A fish is any member of a paraphyletic group of organisms that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups. Most fish are ectothermic ("cold-blooded")(Dell 1691P battery), 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 core temperature. 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) (Dell 310-6321 battery). At 32,000 species, fish exhibit greater species diversity than any other group of vertebrates.
Fish are an important resource 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(Dell 312-0068 battery), serving as deities, religious symbols, and as the subjects of art, books and movies.
Because the term "fish" is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology. The traditional term pisces (also ichthyes) is considered a typological, but not a phylogenetic classification(Dell 312-0078 battery).
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. Fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms(Dell 312-0079 battery). Many fish of the Paleozoic developed external armor that protected them from predators. The first fish with jaws appeared in the Silurian period, after which many (such as sharks) became formidable marine predators rather than just the prey of arthropods.
Diversity of fish
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(Dell 312-0305 battery).
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(Dell 312-0326 battery), 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(Dell 312-0518 battery).
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(Dell 312-0566 battery), as fish. However, according 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 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(Dell 312-0585 battery), 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(Dell 312-0831 battery), 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. 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(Dell BAT30WL battery), 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) (Dell D6400 battery), 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. Mudskippers feed and interact with one another on mudflats and go underwater to hide in their burrows. The catfish Phreatobius cisternarum lives in underground, phreatic habitats, and a relative lives in waterlogged leaf litter(Dell HF674 battery).
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(Dell N3010 battery). More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth.
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(Dell Inspiron N4010 battery).
Traditional classification divide fish into three extant classes, and with extinct forms sometimes classified within the tree, sometimes as their own classes:
Class Agnatha (jawless fish)
Subclass Cyclostomata (hagfish and lampreys)
Subclass Ostracodermi (armoured jawless fish)
Class Chondrichthyes (cartilaginous fish)
Subclass Elasmobranchii (sharks and rays)
Subclass Holocephali (chimaeras and extinct relatives)
Class Placodermi (armoured fish)
Class Acanthodii ("spiny sharks", sometimes classified under bony fishes)
Class Osteichthyes (bony fish) (Dell INSPIRON 1100 battery)
Subclass Actinopterygii (ray finned fishes)
Subclass 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(Dell Inspiron 1200 battery), the fishes has been split up into a more detailed scheme, with the following major groups:
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. (Dell Inspiron 1420 battery)
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(Dell Inspiron 1464 battery), 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: 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 (paired)
Respiration(Dell Inspiron 1564 battery)
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(Dell Inspiron 1764 battery). 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(Dell Inspiron 1520 battery).
Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.
Fish from multiple groups can live out of the water for extended time periods. Amphibious fish such as the mudskipper can live and move about on land for up to several days, or live in stagnant or otherwise oxygen depleted water(Dell Inspiron 1521 battery). 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 breathe air. Catfish of the families Loricariidae, Callichthyidae, and Scoloplacidae absorb air through their digestive tracts. Lungfish, with the exception of the Australian lungfish(Dell inspiron 1525 battery), 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. Gar and 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) (Dell inspiron 1526 battery). 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(Dell Inspiron 1720 battery).
Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud(Dell Inspiron 2000 battery). 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.
Tuna gills inside of the head. The fish head is oriented snout-downwards, with the view looking towards the mouth.
Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish(Dell INSPIRON 2600 battery), 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. (Dell INSPIRON 3800 battery)
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(Dell INSPIRON 4000 battery), 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(Dell Inspiron 5000 battery).
Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the esophagus. 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(Dell INSPIRON 500M battery). 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(Dell INSPIRON 5100 battery).
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(Dell INSPIRON 510M battery).
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
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. (Dell INSPIRON 6000 battery)
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(Dell INSPIRON 600M battery), 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. (Dell Inspiron 6400 battery) 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 and cichlids.
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(Dell INSPIRON 7000 battery). 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. (Dell INSPIRON 700M battery)
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(Dell Inspiron 710m battery), which detects gentle currents and vibrations, and senses the motion of nearby fish and prey. Some fish, such as catfish and sharks, have 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(Dell INSPIRON 8200 battery).
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(Dell INSPIRON 8600 battery), but have a more spherical lens. Their retinas generally have both rod cells and cone cells (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 eyespots. Fish vision shows adaptation to their visual environment(Dell INSPIRON 9100 battery), for example deep sea fishes have eyes suited to the dark environment.
Main article: Hearing in fishes
Hearing is an important sensory system for most species of fish. Fish sense sound using their lateral lines and their ears.
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. (Dell INSPIRON 9200 battery)
In 2003, Scottish scientists at the 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(Dell INSPIRON 9300 battery), similar to what mammals would do. Neurons fired in a pattern resembling human neuronal patterns.
Professor James D. Rose of the University of Wyoming claimed the study was flawed since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours". Rose argues that since fish brains are so different from human brains(Dell Inspiron 9400 battery), 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 same functions." (Dell Inspiron E1505 battery)
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.
Main article: Fish locomotion
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(Dell Inspiron E1705 battery). 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(Dell Inspiron Mini 9 battery). Many bony fish have an internal organ called a swim bladder that adjusts their buoyancy through manipulation of gases.
A 3-tonne (3.0-long-ton; 3.3-short-ton) great white shark off Isla Guadalupe
Although most fish are exclusively ectothermic, there are exceptions.
Certain species of fish maintain elevated body temperatures. Endothermic teleosts (bony fish) are all in the suborder Scombroidei and include the billfishes, tunas, and one species of "primitive" mackerel (Gasterochisma melampus) (Dell Latitude D400 battery). All sharks in the family Lamnidae – shortfin mako, long fin mako, white, porbeagle, and salmon shark – are endothermic, 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. (Dell STUDIO 1450 battery)See also gigantothermy. 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.
Further information: Spawn (biology)
Organs: 1. Liver, 2. Gas bladder, 3. Roe, 4. Pyloric caeca, 5. Stomach, 6. Intestine
Fish reproductive organs include testes and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused. (Dell Vostro 1400 battery) 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. (Dell Vostro 1500 battery)
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(Dell XPS GEN 2 battery). 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(Dell XPS M1210 battery).
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. (Dell XPS M1330 battery)
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 stages. (Dell XPS 1340 battery)
Some fish 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, (Dell XPS M1530 battery) 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(Dell XPS M170 battery).
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 chimaera
An example of zooplankton
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) (Dell XPS M1710 battery), 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(Dell XPS M1730 battery), 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(Dell XPS M2010 battery), splitfins, and lemon shark. Some viviparous fish exhibit oophagy, in which the developing embryos eat other eggs produced by the mother. This has been observed primarily among sharks, such as the shortfin mako and porbeagle, but is known for a few bony fish as well, such as the halfbeak Nomorhamphus ebrardtii. Intrauterine cannibalism is an even more unusual mode of vivipary(Dell Latitude E5400 battery), in which the largest embryos eat weaker and smaller siblings. This behavior is also most commonly found among sharks, such as the grey nurse shark, but has also been reported for Nomorhamphus ebrardtii.
Aquarists commonly refer to ovoviviparous and viviparous fish as livebearers.
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(Dell Latitude E5500 battery). For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature.) They resemble primitive bone marrow in hagfish. Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to chondrichthyes; the epigonal organs (Dell Latitude E6400 battery) (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(Dell Latitude E6500 battery), plasma cells and macrophages develop and are stored. Chondrostean fish (sturgeons, paddlefish and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges (membranes surrounding the central nervous system.) Their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages(Dell Inspiron Mini 12 battery). The chondrostean kidney is an important hemopoietic organ; where erythrocytes, granulocytes, lymphocytes and macrophages develop.
Like chondrostean fish, the major immune tissues of bony fish (or teleostei) include the kidney (especially the anterior kidney), which houses many different immune cells. In addition, teleost fish possess a thymus, spleen and scattered immune areas within mucosal tissues (Dell XPS M140 battery) (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(Dell XPS 13 battery), this system presumably will be where naive (unstimulated) T cells accumulate while waiting to encounter an antigen.
Main article: 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(Dell XPS 16 battery), 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 koi(Dell XPS 1640 battery).
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. (Dell XPS 1645 battery) 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 Etroplus suratensis.
See also: prehistoric fish
Outdated evolutionary view of continual gradation (animation)
Dunkleosteus was a gigantic, 10 meter (33 feet) long prehistoric fish.
Fish do not represent a monophyletic group, and therefore the "evolution of fish" is not studied as a single event. (Dell XPS 1647 battery)
Proliferation of fish was apparently due to the hinged jaw, because jawless fish left very few descendants. Lampreys may approximate 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, improved respiration, or a combination of factors(Dell Latitude 131L battery).
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 case.
The 2006 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, (Dell Latitude C400 battery) and great white sharks. Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the Devil's Hole pupfish occupies only a single 3 by 6 metres (10 by 20 ft) pool. (Dell Latitude C500 battery)
A Whale shark, the world's largest fish, is classified as Vulnerable.
Main article: Overfishing
Overfishing is a major threat to edible fish such as cod and tuna. 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 fishery(Dell Latitude C510 battery).
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. (Dell Latitude C540 battery)
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(Dell Latitude C600 battery), warning that many stocks could be wiped out within fifty years.
See also: Environmental effects 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. (Dell Latitude C610 battery)
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. (Dell Latitude C640 battery) 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
Avatar of Vishnu as a Matsya
Coat of arms of Narva, Estonia
Saint Benno of Meissen depicted with a fish in hand (Church of Saint Benno in Munich)
Main article: Fishkeeping#Conservation_and_Science
Main articles: Fish as food, Fishing industry, Aquaculture, and Fish farming
Recreation(Dell Latitude C800 battery)
Main articles: Fishkeeping, Recreational fishing, and Angling
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, the shark-gods of Hawaiʻi and Matsya of the Hindus(Dell Latitude C810 battery). 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 feature prominently in art and literature(Dell Latitude C840 battery), 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(Dell Latitude D410 battery).
Fish riders in a 1920s poster of the Republic of China.
In the semiotic of Ashtamangala (buddhist symbolism) the golden fish (Sanskrit: Matsya), represents the state of fearless suspension in samsara, perceived as the harmless ocean, referred to as 'buddha-eyes' or 'rigpa-sight'. The fish symbolizes the auspiciousness of all living beings in a state of fearlessness without danger of drowning in the Samsaric Ocean of Suffering(Dell Latitude D420 battery), and migrating from teaching to teaching freely and spontaneously just as fish swim.
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(Dell Latitude D430 battery). 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.
The name of the Canadian city of Coquitlam, British Columbia is derived from Kwikwetlem, which is said to be derived from a Coast Salish term meaning "little red fish".
Main article: Shoaling and schooling
These goldband fusiliers are schooling because their swimming is synchronised
A random assemblage of fish merely using some localised resource such as food or nesting sites is known simply as an aggregation(Dell Latitude D500 battery). 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(Dell Latitude D505 battery). 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.
Cichlids congregating at lekking sites form an aggregation.
Many minnows and characins form shoals.
Anchovies, herrings and silversides are classic examples of schooling fish.
While school and shoal have different meanings within biology, they are often treated as synonyms by non-specialists(Dell Latitude D510 battery), with speakers of British English using "shoal" to describe any grouping of fish, while speakers of American English often using "school" just as loosely.
Fish or fishes?
Though often used interchangeably, these words have different
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom(Dell Latitude D520 battery). They can live in schools and can swim rapidly backwards. Shrimp are an important food source for larger animals from fish to whales. They have a high tolerance to toxins in polluted areas, and may contribute to high toxin levels in their predators. Together with prawns, shrimp are widely caught and farmed for human consumption(Dell Latitude D600 battery).
A number of more or less unrelated crustaceans share the word "shrimp" in their common name. Examples are the mantis shrimp and the opossum or mysid shrimp, both of which belong to the same class (Malacostraca) as decapods including shrimp, but constitute two different orders within it, the Stomatopoda and the Mysidacea(Dell Latitude D610 battery). Triops longicaudatus and Triops cancriformis are also popular animals in freshwater aquaria, and are often called shrimp, although they belong instead to the Notostraca, a quite unrelated group.
Shrimp (Caridea), such as Pandalus borealis, typically have two pairs of claws, and the second segment of the abdomen overlaps the segments on either side. The abdomen shows a pronounced caridean bend(Dell Latitude D620 battery).
Prawns (Dendrobanchiata), such as Penaeus monodon, typically have three pairs of claws, and even-sized segments on the abdomen. There is no pronounced bend in the abdomen.
The class Malacostraca contains about half of the crustaceans. The members of this class have a primitive body plan that can be described as shrimp-like, consisting of a 5-8-7 body plan. They have a small carapace that encloses the head and the thorax(Dell Latitude D630 battery), and have a muscular abdomen for swimming. They also have a thin exoskeleton to maintain a light weight. These general characteristics are common in all members of the class.
The class can be further divided into the decapods, which are even still divided into the dendrobranchiates (prawns) and the carideans (shrimp and snapping shrimp).
The prawns have sequentially overlapping body segments (segment one covers the segment two, segment two covers segment three, etc.), chelate (claw like) first three leg pairs, and have a very basic larval body type(Dell Latitude D800 battery).
The shrimps also have overlapping segments, however, in a different pattern (segment two overlaps segments one and three), only the first two leg pairs are chelate, and they have a more complex larval form.
Biologists distinguish the true shrimp from the true prawn because of the differences in their gill structures(Dell Latitude D810 battery). The gill structure is lamellar in shrimp but branching in prawns. The easiest practical way to separate true shrimps from true prawns is to examine the second abdominal segment. The second segment of a shrimp overlaps both the first and the third segment, while the second segment of a prawn overlaps only the third segment(Dell Latitude D820 battery).
Commercial and culinary definition
While in biological terms shrimps and prawns belong to different suborders of Decapoda, they are very similar in appearance. In commercial farming and fisheries, the terms "shrimp" and "prawn" are often used interchangeably. However, recent aquaculture literature increasingly uses the term "prawn" only for the freshwater forms of palaemonids and "shrimp" for the marine penaeids. (Dell Latitude D830 battery)
In the United Kingdom, the word “prawn” is more common on menus than “shrimp”; while the opposite is the case in North America. The term “prawn” is also loosely used to describe any large shrimp, especially those that come 15 (or fewer) to the pound (such as “king prawns”, yet sometimes known as “jumbo shrimp”) (Dell Latitude 2100 battery). Australia and some other Commonwealth nations follow this British usage to an even greater extent, using the word “prawn” almost exclusively. When Australian comedian Paul Hogan used the phrase, “I'll slip an extra shrimp on the barbie for you” in an American television advertisement, it was intended to make what he was saying easier for his American audience to understand(Dell Latitude 2110 battery), and was thus a deliberate distortion of what an Australian would typically say.
In Britain very small crustaceans with a brownish shell are called shrimp, and are used to make potted shrimp. They are also used in dishes where they are not the primary ingredient.
A fresh catch of brown shrimp, Crangon crangon
As with other seafood, shrimp is high in calcium, iodine and protein but low in food energy. A shrimp-based meal is also a significant source of cholesterol, from 122 mg to 251 mg per 100 g of shrimp, depending on the method of preparation. (Dell Latitude E4300 battery) Shrimp consumption, however, is considered healthy for the circulatory system because the lack of significant levels of saturated fat in shrimp means that the high cholesterol content in shrimp actually improves the ratio of LDL to HDL cholesterol and lowers triglycerides.
Shrimp and other shellfish are among the most common food allergens. The Jewish laws of Kashrut forbid the eating of shrimp. (Dell Vostro 1310 battery) According to the King James version of the Old Testament, it is alright to eat finfish, but shrimp are an abomination and should not be eaten. In Islam, the Shafi'i, Maliki and Hanbali schools allow the eating of shrimp, while the Hanafi school does not allow it in Sunni Islam. Nor does the Shi'ite school (Ja'fari) allow it(Dell Vostro 1320 battery).
Main article: Shrimp fishery
Double-rigged shrimp trawler hauling in the nets
Common commercial methods for catching shrimp and prawns include otter trawls, cast nets, seines, shrimp baiting and dip netting. Trawling involves the use of a system of nets. In some parts of the Pacific Northwest, fishing with baited traps is also common.
The following table shows the yearly weight of shrimp and prawns captured globally in millions of tonnes(Dell Vostro 1510 battery):
The highest rates of incidental catch of non-target species is associated with shrimp trawling. In 1997, the FAO documented the estimated bycatch and discard levels from shrimp fisheries around the world. They found discard rates as high as 20 pounds for every pound of shrimp, with a world average of 5.7 pounds for every pound of shrimp. (Dell Vostro 1520 battery)
Trawl nets in general, and shrimp trawls in particular, have been identified as sources of mortality for species of finfish and cetaceans. Bycatch is often discarded dead or dying by the time it is returned to the sea, and may alter the ecological balance in discarded regions. Worldwide, shrimp trawl fisheries generate about 2% of the world’s catch of fish in weight, but result in more than one third of the global bycatch total(Dell Vostro 2510 battery).
Tanks in a shrimp hatchery
Main articles: Shrimp farm and Freshwater prawn farm
A shrimp farm is an aquaculture business for the cultivation of marine shrimp or prawns for human consumption. Commercial shrimp farming began in the 1970s, and production grew steeply, particularly to match the market demands of the United States, Japan and Western Europe(Dell Vostro 1014 battery). The total global production of farmed shrimp reached more than 1.6 million tonnes in 2003, representing a value of nearly 9 billion U.S. dollars. About 75% of farmed shrimp are produced in Asia, in particular in China, Thailand and in the Philippines (normally shrimp or prawns are caught in lake, river and sea and are not farmed in tanks). The other 25% are produced mainly in Latin America(Dell Inspiron 1410 battery), where Brazil is the largest producer. The largest exporting nation is Thailand. In Southeast Asia, mangroves are continually taken over then discarded after several years, leading to the equivalent of a ton of carbon dioxide emissions per pound of shrimp produced.
Main article: Shrimp marketing
Shrimp are marketed and commercialized with several issues in mind. Most shrimp are sold frozen and marketed based on their categorization of presentation, grading, colour, and uniformity. (Dell Vostro 1014N battery)
A steamed tail-on shrimp
Wikibooks Cookbook has a recipe/module on
Traditional preparation of tiger shrimp curry in Bengali cuisine
Preparing shrimp for consumption usually involves removing the head, shell, tail, and "sand vein".
To de-shell a shrimp, the tail is held while gently removing the shell around the body. The tail can be detached completely at this point, or left attached for presentation purposes.
Removing the "sand vein" (a euphemism for the digestive tract) is referred to as "deveining"(Dell Vostro 1015 battery). The sand vein can be removed by making a shallow cut lengthwise down the outer curve of the shrimp's body, allowing the dark ribbon-like digestive tract to be removed with a pointed utensil. Alternatively, if the tail has been detached, the vein can be pinched at the tail end and pulled out completely with the fingers. On large shrimp, the "blood vein" (a euphemism for the ventral nerve cord) (Dell Inspiron 1088 battery) along the inner curve of the shrimp's body is typically removed as well. The shrimp is then rinsed under cold running water.
Shrimp and prawns are versatile ingredients, and are often used as an accompaniment to fried rice. Common methods of preparation include baking, boiling, frying, and grilling.
Recipes using shrimp form part of the cuisine of many cultures. Strictly speaking, dishes containing scampi should be made from the Norway lobster(Dell Vostro A840 battery), a shrimp-like crustacean more closely related to the lobster than shrimp, but in some places it is quite common for large shrimp to be used instead.
Wet shrimp is commonly used as a flavouring and as a soup base in Asian cuisines while fried shrimp is popular in North America. In Europe, shrimp is very popular, forming a necessary ingredient in Spanish paella de marisco, Italian cacciucco(Dell Vostro A860 battery), Portuguese caldeirada and many other seafood dishes. Shrimp curry is very popular in South Asia and Southeast Asia. Shrimp are also found in Latin and Caribbean dishes such as enchiladas and coconut shrimp. Other recipes include jambalaya, okonomiyaki, poon choi and bagoong. Shrimp are also consumed as salad, by frying, with rice, and as shrimp guvec (a dish baked in a clay pot) in the Western and Southern coasts of Turkey(Dell Inspiron Mini 1012 battery).
Deveining is the removal of the gastrointestinal tract of a shrimp, a common part of preparing them for human consumption. The digestive tract is a dark band running from the head to the tail of the animal, where the spine would be if they were vertebrates. In females, the reproductive canal is also in the same area(SONY PCG-5G2L battery).
One must first cut a slit in the shell at the back of the shrimp. Special deveining tools are sometimes used but knives, skewers, and even toothpicks can be used to devein.
Removing the vein is not essential, as it is not poisonous and is mostly tasteless. Deveining does slightly change the flavour and makes it more consistent. Shrimp also sometimes consume small amounts of sand by accident and the vein thus might be gritty(SONY PCG-5G3L battery).
Most shrimp mature and breed only in a marine habitat, although there are a small number of freshwater species. The females lay 50,000 to 1 million eggs, which hatch after some 24 hours into tiny nauplii. These nauplii feed on yolk reserves within their body and then undergo a metamorphosis into zoeae(SONY PCG-F305 battery). This second larval stage feeds in the wild on algae and after a few days metamorphoses again into the third stage to become myses. At this stage the myses already begin to appear like tiny versions of fully developed adults and feed on algae and zooplankton. After another three to four days they metamorphose a final time into postlarvae: young shrimp having all the characteristics of adults(SONY PCG-5J1L battery). The whole process takes about 12 days from hatching. In the wild, the marine postlarvae then migrate into estuaries, which are rich in nutrients and low in salinity. There they grow and eventually migrate back into open waters when they mature. Most adult shrimp are benthopelagic, living primarily near the sea floor(SONY PCG-5J2L battery).
Common shrimp species include pink, brown and snapping shrimp. Depending on the species and location, they grow from about 1.2 to 30 centimetres (0.47 to 12 in) long, and live between one and 6.5 years.
The fossil record of shrimp is sparse, with only 57 exclusively fossil species known. The earliest of these cannot be assigned to any family, but date from the Lower Jurassic and Cretaceous. (SONY PCG-5K2L battery)
Lysmata debelius, a popular aquarium shrimp
Several types of shrimp are kept in home aquaria. Some are purely ornamental, while others are useful in controlling algae and removing debris. Freshwater shrimp commonly available for aquaria include the Bamboo shrimp, Japanese marsh shrimp (Caridina multidentata, also called "Amano shrimp," as their use in aquaria was pioneered by Takashi Amano) (SONY PCG-5L1L battery), cherry shrimp (Neocaridina heteropoda), and ghost or glass shrimp (Palaemonetes spp.). Popular saltwater shrimp include the cleaner shrimp Lysmata amboinensis, the fire shrimp (Lysmata debelius) and the harlequin shrimp (Hymenocera picta).
The term shrimp originated around the 14th century with the Middle English shrimpe, akin to the Middle Low German schrempen, and meaning to contract or wrinkle; and the Old Norse skorpna, meaning to shrivel up(SONY PCG-6S2L battery).