among the deity, deities said to take the form of a fish are Ika-Roa of the Polynesians, Dagon#Fish-god tradition, Dagon of various ancient Semitic peoples, the shark-gods of Hawaii, Hawaii and Matsya of the Hindus. The astrology, astrological symbol Pisces (astrology), Pisces is based on a constellation of the Pisces (constellation), same name, but there is also a second fish constellation in the night sky, Piscis Austrinus.
Fish feature prominently in art and literature, in movies such as ''Finding Nemo'' and books such as ''The Old Man and the Sea''. Large fish, particularly sharks, have frequently been the subject of Horror film, horror movies and Thriller (genre), thrillers, most notably the novel ''Jaws (novel), Jaws'', which spawned a series of films of the Jaws (film), same name that in turn inspired similar films or parodies such as ''Shark Tale'' and ''Snakehead Terror''. Piranhas are shown in a similar light to sharks in films such as ''Piranha (1978 film), Piranha''; however, contrary to popular belief, the red-bellied piranha is actually a generally timid scavenger species that is unlikely to harm humans. Legends of half-human, half-fish mermaids have featured in folklore, including the stories of Hans Christian Andersen.
Terminology
"Fish" or "fishes"
Though often used interchangeably, in biology these words have different meanings. ''Fish'' is used as a singular noun, or as a plural to describe multiple individuals from a single species. ''Fishes'' is used to describe different species or species groups.
Thus a pond would be said to contain 120 fish if all were from a single species or 120 fishes if these included a mix of several species. The distinction is similar to that between people and peoples.
"True fish" or "finfish"
* In biology, the term ''fish'' is most strictly used to describe any animal with a Vertebral column, backbone,
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s throughout life, and limbs (if any) in the shape of fins. Many types of
aquatic animal
An aquatic animal is any animal, whether invertebrate or vertebrate, that lives in water for most or all of its lifetime. Many insects such as mosquitoes, mayflies, dragonflies and caddisflies have aquatic larvae, with winged adults. Aquatic ...
s with common names ending in "fish" are not fish in this Fish#Diversity, sense; examples include
shellfish
Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
,
cuttlefish
Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
,
starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
,
crayfish
Crayfish are freshwater crustaceans belonging to the clade Astacidea, which also contains lobsters. In some locations, they are also known as crawfish, craydids, crawdaddies, crawdads, freshwater lobsters, mountain lobsters, rock lobsters, m ...
and
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...
. In earlier times, even biologists did not make a distinction – sixteenth century natural historians classified also
seals
Seals may refer to:
* Pinniped, a diverse group of semi-aquatic marine mammals, many of which are commonly called seals, particularly:
** Earless seal, or "true seal"
** Fur seal
* Seal (emblem), a device to impress an emblem, used as a means of a ...
, whales,
amphibian
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s,
crocodile
Crocodiles (family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to include all extant me ...
s, even
hippopotamus
The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two exta ...
es, as well as a host of aquatic invertebrates, as fish.
* In fisheries, the term ''fish'' is used as a collective term, and includes mollusks, crustaceans and any
aquatic animal
An aquatic animal is any animal, whether invertebrate or vertebrate, that lives in water for most or all of its lifetime. Many insects such as mosquitoes, mayflies, dragonflies and caddisflies have aquatic larvae, with winged adults. Aquatic ...
which is harvested.
* The strict biological definition of a fish, above, is sometimes called a ''true fish''. True fish are also referred to as ''finfish'' or ''fin fish'' to distinguish them from other aquatic life harvested in fisheries or aquaculture.
"Shoal" or "school"
An assemblage of fish merely using some localised resource such as food or nesting sites is known simply as an ''aggregation''. When fish come together in an interactive, social grouping, then they may be forming either a ''shoal'' or a ''school'' depending on the degree of organisation. A ''shoal'' is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group. ''Schools'' of fish are much more tightly organised, synchronising their swimming so that all fish move at the same speed and in the same direction. Shoaling and schooling behaviour is believed to provide a variety of advantages.
Examples:
* Cichlids congregating at Lek (mating arena), lekking sites form an ''aggregation''.
* Many minnows and characins form ''shoals''.
* Anchovies, herrings and silversides are classic examples of ''schooling'' fish.
The most common collective nouns for a group of fish in general are school and shoal. Both the words have evolved from the same common Dutch root 'schole' meaning a troop or crowd. While the words "school" and "shoal" have different meanings within biology, the distinctions are often ignored by non-specialists who treat the words as synonyms. Thus speakers of British English commonly use "shoal" to describe any grouping of fish, and speakers of American English commonly use "school" just as loosely.
See also
Notes
References
*
*
Further reading
*
*
* Moyle, Peter B. (1993
''Fish: An Enthusiast's Guide'' University of California Press. – good lay text.
*
*
*
''UCTV'' interview
External links
ANGFA– Illustrated database of freshwater fishes of Australia and New Guinea
*
FishBase online– Comprehensive database with information on over 29,000 fish species
*
Philippines Fishes– Database with thousands of Philippine Fishes photographed in natural habitat
*
United Nation– Fisheries and Aquaculture Department: Fish and seafood utilization
University of Washington Libraries Digital Collections– Digital collection of freshwater and marine fish images
*
{{good article
Fish,
Aquatic ecology
Fishing, Fish
Ichthyology
Seafood
Obsolete vertebrate taxa
Paraphyletic groups