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A fish is any aquatic vertebrate animal that is typically ectothermic (or
cold-blooded), covered with scales, and equipped with two sets of paired
fins and several unpaired fins. Fish are abundant in the sea and in fresh
water, with species being known from mountain streams (e.g., char and
gudgeon) as well as in the deepest depths of the ocean (e.g., gulpers and
Diversity of fish
Main article: Diversity of fish
The term "fish" is most precisely used to describe any non-tetrapod
chordate, (i.e., an animal with a backbone), that has gills throughout
life and has limbs, if any, 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 fishes, coelacanths, and lungfishes.
A typical fish is ectothermic, has a streamlined body that allows it to
swim rapidly, extracts oxygen from the water using gills or an accessory
breathing organ to enable it 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 that are fertilized internally or externally.
Fish come in many shapes and sizes. This is a sea dragon, a close relative
of the seahorse. Their leaf-like appendages enable them to blend in with
floating seaweed.To each of these there are exceptions. Tuna, swordfish,
and some species of sharks show some warm-blooded adaptations, and are
able to raise their body temperature significantly above that of the
ambient water surrounding them. Streamlining and swimming performance
varies from highly streamlined and rapid swimmers which are able to reach
10–20 body-lengths per second (such as tuna, salmon, and jacks) through to
slow but more maneuverable species such as eels and rays that reach 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 have 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 lungfishes and coelacanths), ganoid (various
fossil fishes but also living gars and bichirs, cycloid, and ctenoid
(these last two are found on most bony fish. There are even fishes that
spend most of their time out of water. Mudskippers feed and interact with
one another on mudflats and are only underwater when hiding in their
burrows. The catfish Phreatobius cisternarum lives in underground,
phreatic habitats, and a relative lives in waterlogged leaf litter.
Fish range in size from the 16 m (51 ft) whale shark to the 8 mm (just
over ¼ of an inch) long stout infantfish.
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. In some contexts, especially
in aquaculture, the true fish are referred to as finfish (or fin fish) to
distinguish them from these other animals.
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
Fish are classified into the following major groups:
Subclass Pteraspidomorphi (early jawless fish)
(unranked) Cephalaspidomorphi (early jawless fish)
Infraphylum Gnathostomata (jawed vertebrates)
Class Placodermi (armoured fishes, extinct)
Class Chondrichthyes (cartilaginous fish)
Class Acanthodii (spiny sharks, extinct)
Superclass Osteichthyes (bony fish)
Class Actinopterygii (ray-finned fish)
Order Acipenseriformes (sturgeons and paddlefishes)
Order Polypteriformes (reedfishes and bichirs).
Infraclass Holostei (gars and bowfins)
Infraclass Teleostei (many orders of common fishes)
Class Sarcopterygii (lobe-finned fish)
Subclass Coelacanthimorpha (coelacanths)
Subclass Dipnoi (lungfish)
Some palaeontologists consider that Conodonta are chordates, and so regard
them as primitive fish. For a fuller treatment of classification, see the
The various fish groups taken together account for more than half of the
known vertebrates. There are almost 28,000 known extant species of fish,
of which almost 27,000 are bony fish, with the remainder being about 970
sharks, rays, and chimeras and about 108 hagfishes and lampreys. A third
of all of these species are contained within the nine largest families;
from largest to smallest, these families are Cyprinidae, Gobiidae,
Cichlidae, Characidae, Loricariidae, Balitoridae, Serranidae, Labridae,
and Scorpaenidae. On the other hand, about 64 families are monotypic,
containing only one species. It is predicted that the eventual number of
total extant species will be at least 32,500.
Main article: Fish anatomy
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
The advent of jaws allowed fish to eat a much wider variety of food,
including plants and other organisms. In fish, food is ingested through
the mouth and then broken down in the esophagus. When it enters the
stomach, the food is further broken down and, in many fish, further
processed in finger-like pouches called pyloric caeca. The pyloric caeca
secrete digestive enzymes and absorb nutrients from the digested food.
Organs such as the liver and pancreas add enzymes and various digestive
chemicals as the food moves through the digestive tract. The intestine
completes the process of digestion and nutrient absorption.
Most fish exchange gases by using gills that are located on either side of
the pharynx. Gills are made up of threadlike structures called filaments.
Each filament contains a network of capillaries that allow a large surface
area for the exchange of oxygen and carbon dioxide. Fish exchange gases by
pulling oxygen-rich water through their mouths and pumping it over their
gill filaments. The blood in the capillaries flows in the opposite
direction to the water, causing counter current exchange. They then push
the oxygen-poor water out through openings in the sides of the pharynx.
Some fishes, like sharks and lampreys, possess multiple gill openings.
However, most fishes have a single gill opening on each side of the body.
This opening is hidden beneath a protective bony cover called an
Juvenile bichirs have external gills, a very primitive feature that they
hold in common with larval amphibians.
Swim bladder of a Rudd (Scardinius erythrophthalmus)Many fish can breathe
air. The mechanisms for doing so are varied. The skin of anguillid eels
may be used to absorb oxygen. The buccal cavity of the electric eel may be
used to breathe air. Catfishes of the families Loricariidae,
Callichthyidae, and Scoloplacidae are able to absorb air through their
digestive tracts. Lungfish and bichirs have paired lungs similar to those
of tetrapods and must rise to the surface of the water to gulp fresh air
in through the mouth and pass spent air out through the gills. Gar and
bowfin have a vascularised swim bladder that is used 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
what frogs do). A number of fishes have evolved so-called accessory
breathing organs that are used to 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 more or less
resembling labyrinth organs in form and function, most notably snakeheads,
pikeheads, and the Clariidae family of catfish.
Being able to breathe air is primarily of use to fish that inhabit
shallow, seasonally variable waters where the oxygen concentration in the
water may decline at certain times of the year. At such times, fishes
dependent solely on the oxygen in the water, such as perch and cichlids,
will quickly suffocate, but air-breathing fish can survive for much
longer, in some cases in water that is little more than wet mud. At the
most extreme, some of these air-breathing fish are able to survive in damp
burrows for weeks after the water has otherwise completely dried up,
entering a state of aestivation until the water returns.
Tuna gills inside of the head. The fish head is oriented snout-downwards,
with the view looking towards the mouth.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 will
suffocate. Facultative air breathers, such as the catfish Hypostomus
plecostomus, will only breathe air if they need to and will otherwise rely
solely on their gills for oxygen if conditions are favourable. Most air
breathing fish are not obligate air breathers, as there is an energetic
cost in rising to the surface and a fitness cost of being exposed to
Fish have a closed circulatory system with a heart that pumps the blood in
a single loop throughout the body. The blood goes from the heart to gills,
from the gills to the rest of the body, and then back to the heart. In
most fish, the heart consists of four parts: the sinus venosus, the
atrium, the ventricle, and the bulbus arteriosus. Despite consisting of
four parts, the fish heart is still a two-chambered heart. The sinus
venosus is a thin-walled sac that collects blood from the fish's veins
before allowing it to flow to the atrium, which is a large muscular
chamber. The atrium serves as a one-way compartment for blood to flow into
the ventricle. The ventricle is a thick-walled, muscular chamber and it
does the actual pumping for the heart. It pumps blood to a large tube
called the bulbus arteriosus. At the front end, the bulbus arteriosus
connects to a large blood vessel called the aorta, through which blood
flows to the fish's gills.
As with many aquatic animals, most fish release their nitrogenous wastes
as ammonia. Some of the wastes diffuse through the gills into the
surrounding water. Others are removed by the kidneys, excretory organs
that filter wastes from the blood. Kidneys help fishes control the amount
of ammonia in their bodies. Saltwater fish tend to lose water because of
osmosis. In salt-water fish, the kidneys concentrate wastes and return as
much water as possible back to the body. The reverse happens in freshwater
fish: they tend to gain water continuously. The kidneys of freshwater fish
are specially adapted to pump out large amounts of dilute urine. Some fish
have specially adapted kidneys that change their function, allowing them
to move from freshwater to salt-water.
Main article: Scale (zoology)#Fish scales
The scales of fish originate from the mesoderm (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 when compared
with other vertebrates, typically one-fifteenth the mass of the brain from
a similarly sized bird or mammal. However, some fish have relatively large
brains, most notably mormyrids and sharks, which have brains of about as
massive relative to body weight as birds and marsupials.
The brain is divided into several regions. At the front are the olfactory
lobes, a pair of structure the receive and process signals from the
nostrils via the two olfactory nerves. The olfactory lobes are very large
in fishes that hunt primarily by smell, such as hagfish, sharks, and
catfish. Behind the olfactory lobes is the two-lobed telencephalon, the
equivalent structure to the cerebrum in higher vertebrates. In fishes the
telencephalon is concerned mostly with olfaction. Together these
structures form the forebrain.
Connecting the forebrain to the midbrain is the diencephalon (in the
adjacent diagram, this structure is below the optic lobes and consequently
not visible). The diencephalon performs a number of functions associated
with hormones and homeostasis. The pineal body lies just above the
diencephalon. This structure performs many different functions including
detecting light, maintaining circadian rhythms, and controlling colour
The midbrain or mesencephalon contains the two optic lobes. These are very
large in species that hunt by sight, such as rainbow trout and cichlids.
The hindbrain or metencephalon is particularly involved in swimming and
balance. The cerebellum is a single-lobed structure that is usually very
large, typically the biggest part of the brain. Hagfish and lampreys have
relatively small cerebellums, but at the other extreme the cerebellums of
mormyrids are massively developed and apparently involved in their
The brain stem or myelencephalon is the most posterior part of the brain.
As well as controlling the functions of some of the muscles and body
organs, in bony fish at least the brain stem is also concerned with
respiration and osmoregulation.
Most fish possess highly developed sense organs. Nearly all daylight fish
have well-developed eyes that have color vision that is at least as good
as a human's. Many fish also have specialized cells known as
chemoreceptors that are responsible for extraordinary senses of taste and
smell. Although they have ears in their heads, many fish may not hear
sounds very well. However, most fishes have sensitive receptors that form
the lateral line system. The lateral line system allows for many fish to
detect gentle currents and vibrations, as well as to sense the motion of
other nearby fish and prey. Some fish, such as catfish and sharks, have
organs that detect low levels electric current. Other fish, like the
electric eel, can produce their own electricity.
Fish orient themselves using landmarks and may use mental maps of
geometric relationships based on multiple landmarks or symbols. By
studying fish in mazes, it has been determined that fish routinely use
spacial memory and visual discrimination.
Capacity for pain
Further information: 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 performing
research on rainbow trout concluded that fish exhibit behaviors often
associated with pain. At tests conducted at both the University of
Edinburgh and the Roslin Institute, bee venom and acetic acid were
injected into the lips of rainbow trout, resulted in fish rocking their
bodies and rubbing their lips along the sides and floors of their tanks,
which the researchers believe were efforts to relieve themselves of pain
similar to what mammals would also do. Neurons in the brains of the fish
fired in a pattern resembling that of humans when they experience pain.
Professor James D. Rose of the University of Wyoming critiqued the study,
claiming it was flawed, mainly 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 the fish brain is rather
different from ours, fish are probably not conscious (in the manner humans
are), whence reactions similar to human reactions to pain instead have
other causes. Rose had published his own opinion a year earlier arguing
that fish cannot feel pain as 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 same functions."
Animal protection advocates have raised concerns about the possible
suffering of fish caused by angling. In light of recent research, some
countries, like Germany, have banned specific types of fishing, and the
British RSPCA now formally prosecutes individuals who are cruel to fish.
Most fish move by contracting paired sets of muscles on either side of the
backbone alternately. These contractions form S-shaped curves that move
down the body of the fish. As each curve reaches the back fin, backward
force is created. This backward force, in conjunction with the fins, moves
the fish forward. The fish's fins are used like an airplane's stabilizers.
Fins also increase the surface area of the tail, allowing for an extra
boost in speed. The streamlined body of the fish decreases the amount of
friction as they move through water. Since body tissue is denser than
water, fish must compensate for the difference or they will sink. Many
bony fishes have an internal organ called a swim bladder that adjusts
their buoyancy through manipulation of gases.
Although most fish are
exclusively aquatic and ectothermic, there are exceptions to both cases.
Fish from a number of different groups have evolved the capacity to live
out of the water for extended periods of time. Of these amphibious fish,
some such as the mudskipper can live and move about on land for up to
Also, certain species of fish maintain elevated body temperatures to
varying degrees. Endothermic teleosts (bony fishes) are all in the
suborder Scombroidei and include the billfishes, tunas, and one species of
"primitive" mackerel (Gasterochisma melampus). All sharks in the family
Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon
shark – are known to have the capacity for endothermy, and evidence
suggests the trait exists in family Alopiidae (thresher sharks). The
degree of endothermy varies from the billfish, which warm only their eyes
and brain, to bluefin tuna and porbeagle sharks who maintain body
temperatures elevated in excess of 20 °C above ambient water temperatures.
See also gigantothermy. Endothermy, though metabolically costly, is
thought to provide advantages such as increased contractile force of
muscles, higher rates of central nervous system processing, and higher
rates of digestion.
Fish reproductive organs include testes and ovaries. In most fish
species, gonads are paired organs of similar size, which can be partially
or totally fused. There may also be a range of secondary reproductive
organs that help in increasing a fish's 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 fishes they are confined to
the distal portion of these structures. Fishes can present cystic or
semi-cystic spermatogenesis in relation to the phase of release 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 and
then 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 lungfishes, sturgeons, and bowfins.
Cystovaries are the condition that characterizes most of the 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 process.
Postovulatory follicles are structures formed after oocyte release; they
do not have endocrine function, present a wide irregular lumen, and are
rapidly reabosrbed 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 stages.
Some fish are hermaphrodites, having testes and ovaries either at
different phases in their life cycle or, like hamlets, can be
simultaneously male and female.
Over 97% of all known fishes are oviparous, that is, the eggs develop
outside the mother's body. Examples of oviparous fishes 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 fishes practise internal fertilisation, 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 1mm.
An example of zooplanktonThe newly-hatched young of oviparous fish are
called larvae. They are usually poorly formed, carry a large yolk sac
(from which they gain their nutrition) and are very different in
appearance to juvenile and adult specimens of their species. The larval
period in oviparous fish is relatively short however (usually only several
weeks), and larvae rapidly grow and change appearance and structure (a
process termed metamorphosis) to resemble juveniles of their species.
During this transition larvae use up their yolk sac and must switch from
yolk sac nutrition to feeding on zooplankton prey, a process which is
dependent on zooplankton prey densities and causes many mortalities in
Ovoviviparous fish are ones in which the eggs develop inside the mother's
body after internal fertilization but receive little or no nutrition from
the mother, depending instead on the yolk. Each embryo develops in its own
egg. Familiar examples of ovoviviparous fishes include guppies, angel
sharks, and coelacanths.
Some species of fish are viviparous. In such species the mother retains
the eggs, as in ovoviviparous fishes, but the embryos receive nutrition
from the mother in a variety of different ways. Typically, viviparous
fishes have a structure analogous to the placenta seen in mammals
connecting the mother's blood supply with the that of the embryo. Examples
of viviparous fishes of this type include the surf-perches, splitfins, and
lemon shark. The embryos of some viviparous fishes exhibit a behaviour
known as oophagy where the developing embryos eat 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. Intrauterine cannibalism is
an even more unusual mode of vivipary, where the largest embryos in the
uterus will eat their weaker and smaller siblings. This behaviour is also
most commonly found among sharks, such as the grey nurse shark, but has
also been reported for Nomorhamphus ebrardtii.
Aquarists commonly refer to ovoviviparous and viviparous fishes as
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