Fish are the gill-bearing aquatic craniate animals that lack limbs
with digits. They form a sister group to the tunicates, together
forming the olfactores. Included in this definition are the living
hagfish, lampreys, and cartilaginous and bony fish as well as various
extinct related groups. Tetrapods emerged within lobe-finned fishes,
so cladistically they are fish as well. However, traditionally fish
are rendered paraphyletic by excluding the tetrapods (i.e., the
amphibians, reptiles, birds and mammals which all descended from
within the same ancestry). Because in this manner the term "fish" is
defined negatively as a paraphyletic group, it is not considered a
formal taxonomic grouping in systematic biology. The traditional term
pisces (also ichthyes) is considered a typological, but not a
The earliest organisms that can be classified as fish were soft-bodied
chordates that first appeared during the
Cambrian period. Although
they lacked a true spine, they possessed notochords which allowed them
to be more agile than their invertebrate counterparts.
continue to evolve through the
Paleozoic 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 jaws appeared
Silurian period, after which many (such as sharks) became
formidable marine predators rather than just the prey of arthropods.
Most fish are ectothermic ("cold-blooded"), allowing their body
temperatures to vary as ambient temperatures change, though some of
the large active swimmers like white shark and tuna can hold a higher
Fish can communicate in their underwater environments through the use
of 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 by fish can vary depending
on the species and stimulus involved. They 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.
Fish are abundant in most bodies of water. They can be found in nearly
all aquatic environments, from high mountain streams (e.g., char and
gudgeon) to the abyssal and even hadal depths of the deepest oceans
(e.g., gulpers and anglerfish). With 33,600 described species, fish
exhibit greater species diversity than any other group of
Fish are an important resource for humans worldwide, especially as
food. Commercial and subsistence fishers hunt fish in wild fisheries
(see fishing) or farm them in ponds or in cages in the ocean (see
aquaculture). 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.
3 Anatomy and physiology
3.1.2 Air breathing
3.6 Sensory and nervous system
3.6.1 Central nervous system
3.6.2 Sense organs
3.6.3 Capacity for pain
3.7 Muscular system
3.8 Reproductive system
4 Acoustic communication in fish
4.1 Stridulatory sound producing mechanisms
4.2 Non-stridulatory sound producing mechanisms
5.1 Immune system
6.2 Habitat destruction
6.3 Exotic species
7 Importance to humans
7.1 Economic importance
Fish or fishes
8.2 True fish and finfish
8.3 Shoal or school
9 See also
12 Further reading
13 External links
Main article: Evolution of fish
Dunkleosteus was a gigantic, 10-metre (33 ft) long prehistoric
fish of class Placodermi.
Fish do not represent a monophyletic group, and therefore the
"evolution of fish" is not studied as a single event.
Early fish from the fossil record are represented by a group of small,
jawless, armored fish known as ostracoderms.
Jawless fish lineages are
mostly extinct. An extant clade, the lampreys may approximate ancient
pre-jawed fish. The first jaws are found in
Placodermi fossils. 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
Fish may have evolved from a creature similar to a coral-like sea
squirt, 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), although perhaps the reverse is the
Fish are a paraphyletic group: that is, any clade containing all fish
also contains the tetrapods, which are not fish. For this reason,
groups such as the "Class Pisces" seen in older reference works are no
longer used in formal classifications.
Traditional classification divide fish into three extant classes, and
with extinct forms sometimes classified within the tree, sometimes as
their own classes:
Agnatha (jawless fish)
Cyclostomata (hagfish and lampreys)
Ostracodermi (armoured jawless fish) †
Chondrichthyes (cartilaginous fish)
Elasmobranchii (sharks and rays)
Holocephali (chimaeras and extinct relatives)
Placodermi (armoured fish) †
Acanthodii ("spiny sharks", sometimes classified under bony
Leedsichthys (left), of the subclass Actinopterygii, is the largest
known fish, with estimates in 2005 putting its maximum size at 16
metres (52 ft)
Osteichthyes (bony fish)
Actinopterygii (ray finned fishes)
Sarcopterygii (fleshy 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, the fishes has been split up into a more
detailed scheme, with the following major groups:
Class Myxini (hagfish)
Pteraspidomorphi † (early jawless fish)
Class Petromyzontida or Hyperoartia
Conodonta (conodonts) †
Cephalaspidomorphi † (early jawless fish)
Gnathostomata (jawed vertebrates)
Placodermi † (armoured fish)
Chondrichthyes (cartilaginous fish)
Acanthodii † (spiny sharks)
Osteichthyes (bony fish)
Actinopterygii (ray-finned fish)
Acipenseriformes (sturgeons and paddlefishes)
Polypteriformes (reedfishes and bichirs).
Holostei (gars and bowfins)
Teleostei (many orders of common fish)
Sarcopterygii (lobe-finned fish)
Subclass Dipnoi (lungfish)
† – indicates extinct taxon
Some palaeontologists contend that because
Conodonta are chordates,
they are primitive fish. For a fuller treatment of this taxonomy, see
the vertebrate article.
The position of hagfish 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, that
is a sister group of the Gnathostomata.
The various fish groups account for more than half of vertebrate
species. There are almost 28,000 known extant 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, Gobiidae, Cichlidae, Characidae, Loricariidae,
Balitoridae, Serranidae, Labridae, and Scorpaenidae. About 64 families
are monotypic, containing only one species. The final total of extant
species may grow to exceed 32,500.
Main article: Diversity of fish
The term "fish" most precisely describes any non-tetrapod craniate
(i.e. an animal with a skull and in most cases a backbone) that has
gills throughout life and whose limbs, if any, are in the shape of
fins. Unlike groupings such as birds or mammals, fish are not a
single clade but a paraphyletic collection of taxa, including
hagfishes, lampreys, sharks and rays, ray-finned fish, coelacanths,
and lungfish. Indeed, lungfish and coelacanths are closer
relatives of tetrapods (such as mammals, birds, amphibians, etc.) than
of other fish such as ray-finned fish or sharks, so the last common
ancestor of all fish is also an ancestor to tetrapods. As paraphyletic
groups are no longer recognised in modern systematic biology, the use
of the term "fish" as a biological group must be avoided.
Many types of aquatic animals commonly referred to as "fish" are not
fish in the sense given above; examples include shellfish, cuttlefish,
starfish, crayfish and jellyfish. In earlier times, even biologists
did not make a distinction – sixteenth century natural historians
classified also seals, whales, amphibians, crocodiles, even
hippopotamuses, as well as a host of aquatic invertebrates, as
fish. However, according to the definition above, all mammals,
including cetaceans like whales and dolphins, are not fish. In some
contexts, especially in aquaculture, the true fish are referred to as
finfish (or fin fish) to distinguish them from these other animals.
A relative of the seahorses, the leafy seadragon's appendages allow it
to camouflage (in the form of crypsis) with the surrounding seaweed.
A typical fish is ectothermic, has a streamlined 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,
and lays eggs.
Each criterion has exceptions. Tuna, swordfish, and some species of
sharks 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, and jacks
that can cover 10–20 body-lengths per second to species such as eels
and rays 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.
paired lungs similar to those of tetrapods, gouramis have a structure
called the labyrinth organ that performs a similar function, while
many catfish, such as
Corydoras 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 seahorses,
pufferfish, anglerfish, and gulpers. Similarly, the surface of the
skin may be naked (as in moray eels), or covered with scales of a
variety of different types usually defined as placoid (typical of
sharks and rays), cosmoid (fossil lungfish and coelacanths), ganoid
(various fossil fish but also living gars and bichirs), cycloid, and
ctenoid (these last two are found on most bony fish). There are
even fish that live mostly on land or lay their eggs on land near
water. Mudskippers feed and interact with one another on mudflats
and go underwater to hide in their burrows. A single, undescribed
species of Phreatobius, has been called a true "land fish" as this
worm-like catfish strictly lives among waterlogged leaf
litter. Many species live in underground lakes, underground
rivers or aquifers and are popularly known as cavefish.
Fish range in size from the huge 16-metre (52 ft) whale shark to
the tiny 8-millimetre (0.3 in) stout infantfish.
Fish species diversity is roughly divided equally between marine
(oceanic) and freshwater ecosystems.
Coral reefs in the Indo-Pacific
constitute the center of diversity for marine fishes, whereas
continental freshwater fishes are most diverse in large river basins
of tropical rainforests, especially the Amazon, Congo, and Mekong
basins. More than 5,600 fish species inhabit Neotropical freshwaters
alone, such that Neotropical fishes represent about 10% of all
vertebrate species on the Earth. Exceptionally rich sites in the
Amazon basin, such as Cantão State Park, can contain more freshwater
fish species than occur in all of Europe.
Anatomy and physiology
Fish anatomy and
The anatomy of Lampanyctodes hectoris
(1) – operculum (gill cover), (2) – lateral line, (3) – dorsal
fin, (4) – fat fin, (5) – caudal peduncle, (6) – caudal fin, (7)
– anal fin, (8) – photophores, (9) – pelvic fins (paired), (10)
– pectoral fins (paired)
See also: Aquatic respiration
Most fish exchange gases using gills on either side of the pharynx.
Gills consist of threadlike structures called filaments. Each filament
contains a capillary network that provides a large surface area for
exchanging oxygen and carbon dioxide.
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. The gills push the
oxygen-poor water out through openings in the sides of the pharynx.
Some fish, like sharks and lampreys, possess multiple gill openings.
However, bony fish have a single gill opening on each side. This
opening is hidden beneath a protective bony cover called an operculum.
Juvenile bichirs have external gills, a very primitive feature that
they share with larval amphibians.
Tuna gills inside the head. The fish head is oriented snout-downwards,
with the view looking towards the mouth.
Fish from multiple groups can live out of the water for extended
Amphibious fish such as the mudskipper can live and move
about on land for up to several days,[dubious – discuss] 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 of the electric eel may
Catfish of the families Loricariidae, Callichthyidae, and
Scoloplacidae absorb air through their digestive tracts. Lungfish,
with the exception of the Australian lungfish, and bichirs have paired
lungs similar to those of tetrapods and must surface to gulp fresh air
through the mouth and pass spent air out through the gills.
bowfin have a vascularized swim bladder that functions in the same
way. Loaches, trahiras, and many catfish 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 gouramis and bettas) have a labyrinth organ 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
Fish dependent solely on dissolved oxygen, such as
perch and cichlids, quickly suffocate, while air-breathers survive for
much longer, in some cases in water that is little more than wet mud.
At the most extreme, some air-breathing fish are able to survive in
damp burrows for weeks without water, entering a state of aestivation
(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, must breathe air periodically or they suffocate. Facultative
air breathers, such as the catfish Hypostomus plecostomus, 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.
Didactic model of a fish heart.
Fish have a closed-loop circulatory system. The heart 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, a thin-walled sac that collects
blood from the fish's veins before allowing it to flow to the second
part, the atrium, 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, a large tube,
and then out of the heart. The bulbus arteriosus connects to the
aorta, through which blood flows to the gills for oxygenation.
Jaws allow fish to eat a wide variety of food, including plants and
Fish ingest food through the mouth and break it down
in the esophagus. In the stomach, food is further digested and, in
many fish, processed in finger-shaped pouches called pyloric caeca,
which secrete digestive enzymes and absorb nutrients. Organs such as
the liver and pancreas add enzymes and various chemicals as the food
moves through the digestive tract. The intestine completes the process
of digestion and nutrient absorption.
As with many aquatic animals, most fish release their nitrogenous
wastes as ammonia. Some of the wastes diffuse through the gills. Blood
wastes are filtered by the kidneys.
Saltwater fish tend to lose water because of osmosis. Their kidneys
return water to the body. The reverse happens in freshwater fish: 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.
The scales of fish originate from the mesoderm (skin); they may be
similar in structure to teeth.
Sensory and nervous system
Dorsal view of the brain of the rainbow trout
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 sharks, which have brains
about as massive relative to body weight as birds and marsupials.
Fish brains are divided into several regions. At the front are the
olfactory lobes, a pair of structures that receive and process signals
from the nostrils via the two olfactory nerves. 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 structural equivalent to the cerebrum in
higher vertebrates. In fish the telencephalon is concerned mostly with
olfaction. Together these structures form the forebrain.
Connecting the forebrain to the midbrain is the diencephalon (in the
diagram, this structure is below the optic lobes and consequently not
visible). The diencephalon performs functions associated with hormones
and homeostasis. The pineal body lies just above the diencephalon.
This structure detects light, maintains circadian rhythms, and
controls color changes.
The midbrain (or mesencephalon) contains the two optic lobes. These
are very large in species that hunt by sight, such as rainbow trout
The hindbrain (or metencephalon) 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 lampreys have
relatively small cerebellae, while the mormyrid cerebellum is massive
and apparently involved in their electrical sense.
The brain stem (or myelencephalon) is the brain's posterior. As
well as controlling some muscles and body organs, in bony fish at
least, the brain stem governs respiration and osmoregulation.
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). Many fish also have chemoreceptors that are
responsible for extraordinary senses of taste and smell. Although they
have ears, many fish may not hear very well. Most fish have sensitive
receptors that form the lateral line system, which detects gentle
currents and vibrations, and senses the motion of nearby fish and
prey. Some fish, such as catfish and sharks, have the Ampullae of
Lorenzini, organs that detect weak electric currents on the order of
millivolt. Other fish, like the South American electric fishes
Gymnotiformes, 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.
Main article: Vision in fishes
Vision is an important sensory system for most species of fish. Fish
eyes are similar to those of terrestrial vertebrates like birds and
mammals, but have a more spherical lens. Their retinas generally have
both rods and cones (for scotopic and photopic vision), and most
species have colour vision. Some fish can see ultraviolet and some can
see polarized light. Amongst jawless fish, the lamprey has
well-developed eyes, while the hagfish has only primitive
Fish vision shows adaptation to their visual
environment, for example deep sea fishes have eyes suited to the dark
Sensory systems in fish
Sensory systems in fish § Hearing
Hearing is an important sensory system for most species of fish. Fish
sense sound using their lateral lines and their ears.
Capacity for pain
Pain in fish
Experiments done by William Tavolga provide evidence that fish have
pain and fear responses. For instance, in Tavolga's experiments,
toadfish 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
University of Edinburgh and the
Roslin Institute concluded that rainbow trout exhibit behaviors often
associated with pain in other animals.
Bee venom and acetic acid
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
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.
However, animal behaviorist
Temple Grandin argues that fish could
still have consciousness without a neocortex because "different
species can use different brain structures and systems to handle the
Animal welfare advocates raise concerns about the possible suffering
of fish caused by angling. Some countries, such as Germany have banned
specific types of fishing, and the British
RSPCA now formally
prosecutes individuals who are cruel to fish.
Swim bladder of a rudd (Scardinius erythrophthalmus)
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 that adjusts their buoyancy through
manipulation of gases.
Although most fish are exclusively ectothermic, there are exceptions.
The only known bony fishes (infraclass Teleostei) that exhibit
endothermy are in the suborder
Scombroidei – which includes the
billfishes, tunas, and the butterfly kingfish, a basal species of
mackerel – and also the opah. The opah, a lampriform, was
demonstrated in 2015 to utilize "whole-body endothermy", generating
heat with its swimming muscles to warm its body while countercurrent
exchange (as in respiration) minimizes heat loss. It is able to
actively hunt prey such as squid and swim for long distances due to
the ability to warm its entire body, including its heart, which is
a trait typically found in only mammals and birds (in the form of
homeothermy). In the cartilaginous fishes (class Chondrichthyes),
sharks of the families
Lamnidae (porbeagle, mackerel, salmon, and
great white sharks) and
Alopiidae (thresher sharks) exhibit
endothermy. The degree of endothermy varies from the billfishes, which
warm only their eyes and brain, to the bluefin tuna and the porbeagle
shark, which maintain body temperatures in excess of 20 °C
(68 °F) above ambient water temperatures.
Endothermy, though metabolically costly, is thought to provide
advantages such as increased muscle strength, higher rates of central
nervous system processing, and higher rates of digestion.
Fish reproduction and Spawn (biology)
Organs: 1. Liver, 2. Gas bladder, 3. Roe, 4. Pyloric caeca, 5.
Stomach, 6. Intestine
Fish reproductive organs include testicles and ovaries. In most
species, gonads are paired organs of similar size, which can be
partially or totally fused. There may also be a range of secondary
organs that increase reproductive fitness.
In terms of spermatogonia distribution, the structure of teleosts
testes has two types: in the most common, spermatogonia occur all
along the seminiferous tubules, while in atherinomorph fish they are
confined to the distal portion of these structures.
Fish can present
cystic or semi-cystic spermatogenesis 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 oocytes are released directly
into the coelomic cavity and then enter the ostium, then through the
oviduct and are eliminated. Secondary gymnovarian ovaries shed ova
into the coelom from which they go directly into the oviduct. In the
third type, the oocytes are conveyed to the exterior through the
oviduct. Gymnovaries are the primitive condition found in
lungfish, sturgeon, and bowfin. Cystovaries characterize most
teleosts, where the ovary lumen has continuity with the oviduct.
Secondary gymnovaries are found in salmonids and a few other teleosts.
Oogonia 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,
ooplasm, and the surrounding layers characterize the oocyte maturation
Postovulatory follicles are structures formed after oocyte release;
they do not have endocrine function, present a wide irregular lumen,
and are rapidly reabsorbed in a process involving the apoptosis of
follicular cells. A degenerative process called follicular atresia
reabsorbs vitellogenic oocytes not spawned. This process can also
occur, but less frequently, in oocytes in other development
Some fish, like the California sheephead, are hermaphrodites, having
both testes and ovaries either at different phases in their life cycle
or, as in hamlets, have them simultaneously.
Over 97% of all known fish are oviparous, that is, the eggs
develop outside the mother's body. Examples of oviparous fish include
salmon, goldfish, cichlids, tuna, and eels. In the majority of these
species, fertilisation takes place outside the mother's body, with the
male and female fish shedding their gametes into the surrounding
water. However, a few oviparous fish practice internal fertilization,
with the male using some sort of intromittent organ to deliver sperm
into the genital opening of the female, most notably the oviparous
sharks, such as the horn shark, and oviparous rays, such as skates. In
these cases, the male is equipped with a pair of modified pelvic fins
known as claspers.
Marine fish can produce high numbers of eggs which are often released
into the open water column. The eggs have an average diameter of 1
millimetre (0.039 in).
Egg of lamprey
Egg of catshark (mermaids' purse)
Egg of bullhead shark
Egg of chimaera
Ovary of fish (Corumbatá).
The newly hatched young of oviparous fish are called larvae. 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
guppies, angel sharks, and coelacanths.
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 mammals connecting
the mother's blood supply with that of the embryo. Examples of
viviparous fish include the surf-perches, 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
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
Aquarists commonly refer to ovoviviparous and viviparous fish as
Acoustic communication in fish
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
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 sound producing mechanisms
French grunts - Haemulon flavolineatum
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
Non-stridulatory sound producing mechanisms
Some fish species can create noise by engaging specialized muscles
that contract and cause swimbladder vibrations.
The fish species Opsanus tao, commonly known as '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 don’t produce sounds and lack sound-producing
Fish diseases and parasites
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 that traps and inhibits the growth of microorganisms. If
pathogens breach these defenses, fish can develop an 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 and koi herpes virus in
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 and 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, Etroplus maculatus, the cleaner, and the much larger
Immune organs vary by type of fish. In the jawless fish (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
Cartilaginous fish (sharks and rays) have a more advanced
immune system. They have three specialized organs that are unique to
Chondrichthyes; 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
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
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 evolved in an ancestor of all jawed
IUCN Red List
IUCN Red List names 1,173 fish species that are threatened
with extinction. Included are species such as Atlantic cod,
Devil's Hole pupfish, coelacanths, 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
Devil's Hole pupfish occupies only a single 3 by 6
metres (10 by 20 ft) pool.
Whale sharks, the largest species of fish, are classified as
Main article: Overfishing
Overfishing is a major threat to edible fish such as cod and
Overfishing eventually causes population (known as
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
One well-studied example of fishery collapse is the Pacific sardine
Sadinops sagax caerulues fishery off the California coast. From a 1937
peak of 790,000 long tons (800,000 t) the catch steadily declined
to only 24,000 long tons (24,000 t) 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.
See also: Environmental impact of fishing
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 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.
Introduction of non-native species has occurred in many habitats. One
of the best studied examples is the introduction of
Nile perch into
Lake Victoria in the 1960s.
Nile perch gradually exterminated the
lake's 500 endemic cichlid species. Some of them survive now in
captive breeding programmes, but others are probably extinct.
Carp, snakeheads, tilapia, European perch, brown trout, rainbow
trout, and sea lampreys are other examples of fish that have caused
problems by being introduced into alien environments.
Importance to humans
Fishing industry, Aquaculture, and
These fish-farming ponds were created as a cooperative project in a
Throughout history, humans have utilized fish as a food source.
Historically and today, most fish protein has come by means of
catching wild fish. However, aquaculture, or fish farming, which has
been practiced since about 3,500 BCE. in 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 the sea. In a similar manner, fish have
been tied to trade.
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. 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, flounder, and salmon. However, the term fishery is
broadly applied, and includes more organisms than just fish, such as
mollusks and crustaceans, which are often called "fish" when used as
Main articles: Fishkeeping, Recreational fishing, and Angling
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
Recreational fishing is fishing for pleasure or competition; it can be
contrasted with commercial fishing, which is fishing for profit. The
most common form of recreational fishing is done with a rod, reel,
line, hooks and any one of a wide range of baits.
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 accurately,
and retrieve at the right speed while considering water and weather
conditions, species, fish response, time of the day, and other
Fish in culture
Vishnu as a Matsya
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 movies
and thrillers, most notably the novel Jaws, which spawned a series of
films of the same name that in turn inspired similar films or parodies
Shark Tale and Snakehead Terror. Piranhas are shown in a
similar light to sharks in films such as Piranha; however, contrary to
popular belief, the red-bellied piranha is actually a generally timid
scavenger species that is unlikely to harm humans. In the Book of
Jonah a "great fish" swallowed
Jonah the Prophet. Legends of
half-human, half-fish mermaids have featured in folklore, including
the stories of Hans Christian Andersen.
The ichthus is a Christian symbol of a fish signifying that the person
who uses it is a Christian.
Fish themes have symbolic significance in many religions. The fish is
used often as a symbol by Christians to represent Jesus, or
Christianity in general; the gospels also refer to "fishers of
men" and feeding the multitude. In the dhamma of
Buddhism the fish
symbolize happiness as they have complete freedom of movement in the
water. Often drawn in the form of carp which are regarded in the
Orient as sacred on account of their elegant beauty, size and
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 Old Babylonian and Neo-Assyrian periods. Starting
during the Kassite Period and lasting until the early Persian Period,
healers and exorcists dressed in ritual garb resembling the bodies of
fish. During the Seleucid Period, the legendary Babylonian culture
hero Oannes, described by Berossus, was said to have dressed in the
skin of a fish.
Among the deities said to take the form of a fish are
Ika-Roa of the
Polynesians, Dagon of various ancient Semitic peoples, the shark-gods
of Hawaiʻi and
Matsya of the Hindus. The astrological symbol Pisces
is based on a constellation of the same name, but there is also a
second fish constellation in the night sky, Piscis Austrinus.
Fish or fishes
Though often used interchangeably, in biology these words have
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
that contained a single species might be said to contain 120 fish. But
if the pond contained a total of 120 fish from three different
species, it would be said to contain three fishes. The distinction is
similar to that between people and peoples.
True fish and finfish
In biology, the term fish is most strictly used to describe any animal
with a backbone that has gills throughout life and has limbs, if any,
in the shape of fins. Many types of aquatic animals with common
names ending in "fish" are not fish in this sense; examples include
shellfish, cuttlefish, starfish, crayfish and jellyfish. In earlier
times, even biologists did not make a distinction—sixteenth century
natural historians classified also seals, whales, amphibians,
crocodiles, even hippopotamuses, 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 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
Shoal or school
Main article: Shoaling and schooling
These goldband fusiliers are schooling because their swimming is
A random 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
Cichlids congregating at lekking sites form an aggregation.
Many minnows and characins form shoals.
Anchovies, herrings and silversides are classic examples of schooling
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.
Main article: Outline of fish
For a topical guide to sharks, see Outline of sharks
Angling (sport fishing)
Catch and release
Deep sea fish
Fish acute toxicity syndrome
Fish as food
Fishing (fishing for food)
List of fish common names
List of fish families
Mercury in fish
Otolith (Bone used for determining the age of a fish)
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^ "World Review of Fisheries and Aquaculture" (PDF). fao.org. Food and
Agriculture Organization of the United Nations. Archived (PDF) from
the original on 28 August 2015. Retrieved 13 August 2015.
^ Zollinger, Sue Anne (3 July 2009). "Piranha–Ferocious Fighter or
Scavenging Softie?". A Moment of Science. Indiana Public Media.
Archived from the original on 17 October 2015. Retrieved 1 November
^ Coffman, Elesha (8 August 2008). "What is the origin of the
Christian fish symbol?". Christianity Today. Archived from the
original on 29 August 2015. Retrieved 13 August 2015.
^ Matthew 4:19
^ a b c d e Black, Jeremy; Green, Anthony (1992). Gods, Demons and
Symbols of Ancient Mesopotamia: An Illustrated Dictionary. The British
Museum Press. pp. 82–83. ISBN 0-7141-1705-6. Archived from
the original on 20 February 2018.
^ "Piscis Austrinus". allthesky.com. The Deep Photographic Guide to
the Constellations. Archived from the original on 25 November 2015.
Retrieved 1 November 2015.
^ Pauly, Daniel (13 May 2004). "Fish(es)". Darwin's Fishes: An
Encyclopedia of Ichthyology, Ecology, and Evolution. Cambridge
University Press. p. 77. ISBN 978-1-139-45181-9. Archived
from the original on 8 February 2016.
^ Nelson, Joseph S and Paetz, Martin Joseph (1992) The Fishes of
Alberta Archived 7 April 2014 at the Wayback Machine. page 400,
University of Alberta. ISBN 9780888642363.
^ Helfman, Collette & Facey 1997, p. 5
^ Nelson, Joseph S. (2006). Fishes of the World. John Wiley &
Sons, Inc. p. 2. ISBN 0-471-25031-7.
^ FAO: Fisheries glossary Archived 29 November 2013 at the Wayback
^ Helfman, G.; Collette, B.; Facey, D. (1997). The Diversity of
Fishes. Blackwell Publishing. p. 375.
^ Pitcher T. J. and Parish J. K. (1993) "Functions of shoaling
behaviour in teleosts" Archived 5 April 2017 at the Wayback Machine.
In: Pitcher T. J. (Ed) Behaviour of teleost fishes. Chapman and Hall,
New York, pp 363–440
Eschmeyer, William N.; Fong, Jon David (2013). "Catalog of Fishes".
California Academy of Sciences.
Helfman, G.; Collette, B.; Facey, D. (1997). The Diversity of Fishes
(1st ed.). Wiley-Blackwell. ISBN 978-0-86542-256-8.
Moyle, Peter B.; Cech, Joseph J. (2003). Fishes, An Introduction to
Ichthyology (5th ed.). Benjamin Cummings.
Nelson, Joseph S. (2006).
Fishes of the World (PDF) (4th ed.). John
Wiley & Sons. ISBN 9780471756446. Archived from the original
(PDF) on 5 March 2013. Retrieved 30 April 2013.
Helfman, G.; Collette, B.; Facey, D.; Bowen, B. (2009). The Diversity
of Fishes: Biology, Evolution, and Ecology (2nd ed.). Wiley-Blackwell.
Moyle, Peter B. (1993) Fish: An Enthusiast's Guide University of
California Press. ISBN 9780520916654 – good lay text.
Shubin, Neil (2009) Your inner fish: A journey into the 3.5 billion
year history of the human body Vintage Books. ISBN 9780307277459.
Wikimedia Commons has media related to Fish, Actinopterygii, Marine
aquarium fish and
Freshwater aquarium fish.
Look up fish in Wiktionary, the free dictionary.
Wikispecies has information related to Actinopterygii
Wikisource has the text of the 1911 Encyclopædia Britannica article
ANGFA – Illustrated database of freshwater fishes of Australia and
Fischinfos.de – Illustrated database of the freshwater fishes of
Germany (in German) at the
Wayback Machine (archived 2011-11-30)
FishBase online – Comprehensive database with information on over
29,000 fish species
Fisheries and Illinois
Aquaculture Center – Data outlet for
fisheries and aquaculture research center in the central US at
Archive.is (archived 15 December 2012)
Philippines Fishes – Database with thousands of Philippine Fishes
photographed in natural habitat
Fish Conservancy – Conservation and study of North
American freshwater fishes
United Nation – Fisheries and
University of Washington Libraries Digital Collections[permanent dead
link] – Digital collection of freshwater and marine fish images
Davenport, Charles B.; Ingersoll, Ernest (1905). "Fish". New
Live webcam feed of a goldfish tank at
Archive.is (archived 16
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