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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 muc ...
,
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 are ...
-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, a ...
,
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 and ...
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 cartilag ...
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. Tel ...
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 fiv ...
s that first appeared during the
Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, 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 ...
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 consis ...
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 Paleozo ...
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 Selachimo ...
s) became formidable marine predators rather than just the prey of
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
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 a ...
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, exert ...
. 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 Barranquilla ...
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, ...
) to the
abyssal The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean an ...
and even
hadal The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. The hadal zone ranges from around below sea level, and exists in long, narrow, topographic V-shaped depressions. The cumula ...
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 The Liparidae, commonly known as snailfish or sea snails, are a Family (biology), family of Saltwater fish, marine Scorpaeniformes, scorpaeniform fishes. Widely distributed from the Arctic Ocean, Arctic to Antarctic Oceans, including the ocea ...
), 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/ri ...
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 used fo ...
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 theraps ...
s (
amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
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 rhynchocephalians ( ...
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 lightweigh ...
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 or ...
s) emerged within
lobe-finned fishes 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 includ ...
, 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 chara ...
they are fish as well. However, traditionally fish (pisces or ichthyes) are rendered
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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 tre ...
, 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, ...
" 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 them ...
, 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, Engli ...
(
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 **Ger ...
;
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 branc ...
, and is related to the
Latin Latin (, or , ) is a classical language belonging to the 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 the power of the ...
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 writt ...
, 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-E ...
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 Fo ...
, 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 ...
,
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 umbrella- ...
,
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 envir ...
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 th ...
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 (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
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 (biology), class in the subphylum Tunicate, Tunicata of sac-like marine (ocean), marine invertebrate filter feeders. Ascidians ar ...
, 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 (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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 term, ...
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 theraps ...
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, alt ...
, 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 d ...
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 term, ...
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 color ...
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 de ...
(†).


Taxonomy

Fishes are a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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 ceratod ...
(
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, a ...
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 Ostracoderms () are the Armour (zoology), armored Agnatha, 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 cl ...
(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 gil ...
(
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 Selachimo ...
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 primiti ...
(
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 (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
(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, which ...
("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 cartilage ...
(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, a ...
) * 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 al ...
† (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, which ...
a (conodonts) † * Class
Cephalaspidomorphi Cephalaspidomorphs are a group of jawless fishes named for ''Cephalaspis'' of the osteostracans. Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought ...
† (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, ...
† * 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, living ...
(jawed vertebrates) ** Class
Placodermi Placodermi (from Greek πλάξ 'plate' and δέρμα 'skin', literally 'Plate (animal anatomy), plate-skinned') is a Class (biology), class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devoni ...
† (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, which ...
† (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 cartilage ...
(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 div ...
(
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 The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretace ...
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 elongla ...
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 Pub ...
(
reedfish The reedfish, ropefish (more commonly used in the United States), or snakefish, ''Erpetoichthys calabaricus'', is a species of fish in the bichir family and order. It is the only member of the genus ''Erpetoichthys''. It is native to fresh and ...
es 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 Pub ...
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 f ...
***** 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 (Le ...
(
gar Gars are members of the family Lepisosteidae, which are the only surviving members of the Ginglymodi, an ancient holosteian group of ray-finned fish, which first appeared during the Triassic, over 240 million years ago. Gars comprise seven livin ...
s 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 Halecomorphi ...
s) ***** Infraclass
Teleostei Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest class (biology), infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of a ...
(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 therapsids ...
) † – 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 fiv ...
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, ...
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, a ...
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 ceratod ...
, 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 verte ...
,
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 st ...
,
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 The hillstream loaches or river loaches are a family, the Balitoridae, of small fish from South, Southeast and East Asia. The family includes about 202 species. They are sometimes sold as "lizardfish" or (in Germany) "flossensaugers". Many of the ...
,
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 case ...
, 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 "unispec ...
, 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 appropriate s ...
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 last individual of the species, although the capacity to breed and ...
(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 Extinction, extinct species of fish that was formerly native to the Yangtze and Yellow River basins in China. Wit ...
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 by t ...
.


Diversity

Pacific_hagfish_Myxine.jpg, Agnatha
(
Pacific hagfish The Pacific hagfish (''Eptatretus stoutii'') is a species of hagfish. It lives in the mesopelagic to abyssal zone, abyssal Pacific ocean, near the ocean floor. It is a Agnatha, jawless fish and has a body plan that resembles early Paleozoic Era, ...
) 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'' morph ...
) 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 theraps ...
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 muc ...
(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 are ...
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 or ...
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 term, ...
but a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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, a ...
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 hor ...
,
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 theraps ...
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, arbore ...
, 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 tre ...
, 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 anim ...
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 envir ...
,
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 ...
,
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, mu ...
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 umbrella- ...
. 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 tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s,
crocodile Crocodiles (family (biology), 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 inclu ...
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 extan ...
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 them ...
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 a ...
, 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. Swordfis ...
, 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 Selachimo ...
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 list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
, 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 ...
(typical of sharks and rays), cosmoid (fossil lungfish and coelacanths),
ganoid 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 ...
(various fossil fish but also living
gar Gars are members of the family Lepisosteidae, which are the only surviving members of the Ginglymodi, an ancient holosteian group of ray-finned fish, which first appeared during the Triassic, over 240 million years ago. Gars comprise seven livin ...
s 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 Pub ...
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 curve ...
, 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 cartilag ...
). 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 and ...
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 river A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth. It is distinct from an aquifer, which may flow like a river but is contained within a per ...
s 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 characterist ...
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 hypog ...
. Fish range in size from the huge
whale shark The whale shark (''Rhincodon typus'') is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of .McClain CR, Balk MA, Benfield MC, Branch TA, Chen C, Cosgrove J, D ...
to the tiny stout infantfish. 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 appropriate s ...
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 include ...
ecosystems.
Coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...
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, the ...
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 equatori ...
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 technology c ...
, Congo, and
Mekong The Mekong or Mekong River is a trans-boundary river in East Asia and Southeast Asia. It is the world's List of rivers by length, twelfth longest river and List of longest rivers of Asia, the third longest in Asia. Its estimated length is , ...
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 bioge ...
al freshwaters alone, such that Neotropical fishes 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, ...
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. Loca ...
, 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, ...
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 appropriate s ...
. 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 d ...
shows the
evolutionary relationships 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. Variation t ...
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 therapsids ...
).


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 are ...
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 struc ...
. 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 body: ...
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 arc ...
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 as wel ...
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 transpar ...
. 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 Selachimo ...
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 cartilag ...
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 Pub ...
s have external gills, a very primitive feature that they share with larval
amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
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 and ...
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 volts ...
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 Pub ...
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 theraps ...
s and must surface to gulp fresh air through the mouth and pass spent air out through the gills.
Gar Gars are members of the family Lepisosteidae, which are the only surviving members of the Ginglymodi, an ancient holosteian group of ray-finned fish, which first appeared during the Triassic, over 240 million years ago. Gars comprise seven livin ...
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 Halecomorphi ...
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 ...
, 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 (Osphronemidae). The best known ''Betta'' species is ''B. splendens,'' commonly known as the Siamese fighting fish and often kept ...
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 this ...
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 ...
(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 Lepid ...
, ''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 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 carbon dioxide t ...
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 ve ...
, 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 b ...
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 ...
, 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 the ...
. 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. A ...
s and absorb nutrients. Organs such as the
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
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 end ...
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 was ...
. 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; blood ...
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 of ...
. 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 of s ...
: 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 E ...
(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 Selachimo ...
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 a po ...
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 (OF ...
, 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 tr ...
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. In ...
, 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 ...
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, it ...
. 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 ol ...
(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 English, British also homoeostasis) Help:IPA/English, (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physics, physical, and chemistry, chemical conditions maintained by organism, living systems. Thi ...
. The
pineal body The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycl ...
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., endogenous) and responds to ...
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 coasta ...
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 this ...
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 The myelencephalon or afterbrain is the most posterior region of the embryonic hindbrain, from which the medulla oblongata develops. Development Neural tube to myelencephalon During fetal development, divisions of the neural tube that give ...
) 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 by ...
). 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 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 ...
, 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 is ...
and
hearing Hearing, or auditory perception, is the ability to perceive sounds In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psycholog ...
.
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 actinopterygia ...
,
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 (the ...
, 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 un ...
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, ...
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 then ...
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 visua ...
), 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 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
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 the ...
. 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, cyclostomes ...
, 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, a ...
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 flashlight ...
es have eyes suited to the dark environment.


Hearing

Hearing Hearing, or auditory perception, is the ability to perceive sounds In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psycholog ...
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-s ...
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 lifesty ...
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 le ...
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 genus a ...
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. The ...
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 15 ...
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 monophyly, monophyletic lineage within the ...
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 sti ...
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 component ...
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 an ...
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 Dyad (sociology), dyadic Intimate relationship, relationship in which an individual has only one Significant other, partner during their lifetime. Alternately, only one partner at any one time (Monogamy#Serial monogamy, ...
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 (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...
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 a ...
, there are exceptions. The only known bony fishes (infraclass
Teleostei Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest class (biology), infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of a ...
) 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 Scombroidei is a suborder of the Perciformes, the largest order of fish. The suborder includes the barracuda, tuna, and mackerel, as well as the billfish The term billfish refers to a group of saltwater predatory fish characterised by pro ...
– 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 Istiophor ...
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 famil ...
. The opah, a
lampriform Lampriformes is an order of ray-finned fish. Members are collectively called lamprids (which is more properly used for the Lampridae) or lampriforms, and unite such open-ocean and partially deep-sea Teleostei as the crestfishes, oarfish, opahs, ...
, 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 Homeothermy, homothermy or homoiothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. This internal body temperature is often, though not necessarily, higher than the immediate environmen ...
). 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 ''lam ...
(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 of the genus ''Thunnus ''Thunnus'' is a genus of ocean-dwelling, ray-finned bony fish from the mackerel family, Scombridae. More specifically, ''Thunnus'' is one of five ...
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 th ...
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 testostero ...
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 A spermatogonium (plural: ''spermatogonia'') is an undifferentiated male germ cell. Spermatogonia undergo spermatogenesis to form mature spermatozoa in the seminiferous tubules of the testis. There are three subtypes of spermatogonia in humans: * ...
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. Tel ...
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 tubule ...
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, it r ...
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 , abbreviated as OVA and sometimes as OAV (original animation video), are Japanese animated films and series made specially for release in home video formats without prior showings on television or in theaters, though the first part of an OVA s ...
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 The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretace ...
, 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 Halecomorphi ...
. 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 whitefis ...
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 larg ...
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 Nucle ...
, 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 neu ...
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 incl ...
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 Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
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 ...
,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 list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
,
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 ...
,
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 this ...
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 t ...
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 en ...
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 n ...
( mermaids' purse) File:Oeufs002b,55.png, Egg of bullhead shark 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. The ...
. They are usually poorly formed, carry a large 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) to become juveniles. During this transition larvae must switch from their yolk sac to feeding on zooplankton 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. Each embryo develops in its own egg. Familiar examples of ovoviviparous fish include Guppy, guppies, angel sharks, 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. In such species the mother retains the eggs and nourishes the embryos. Typically, viviparous fish have a structure analogous to the placenta seen in placental mammals, mammals connecting the mother's blood supply with that of the embryo. Examples of viviparous fish include the Embiotocidae, surf-perches, Goodeidae, splitfins, and lemon shark. 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 and porbeagle, but is known for a few bony fish as well, such as the halfbeak ''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, but has also been reported for ''Nomorhamphus ebrardtii''. Fishkeeping, Aquarists 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'', 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'', 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 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 (skin), epidermis that traps and inhibits the growth of microorganisms. If pathogens breach these defenses, fish can develop an inflammation, inflammatory response that increases blood flow to the infected region and delivers white blood cells that attempt to destroy pathogens. Specific defenses respond to particular pathogens recognised by the fish's body, i.e., an immune response. In recent years, vaccines 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 list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
and koi herpes virus in koi. Some species use cleaner fish to remove external parasites. The best known of these are the bluestreak cleaner wrasses of the genus ''Labroides'' found on coral reefs in the Indian Ocean, Indian and Pacific Ocean, Pacific 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, ''Orange chromide, Etroplus maculatus'', 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, cyclostomes ...
(lampreys and hagfish), true lymphoid organs are absent. These fish rely on regions of lymphoid tissue within other organs to produce immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature.) They resemble primitive bone marrow in hagfish. Cartilaginous fish (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 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 and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored. Chondrostean fish (sturgeons, paddlefish, and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system.) Their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop. Like chondrostean fish, the major immune tissues of bony fish (or teleostei) 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. Although not confirmed as yet, this system presumably will be where naive (unstimulated) T cells accumulate while waiting to encounter an antigen. B and T lymphocytes bearing immunoglobulins and T cell receptors, respectively, are found in all jawed fishes. Indeed, the adaptive immune system as a whole adaptive immune system#Evolution, evolved in an ancestor of all jawed vertebrates.


Conservation

The 2006 International Union for Conservation of Nature, IUCN IUCN Red List, Red List names 1,173 fish species that are threatened with extinction. Included are species such as Gadus morhua, Atlantic cod, Cyprinodon diabolis, Devil's Hole pupfish,
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 sharks. 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 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 dynamics of fisheries, population (known as Fish stocks, stock) 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 ''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 and the fishing industry 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, the building of dams, removal of water for use by humans, and the introduction of invasive species, exotic species. An example of a fish that has become endangered because of habitat change is the pallid sturgeon, a North American freshwater fish that lives in rivers damaged by human activity.


Exotic species

Introduction of invasive species, non-native species occurs in many habitats. A notable case in point is the Mediterranean Sea 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 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 discharge and the environment, ballast water, among them the invasive comb jelly ''Mnemiopsis leidyi'', the dangerous bacterium ''Vibrio cholerae'', or the fouling zebra mussel. 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 into Lake Victoria in the 1960s. Nile perch gradually exterminated the lake's 500 Endemism, endemic
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 this ...
species. Some of them now survive in captive breeding programmes, but others are probably extinct. Carp, Channidae, snakeheads, tilapia, perch, European perch, brown trout,
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 coasta ...
, and sea lampreys 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 food, fish as a food source for dietary protein. 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 nations and regions heavily dependent on seafood. In a similar manner, fish have been tied to primary industry and associated food industry, food, fodder, feed, pharmaceutical industry, pharmaceutical secondary industry, production and service industries. Catching fish for the purpose of food or sport is known as fishing, while the organized effort by humans to catch fish is called a fishery (which also describes the fishing ground, 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, cod, anchovy,
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, and
salmon Salmon () is the common name for several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...
. However, the term fishery is broadly applied, and includes more organisms than just fish, such as mollusca, mollusks 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 envir ...
" 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 are ...
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 anim ...
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 envir ...
,
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 ...
,
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, mu ...
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 umbrella- ...
. 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 tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s,
crocodile Crocodiles (family (biology), 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 inclu ...
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 extan ...
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 anim ...
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