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Atlantic herring
Atlantic herring, Clupea harengus
The most abundant species of fish in the world.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
(unranked) Craniata
Subphylum: Vertebrata
Cuvier, 1812

Agnatha (jawless vertebrates)

  • Myxinoidea (hagfish)
  • Cephalaspidomorphi
    • Petromyzontida (lampreys)
  • Conodonta (extinct)
  • Pteraspidomorphi (extinct)
  • Thelodonti (extinct)
  • Anaspida (extinct)
  • Galeaspida (extinct)
  • Pituriaspida (extinct)
  • Osteostraci (extinct)

Gnathostomata (jawed vertebrates)

A fish is a poikilothermic (cold-blooded), water dwelling vertebrate with gills throughout life, and limbs—if present—in the form of fins. Poikilothermic refers to the fact that the internal temperatures of fish vary, often matching the ambient temperature of the environment.

There are over 27,000 known extant (living) fish species, making them the most diverse group of vertebrates, with more than one-half of the total vertebrate species. A heterogeneous assemblage, modern fish are conventionally divided into the jawless fish (class or superclass Agnatha, about 75 species including lampreys and hagfish), the cartilaginous fish (class Chondrichthyes, about 800 species including sharks and rays), and the bony fish (class Osteichthyes, with over 26,000 species). Some individuals only use the term fish when referring to the jawed bony fish, and do not include Agnatha or Chondrichthyes.

Since the dawn of humanity, people and fish have been linked. Fish provide humans much of their protein, offer recreational use via fishing, provide a sense of beauty as ornamental fish, and even serve in religious symbolism. The relationship has not been as favorable for fish: in 1999, seventy percent of the world's major fish species were determined to be fully- or over-exploited (Brown et. al. 1999). (See Fish and humans.)

The study of fish is called ichthyology.


Terminology: "fish" vs. "fishes"

Both "fish" and "fishes" are acceptable plurals, and both forms are common. For example, the later usage can be found throughout Nelson's Fishes of the World, 3rd Edition (1994), and within the Guidelines for Use of Fishes in Field Research (1988) by the American Society of Ichthyologists and Herpetologists, the American Fisheries Society, and the American Institute of Fisheries Research Biologists. Examples of the use of fish as the plural are quite common in popular literature.

Although both plural forms are acceptable, a common preference among biologists is to use the term "fishes" when speaking about two or more kinds of fish (species), and the term "fish" when referring to two or more individual fish organisms. For example, the publication Guidelines of the American Fisheries Society (2005) offers a preference for "fishes" as a synonym for "species of fish." Thus, in the statement "there are over 27,000 fishes in the world," the reference would be to over 27,000 fish species in the world. In the statement, "there are several million fish of the species Gadus morhua," the reference is that G. morhua comprises several million individuals. The usage of the two words is similar to that of the words "people" and "peoples." Of course, languages change over time and one may see a trend in the use of these terms as well.

Characteristics of fish

Fish range in size from the 14m (45ft) whale shark to the 7mm (just over 1/4 of an inch) long stout infantfish and the 13mm Philippine goby. Fish can be found in almost all large bodies of water in salt, or brackish, or fresh water, at depths from just below the surface to several thousand meters. However, hyper-saline lakes like the Great Salt Lake of the United States do not support fish. Some species of fish have been specially bred to be kept and displayed in an aquarium, and can survive in the home environment.

Hagfish, while generally classified in Agnatha ("jawless") and as fish, actually lack vertebrae. For this reason, hagfish, which are also commonly known as "slime eels," are sometimes not considered to be fish. The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage. Hagfish are a staple food in Korea. They are classified in the order Myxini and the family Myxinidae. Both hagfish and lamprey have slimy skin without scales or plates. They also have a notochord that remains throughout life; circular, jawless mouths; and unpaired fins. Hagfish are found in the oceans and lampreys are found in both freshwater and ocean environments. Most lampreys are parasitic.

Fish belonging to the class Chondrichthyes are distinguished by cartilage skeletons, as opposed to skeletons of bone. They have movable jaws and paired fins. Almost all of the Chondrichthyes—sharks, rays, skates, and chimaeras—are found in ocean environments.

Northern pike, a ray-finned fish

Most fish species (about 95 percent) are placed in the class Osteichthyes (which some taxonomies consider a superclass). They have bony internal skeletons and skins with scales. (As a general rule for the taxon. Not all bony fish have scales, and scales may be absent or present in two closely related species (Nelson 1994); Catfish is an example of an order of fish that lack scales.) The Osteichthyes taxon include coelacanths (lobe-finned fish), lungfish, and ray-finned fish. Coelacanths were thought to have been extinct for millions of years until fishermen caught one in 1938. Lungfish have lungs, as well as gills. Ray-finned fish are what many people refer to when they use the term fish, as these are our most familiar fish, including bass, eels, and many sports fish. Ray-finned fish have fins that are supported by bones (rays).

Many types of aquatic animals named "fish," such as jellyfish, starfish, and cuttlefish, are not true fish. A number of sea dwelling creatures, like dolphins and whales, are actually mammals.

While fish are poikilothermic in that they do not maintain constant internal temperatures and the temperature often mirrors the ambient temperature, certain species of fish maintain elevated body temperatures to varying degrees. These include teleosts (bony fishes) in the suborder Scombroidei and 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 this capacity, and evidence suggests the trait exists in family Alopiidae (thresher sharks). The degree of being able to have elevated temperatures varies from the billfish, which warm only their eyes and brain, to bluefin tuna and porbeagle sharks, which can elevate body temperatures in excess of 20 °C above ambient water temperatures. In many cases, this phenomena has been traced to heat exchange, as warmer blood being returned to the gills in small veins runs close to colder, oxygenated blood in narrow arteries leaving the gills. This ability to have elevated temperatures allows fish to be active in colder waters and to have enhanced swimming ability because of the warmer muscles. In general, most fish can survive only at a relatively small range of body temperatures, but may adjust their depth in large bodies of water in order to find preferable ranges.


Fish are a very diverse assemblage, so much so that the term fish is itself more one of convenience than a taxonomic rank (Nelson 1994). It is used to designate a paraphyletic group, whereby the most recent common ancestor is included but not all descendants, with tetrapods (four-legged vertebrates, or descendants of four-legged vertebrates) being excluded. It is thus not surprising that different taxonomists may classify fish differently.

Latimeria chalumnae (coelacanth, a lobe-finned fish)

Vertebrates are generally classified into two groups, the Agnatha (jawless vertebrates) and the Gnathostomata (jawed vertebrates). The later group includes fish with hinged jaws, but also includes amphibians, reptiles, birds, and mammals (the tetrapods). In most taxonomies, Agnatha and Gnathostomata are each considered a superclass, although sometimes Agnatha is considered a class (see taxonomy).

The Agnatha, in addition to including the modern day lampreys (Petromyzontiformes) and hagfish (Myxiniformes), also includes several extinct orders.

Within Gnathostomata, several classes of fish are recognized. Two of these classes have living representatives, the Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). In some taxonomies, Osteichthyes is considered a superclass.

Within the Osteichthyes, two extant subclasses (or classes) are generally recognized, the Sarcopterygii (lobe-finned fish) and the Actinopterygii (ray-finned or spiny-finned fish). The coelacanths are generally placed within the Sacropterygii subclass. The Actinopterygii are generally divided into the Chondrostei and the Neopterygii, the latter of which includes the Teleostei (modern bony fishes), a classification into which most fish fit today.

A general grouping of fish, without reference to the names of ranks of taxa (superclass, class, subclass, etc.) is presented above, in the image box. Below is presented a more detailed taxonomic scheme with the rank names, as derived from that offered by Benton (2004), in his text Vertebrate Paleontology:

    • Class 'Agnatha'
      • Subclass Myxinoidea (hagfish)
      • Subclass Petromyzontida (lampreys)
      • Subclass †Conodonta
      • Subclass †Pteraspidomorphi
      • Order †Thelodonti
      • Order †Anaspida
      • Subclass unnamed
        • Order †Galeaspida
        • Order †Pituriaspida]
        • Order †Osteostraci
    • Infraphylum Gnathostomata (jawed vertebrates)
      • Class †Placodermi
      • Class Chondrichthyes (cartilaginous fish)
        • Subclass Elasmobrachii (sharks, rays, skates)
        • Subclass Subterbranchialia
          • Superorder Holocephali (chimaeras)
      • Class †Acanthodii
      • Class Osteichthyes (bony fish)
        • Subclass Actinopterygii (ray-finned fish)
          • Superdivision Chondrostei (sturgeons, paddlefish)
          • Superdivision Neopterygii (teleosts—modern bony fish)
        • Subclass Sarcopterygii (lobe-finned fish)
          • Order Dipnoi (lungfish)
          • Infraclass Crossopterygii

† = extinct (the group lacks any living members)

Each year, biologists find 200 to 300 species of fish that had not been previously known (Grady 2006).

Fish and humans

Throughout history, humans have utilized fish as a food source. Historically and today, most fish protein has come by means of catching wild fish. However, aquaculture, or fish farming, which has been practiced since about 3,500 B.C.E. in China, is becoming increasingly important in many nations. Overall, about one-sixth of the world's protein is estimated to be provided by fish. That proportion is considerably elevated in some developing nations and regions heavily dependent on the sea. In a similar manner, fish have been tied to trade. One of the world’s longest lasting trade histories is the trade of dry cod from the Lofoten area in northern Norway to the southern parts of Europe. This trade in cod has been going on for more than 1000 years.

Fish are also caught for sport. Indeed, in many aquatic environments today, including most freshwaters, there are more fish caught for sport than via commercial fisheries.

Catching fish for the purpose of food or sport is known as fishing, while the organized effort by humans to catch fish is called a fishery. Fisheries are a huge global business and provide income for millions of people. The annual yield from all fisheries worldwide is about 100 million tons, with popular species including herring, cod, anchovy, tuna, flounder, and salmon. However, the term fishery is broadly applied, and includes more organisms than just fish, such as mollusks and crustaceans, which are often called "fish" when used as food.

Fish have been recognized as a source of beauty for almost as long as used for food, appearing in cave art, being raised as ornamental fish in ponds, and displayed in aquariums in homes, offices, or public settings. As of 2006, there were an estimated 60 million aquarium enthusiasts worldwide.

Basking shark, the second largest living fish (after the whale shark) is a filter feeder that eats zooplankton.

Because of the popularity of fish for food, sport, and hobby, overfishing is a threat to many species of fish. In the May 15, 2004 issue of the journal Nature, it was reported that all large oceanic fish species worldwide had been so systematically overcaught that fewer than 10 percent of 1950 levels remained. Particularly imperiled were sharks, Atlantic cod, Bluefin tuna, and Pacific sardines.

Some fish pose dangers to humans. Although the sharks may be among the most feared, there are actually few shark species that are known to attack humans. The largest sharks, the whale shark and basking shark, are actually plankton feeders. The International Shark Attack File reports there are only about 10-15 deaths each year worldwide. This compares to about 1,000 deaths annually from crocodiles and 60,000 from snakebites.

On the other hand, Smith and Wheeler (2006) suggest that, in contrast to prior estimates of 200 venomous fishes, 1,200 species of fish should be presumed venomous. Most of these venomous fishes come from off the coast of eastern and southern Africa, Australia, Indonesia, Phillipines, Polynesia, and southern Japan (Grady 2006). About 50,000 people a year suffer from fish stings or envenomations (Grady 2006). Perhaps the most dangerous venomous fish is the stonefish, which can release a venomous toxin from spikes on its back when it is provoked or frightened. This toxin can be fatal to humans if not treated promptly. The pufferfish, often better known by the Japanese name Fugu, poses risks to humans because this species contains a highly toxic poison in the internal organs. Despite this, it is considered a delicacy in Japan. The pufferfish needs to be very specially prepared to be safe for eating. Every year a number of people die from consuming this fish.

Barracudas, sea bass, moray eels, and stingrays are among other fish that pose risks to humans in the aquatic environment.

Fish have a prominent role in in human religious culture, particularly Christianity. The fish is used often as a symbol by Christians to represent Jesus or Christianity. The Greek word for fish (ichthus, spelled Iota Chi Theta Upsilon Sigma) is taken as an acrostic, spelling out Jesus and his titles: "Jesus Christ, Son of God, Savior" (Iesous CHristos THeou Uiou Soter), or similar combinations. The gospels of the New Testament contain many references to fishing, both literal, as when Jesus directed Peter where to cast his nets, and metaphorical, as when Jesus is recorded as stating, "Follow me, and I will make you fishers of men" (Matthew 4:19).

Centropristis striata


  • American Fisheries Society. 2005. AFS Publications Style Guide. Bethesda, MD: American Fisheries Society.
  • American Society of Ichthyologists and Herpetologists, American Fisheries Society, and American Institute of Fisheries Research Biologists. 1988. Guidelines for use of fishes in field research. Fisheries 13(2):16-23.
  • Benton, M. J. 2004. Vertebrate Palaeontology (3rd edition). Oxford, UK: Blackwell Publishers.
  • Brown, L. R., M. Renner, and B. Halweil. 1999. Vital Signs, 1999. New York: W. W. Norton and Company.
  • Grady, D. 2006. Venom runs thick in fish families, researchers learn. New York Times online August 22, 2006.
  • Lagler, K. F., J. E. Bardach, and R. R. Miller. 1962. Ichthyology. New York: John Wiley & Sons.
  • Nelson, J. S. 1994. Fishes of the World (3rd edition). New York: John Wiley & Sons.
  • Rasmussen, A. S., A. Janke, and U. Arnason. 1998. The mitochondrial DNA molecule of the hagfish (Myxine glutinosa) and vertebrate phylogeny. Journal of Molecular Evolution 46(4):382-388.
  • Smith, W. L., and W. C. Wheeler. 2006. Venom evolution widespread in fishes: A phylogenetic road map for the bioprospecting of piscine venoms. Journal of Heredity 97(3):206-217.


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