'Fish' are
aquatic vertebrates that are
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).
They 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 fish 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.
Definition
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.
[1] 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.
[2][3]
A typical fish is
cold-blooded; 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.
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
However, 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.
[4] 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.
[5] 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.
[6] 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.
[7] 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.
[8] The
catfish ''
Phreatobius cisternarum'' lives in underground,
phreatic habitats, and a relative lives in waterlogged
leaf litter.
[9][10]
Fish range in size from the 16 m (51 ft)
whale shark to a 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; see
Fish (disambiguation).
Classification
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.
Fish are classified into the following major groups:
★ Subclass
Pteraspidomorphi (early jawless fish)
★ Class
Thelodonti
★ Class
Anaspida
★ (unranked)
Cephalaspidomorphi (early jawless fish)
★
★ (unranked)
Hyperoartia
★
★
★
Petromyzontidae (
lampreys)
★
★ Class
Galeaspida
★
★ Class
Pituriaspida
★
★ Class
Osteostraci
★ 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)
★
★
★ 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
vertebrate article.
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.
[11] 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.
[12]
Fish anatomy
Main articles: Fish anatomy
2.png)
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)
Digestive system
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 fingerlike 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.
Respiratory system
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
operculum.
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.
[13] 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 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 predators.
Circulatory system
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.
[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.]
Homeothermy
Although most fish are exclusively aquatic and cold-blooded, 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.
Excretory system
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 saltwater 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 saltwater.
Muscular system
Fish locomotion
Main articles: Fish locomotion
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 more dense than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a swim bladder that adjust their buoyancy through manipulation of gases.
Reproductive system
Organs
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.
In terms of spermatogonia distribution, the structure of teleosts testes has two types: in the most commmon, 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.[14]
Fish ovaries may be of two types: gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then eliminated. In the second type, the oocytes are conveyed to the exterior through the oviduct.[15] 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.
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, either having testes and ovaries at different phases in the life cycle. However, there are even some fish, such as hamlets, that are simultaneously male and female.
Reproductive method
Over 97% of all known fishes are oviparous,[16] 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.
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''.[17] 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''.[17]
Aquarists commonly refer to ovoviviparous and viviparous fishes as livebearers.
Immune system
Types of immune organs vary between different types of fish.[19]
In the jawless fish (lampreys and hagfishes), true lymphoid organs are absent. Instead, these fish rely on regions of lymphoid tissue within other organs to produce their 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) resemble primitive bone marrow in hagfish.
Cartilaginous fish (sharks and rays) have a more advanced immune system than the jawless fish. They have three specialized organs that are unique to chondrichthyes; the epigonal organs (lymphoid tissue similar to bone marrow of mammals) that surround the gonads, the Leydig’s organ within the walls of their esophagus, and a spiral valve in their intestine. All these organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored.
Chondrostean fish (sturgeons, paddlefish and birchirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system) and their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.
Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), where many different immune cells are housed.[20] 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.[21][22] Recently, a lymphatic system similar to that described 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 will accumulate while waiting to encounter an antigen.[23]
Evolution
The early fossil record on fish is not very clear. It appears it was not a successful enough animal early in its evolution to leave many fossils. However, this would eventually change over time as it became a dominant form of sea life and eventually branching to include land vertebrates such as amphibians, reptiles, and mammals.
The formation of the hinged jaw appears to be what resulted in the later proliferation of fish because un-jawed fish left very few ancestors.[24] 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, see Neoteny), although the reversal of this case is also possible. Candidates for early fish include Agnatha such as Haikouichthys, Myllokunmingia, Pikaia, and Conodonts.
Fish disease
Like other animals, fish can suffer from a wide variety of diseases and parasites. To prevent disease they have a variety of 'non-specific defences' and 'specific defences'. Non-specific defences include the skin and scales, as well as the mucus layer secreted by the epidermis that traps microorganisms and inhibits their growth. Should pathogens breach these defences, fish can develop an inflammatory response that increases the flow of blood to the infected region and delivers the white blood cells that will attempt to destroy the pathogens. Specific defences are specialised responses to particular pathogens recognised by the fish's body, in other words, an immune response.[25] In recent years, vaccines have become widely used in aquaculture and also with ornamental fish, for example the vaccines for furunculosis in farmed
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