Georgia Aquarium - Giant Grouper edit

A giant grouper at the Georgia Aquarium

Fish are aquatic vertebrate that are typically 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 wide as deities and religious symbols to subjects of books and popular movies.

What is a fish?

The term "fish" is most precisely used to describe any non-tetrapod chordate, i.e., an animal with a backbone but lacking four limbs (or having ancestors that had four limbs). Unlike groupings such as birds or mammals, fish are not a single clade but a paraphyletic collection of taxa including hagfishes, lampreys, sharks, rays, lungfishes and coelacanths, sturgeons, gars, and advanced ray-finned fishes. [1]

A typical fish is cold-blooded; has a streamlined body that allows it to swim rapidly; extracts oxygen from the water using gills; has two sets of paired fins, one or two dorsal fins, an anal fin, and a tail fin; has jaws; has skin that is covered with scales; and lays eggs that are fertilised 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 and some species of sharks are warm-blooded, and able to raise their body temperature significantly above that of the ambient water surrounding them. [2] 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 manoeuvrable species such as eels and rays that reach no more than 0.5 body-lengths per second. [3] 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. [4] 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. [5] 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. [6] The catfish Phreatobius cisternarum lives in waterlogged leaf litter [7], [8]

The various fish groups taken together account for more than half of the known vertebrates. There are at least 24,600 known species of fish, of which over 23,000 are bony fish, with the remainder being about 850 sharks, rays, and chimeras and about 85 hagfishes and lampreys. [9] They 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. These include cuttlefish, jellyfish, inkfish, and starfish. Marine invertebrates that are consumed as food are commonly called shellfish. Whales and dolphins have been called fish as well, although they are mammals. This usage is no longer common in English.


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:

Some palaeontologists consider that Conodonta are chordates, and so regard them as primitive fish.

For a fuller treatment of classification, see the vertebrate article.

Fish anatomy

Main article: Fish anatomy
Lampanyctodes hectoris (Hector's lanternfish)2

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 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 ceca. The pyloric ceca 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. Some fishes, such as lungfish, have developed an adaptation known as a labyrinth that allows them to survive in oxygen-poor areas or places where bodies of water constantly dry up. These species of fish possess specialized organs that serve as lungs. A tube brings air containing oxygen to this organ by way of the fish's mouth. Some kinds of lungfish are so dependent on receiving oxygen from the air that they will suffocate if not allowed to reach the surface of the water.

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 fishes, the heart consists of four parts: the sinus venosus, the atrium, the ventricle, and the bulbus arteriosus. Despite consisting of four parts, the fish heart is still a two-chambered heart. The sinus venosus is a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the atrium, which is a large muscular chamber. The atrium serves as a one-way compartment for blood to flow into the ventricle. The ventricle is a thick-walled, muscular chamber and it does the actual pumping for the heart. It pumps blood to a large tube called the bulbus arteriosus. At the front end, the bulbus arteriosus connects to a large blood vessel called the aorta, through which blood flows to the fish's gills.


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 fishes 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.

Sensory and nervous system

Fish have well-developed nervous systems that organize around a central brain, that is divided into different parts. The most anterior, or front, end of the brain are the olfactory bulbs, which are involved in the fish's sense of smell. Unlike most vertebrates, the cerebrum of the fish primarily processes the sense of smell rather than being responsible for all voluntary actions. The optic lobes process information from the eyes. The cerebellum coordinates body movements while the medulla oblongata controls the functions of internal organs. Most fishes possess highly developed sense organs. Nearly all daylight fish have well-developed eyes that have color vision that is at least good as a human's. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the lateral line system. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey. In 2003, it was found by Scottish scientists at Edinburgh University performing research on rainbow trout that fish exhibit behaviors often associated with pain. Some fishes, such as catfish and sharks, have organs that detect low levels electric current. Other fishes, like the electric eel, can produce their own electricity.

Muscular system

Fish locomotion

Main article: 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


The eggs of fish are fertilized either externally or internally, depending on species. The female usually lays the eggs, and the embryos in the eggs develop and hatch outside her body. These kind of fish are called oviparous fish. Oviparous fish develop by obtaining food from the yolk in the egg. Salmon, for example, are oviparous.

Ovoviviparous fish keep the eggs inside of the mother's body after internal fertilization. Each embryo develops in its own egg. The young are "born alive" like most mammals.

Some species of fish, such as various sharks, are viviparous. Viviparous fish allow their embryos to stay in the mother's body like ovoviviparous fish. However, the embryos of viviparous fish obtain needed substances from the mother's body, not through material in the egg. The young of viviparous species are also "born alive".

Livebearers is a term used to describe fish who give birth to live young. The eggs are fertilized internally by a male through an organ called a gonopodium. The eggs are kept inside the female until they hatch. The female then releases the fry into the water. Livebearer fry do not have egg yolks and can swim by themselves in under 24 hours. The most common livebearing species are the guppy, platy, Poecilia (molly) and swordtail.

Immune system

Types of immune organs vary between different types of fish.[10] 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[11]. 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[12][13]. 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. [14]


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[1]. 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, and Pikaia.

Fish disease

Main article: Fish diseases

Importance of Fish to Humans

Economic importance

Main article: Aquaculture
Main article: Fishing
Main article: Fish farming


Main article: Angling
Main article: Fishkeeping
Main article: Sport fishing

Danger to Fish Populations

Ignoring the fact that the human population is increasing exponentially, humans are already putting too much strain on the ocean. Unregulated overfishing, as well as human-caused global warming (which has caused a rise in ocean temperatures), has already made some fish species endangered. However, the main concern is that unchecked abuse of the marine ecosystem will result in a collapse of global fish populations perhaps as early as 2050, severely impacting cultures which have fish in their diet [15]. Returning to the fact that the human population is increasing exponentially: not only might there be a global collapse of fishing, but the problem could be twice as bad if the human population doubles between now and then. (Also see carrying capacity.)

Fish in culture

Through the ages, many cultures have featured fish in their legends and myths, from the "great fish" that swallowed Jonah the Prophet through to the half-human, half-fish mermaid around which books and movies have been centred (e.g., Splash). 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.[16] 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.

"Fish" or "fishes", "school" or '"shoal"?

Though often used interchangeably, these pairs of words actually mean different things. Fish is used either as singular noun or to describe a group of specimens from a single species. Fishes describes a group containing more than one species. [17] Hence, as plurals, these words could be used thus:

  • My aquarium contains three different fishes: guppies, platies, and swordtails.
  • The North Atlantic stock of Gadus morhua is estimated to contain several million fish.

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 (see article on swarming, the term used to cover such behaviours in animals). [18]

  • Cichlids congregating at lekking sites form an aggregation.
  • Many minnows and characins form shoals.
  • Classic examples of schooling fish are anchovies, herrings, and silversides.

It should be noted that 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.


  1. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0-86542-256-7
  2. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 83-86, 1997, ISBN 0-86542-256-7
  3. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 103, 1997, ISBN 0-86542-256-7
  4. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 53-57, 1997, ISBN 0-86542-256-7
  5. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, pp 33-36, 1997, ISBN 0-86542-256-7
  6. Froese, R. and D. Pauly. Editors.. Species Summary: Periophthalmus barbarus. FishBase. Retrieved on 2006-11-26.
  7. Froese, R. and D. Pauly. Editors.. Species Summary: Phreatobius cisternarum. FishBase. Retrieved on 2006-11-26.
  8. Planet Catfish. Cat-eLog: Heptapteridae: Phreatobius: Phreatobius sp. (1). Planet Catfish. Retrieved on 2006-11-26.
  9. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0-86542-256-7
  10. A.G. Zapata, A. Chiba and A. Vara. Cells and tissues of the immune system of fish. In: The Fish Immune System: Organism, Pathogen and Environment. Fish Immunology Series. (eds. G. Iwama and T.Nakanishi,), New York, Academic Press, 1996, pages 1-55.
  11. D.P. Anderson. Fish Immunology. (S.F. Snieszko and H.R. Axelrod, eds), Hong Kong: TFH Publications, Inc. Ltd., 1977.
  12. S. Chilmonczyk. The thymus in fish: development and possible function in the immune response. Annual Review of Fish Diseases, Volume 2, 1992, pages 181-200.
  13. J.D. Hansen and A.G. Zapata. Lymphocyte development in fish and amphibians. Immunological Reviews, Volume 166, 1998, pages 199-220.
  14. Kucher et al.,. Development of the zebrafish lymphatic system requires VegFc signalling. Current Biology, Volume 16, 2006, pages 1244-1248.
  15. Ocean study predicts the collapse of all seafood fisheries by 2050. Retrieved on 2006-1-13.
  16. Jaffrey, M.: A Taste of India, Atheneum, p 148, 1988, ISBN 0-689-70726-6
  17. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 3, 1997, ISBN 0-86542-256-7
  18. Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 375, 1997, ISBN 0-86542-256-7

See also

External links


  • FishBase online - Comprehensive database with information on over 29,000 fish species


  • ANGFA - Illustrated database of freshwater fishes of Australia and New Guinea
  • Ecology Asia - Photos and facts on freshwater fishes of Southeast Asia
  • - Illustrated database of the freshwater fishes of Germany (in German)
  • The Native Fish Conservancy - Conservation and study of North American freshwater fishes


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