Fish are aquatic vertebrate animals that are typically ectothermic (previously 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 anglerfish).
Fish are of tremendous importance as food for people around the world, either collected from the wild (see fishing) or farmed in much the same way as cattle or chickens (see aquaculture). Fish are also exploited for recreation, through angling and fishkeeping, and are commonly exhibited in public aquaria.
Fish have an important role in many cultures through the ages, ranging as widely as deities and religious symbols to subjects of books and popular movies.
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 breath 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.
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.
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.
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.
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.
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 surface predators.
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 changes.
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 electrical sense.
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.
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.
The 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 larvae.
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.
The proliferation was apparently due to the formation of the hinged jaw because jawless fish left very few descendants. Lampreys may be a rough representative of pre-jawed fish. The first jaws are found in Placodermi fossils. It is unclear if the advantage of a hinged jaw is greater biting force, respiratory-related, or a combination.
Some speculate that fish may have evolved from a creature similar to a coral-like Sea squirt, whose larvae resemble primitive fish in some key ways. The first ancestors of fish may have kept the larval form into adulthood (as some sea squirts do today), although maybe the reverse of this is case. Candidates for early fish include Agnatha such as Haikouichthys, Myllokunmingia, Pikaia, and Conodonts.
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 several days.
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.
Some fish will also take advantage of cleaner fish for removal of external parasites. The best known of these are the Bluestreak cleaner wrasses of the genus Labroides found on coral reefs in the Indian Ocean and Pacific Ocean. These small fish maintain so-called "cleaning stations" where other fish, known as hosts, will congregate and perform specific movements to attract the attention of the cleaner fish. Cleaning behaviours have been observed in a number of other fish groups, including an interesting case between two cichlids of the same genus, Etroplus maculatus, the cleaner fish, and the much larger Etroplus suratensis, the host.
As of 2006, the IUCN Red List describes 1,173 species of fish as being threatened with extinction. Included on this list are species such as Atlantic cod, Devil's Hole pupfish, coelacanths, and great white sharks. Because fish live underwater they are much 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 areas. For example, the Devil's Hole pupfish occupies only a single 3 m by 6 m pool.
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 living in rivers that have all been changed by human activity in a variety of different ways.
In the Book of Jonah a "great fish" swallowed Jonah the Prophet. Legends of half-human, half-fish mermaids have featured in stories like those of Hans Christian Andersen and movies like Splash (See Merman, Mermaid).
Among the deities said to take the form of a fish are Ika-Roa of the Polynesians, Dagon of various ancient Semitic peoples, and Matsya of the Dravidas of India. 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 have been used figuratively in many different ways, for example the ichthys used by early Christians to identify themselves, through to the fish as a symbol of fertility among Bengalis. Fish have also featured prominently in art and literature, as 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 such as Shark Tale, Snakehead Terror, and Piranha.
The golden fish (Sanskrit: Matsya), represents in the semiotic of Ashtamangala,(buddhist symbolism) the state of fearless suspension in samsara, thus perceived as the harmless ocean, referred to as 'buddha-eyes' or ' rigpa-sight] '. The fishes symbolises the auspiciousness of all living beings in a state of fearlessness without danger of drowning in the Samsaric Ocean of Suffering, and migrating from teaching to teaching freely and spontaneously just as fish swim.
The two fishes originally represented the two main sacred rivers of India - the Ganges and Yamuna. These rivers are associated with the lunar and solar channels which originate in the nostrils and carry the alternating rhythms of breath & prana. They have religious significance in Hindu, Jain and Buddhist traditions but also in Christianity who is first signified by the sign of the fish, and especially referring to feeding the multitude in the desert. In the dhamma of Buddha the fish symbolize happiness as they have complete freedom of movement in the water. They represent fertility and abundance. Often drawn in the form of carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span.
A random assemblage of fishes 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.
While school and shoal have different meanings within biology, they are often treated as synonyms by non-specialists, with speakers of British English using "shoal" to describe any grouping of fish, while speakers of American English often using "school" just as loosely.