There are over 20,000 living species of fish. They range in size from the .31-in. (7.9-mm) Paedocypris that lives in tropical swamps in Sumatra to the 45-ft (14-m) whale shark. Many are brightly colored, and many have shapes and patterns that serve as camouflage. They are found in all marine, fresh, and brackish waters throughout the world and at all depths. Members of different species of fish tolerate water temperatures ranging from freezing to over 100°F; (38°C;). Most are confined either to saltwater or to freshwater, but some are physiologically adapted to moving from one to the other. A number of fishes that are born in freshwater spend their adult lives in the ocean, returning to their birthplace to spawn; the reverse of this migration occurs in some fishes born in the ocean. Many fishes stay in tightly organized groups, called schools; others are solitary and congregate only for feeding and spawning. Fish may be carnivorous, herbivorous, or omnivorous. Some fish are scavengers on lake or ocean bottoms. Fish are a major source of human food as well as of oil, fertilizer, and feed for domestic animals (see fishing).

A number of aquatic invertebrate animals and groups have common names that include the term fish (for example, crayfish and shellfish), but these do not resemble and are not related to true fishes. Furthermore, there are members of the terrestrial vertebrate classes, such as whales and sea snakes, that have adopted an aquatic way of life; these may superficially resemble fishes and are sometimes erroneously called fishes, but they are air-breathers, and their anatomical structure reveals their relationship to land animals.

Characteristic Anatomical Features

A typical fish is torpedo-shaped, with a head containing a brain and sensory organs, a trunk with a muscular wall surrounding a cavity containing the internal organs, and a muscular post-anal tail. Most fish propel themselves through the water by weaving movements of their bodies and control their direction by means of the fins. All have skins covered with slimy glandular secretions that decrease friction with the water; in addition, nearly all have scales, which together with the secretions form a nearly waterproof coating. All fishes have a lateral line system of sensory organs for detecting pressure changes in the water. All have water-breathing organs called gills located in passages leading from the throat, or pharynx, to the exterior; a few fishes also have air-breathing lungs as an additional means of respiration. In all but the most primitive class, the gill passages are supported by skeletal structures called gill arches. Plankton-feeding fish have structures called gill rakers attached to the gill arches; these strain minute organisms from the water as it passes out of the pharynx. Fish breathe by taking water into the mouth and forcing it out through the gill passages; as the water passes over the thin-walled gills, dissolved oxygen diffuses into the gill capillaries and carbon dioxide diffuses out. The circulatory system is closed, and the heart is two-chambered; the blood is red. With few exceptions, fish are cold-blooded; that is, they cannot regulate their body temperature, which is the same as that of the environment.

Reproduction

Methods of reproduction are varied. Sharks have internal fertilization, and most give birth to live young. Those that lay eggs produce large ones with tough shells. Since embryonic development is well-protected in these fish, they produce a relatively small number of young, only seven or eight at a time in some species. A few of the bony fishes, including some aquarium species, are live bearers, but most lay small, unprotected eggs that are fertilized after deposition in water. In most marine species the eggs float freely in the currents, where they are eaten by other animals. An enormous number of eggs is therefore necessary to ensure the maturation of a few; in many species a female produces as many as 5 million eggs in one spawn. The eggs of most marine fishes contain oil droplets that buoy them up, while those of most freshwater fishes are heavy, with sticky surfaces that adhere to objects in the water. Most freshwater species build nests for the protection of the eggs, and in some the adults guard the nests.

Types of Fish

The primitive fishes of the class Agnatha lack jaws and the paired pelvic and pectoral fins characteristic of more advanced fishes. This largely extinct class includes two living groups, the bloodsucking lampreys and the scavenging hagfishes. Fishes of the extinct class Placodermi were the first vertebrates to develop jaws and paired fins. These fish had bony skeletons and were covered with bony armor. A branch of this group probably gave rise to the two main modern classes of fish, the cartilaginous fish and the bony fish.

The cartilaginous fish (sharks, rays, and chimaeras) are distinguished from the bony fish by their cartilage skeletons, by the absence of either a swim bladder or lungs, by the construction of their tail fins, and by the absence in most of a gill covering, or operculum. The skin of members of this group is covered with imbedded toothlike structures called denticles, giving it a rough, sandpapery quality. Sharks are almost exclusively marine in distribution.

The bony fishes are distinguished from other living fishes by their bone skeletons and by the presence of either a swim bladder (which functions as a float) or, in a few fishes, lungs. The bony fishes are divided into two subclasses, the fleshy-finned fish and the ray-finned fish. The latter group includes over 95% of all living fish species.

The earliest bony fishes were fleshy-finned. They evolved during a period of widespread drought and stagnation and gave rise to the amphibians (the first terrestrial vertebrates) on the one hand and to the ray-finned fish on the other. The only surviving fleshy-finned fishes are the lungfishes and one species of coelacanth (see lobefin). These fishes retain some of the traits of ancestral bony fishes: fleshy fins with supporting bones (precursors of the limbs of land vertebrates), internal nostrils, and lungs.

Ray-finned fishes, now predominant in both fresh and marine waters, represent an advanced adaptation of the bony fishes to strictly aquatic conditions; they are the most highly successful and diverse of the fishes. In nearly all of these fishes the lung has evolved into a hydrostatic organ, the swim bladder. The fins in this group consist of a web of skin supported by horny rays. Each ray is moved by a set of muscles, giving the fin great flexibility. Most ray-finned fish have overlapping scales made of very thin layers of bone. Their skeletal structure is light but strong and most have excellent vision.

Named for his father's friend Alexander Hamilton, and heir to the Federalist tradition, Fish naturally gravitated to politics as a Whig. He served as U.S. Representative (1843-45) and was elected lieutenant governor of New York in 1847 and governor, for a two-year term, in 1848. From 1851 to 1857, Fish was a U.S. Senator, serving on the foreign relations committee in 1855-57. A moderate antislavery man, he opposed both abolitionist and proslavery excesses and deplored the breakup of the Whigs as a national party. Slow to join the new Republican party, he lost his national political standing but became prominent in civic activities in New York.

Fish was one of many to lionize the victorious Civil War general Ulysses S. Grant, but his appointment (Mar., 1869) as Grant's Secretary of State, to succeed the grossly miscast Elihu B. Washburne, came as a surprise. He accepted reluctantly and expected to hold the office for only a few months, but actually remained in the cabinet longer than any other member, serving through both of Grant's administrations.

Fish was one of the ablest of U.S. Secretaries of State. Grant was much impressed with Fish's character and ability, and he called upon Fish's aid in the administration of domestic affairs as well. Fish's greatest achievement as Secretary was bringing about the treaty (see Washington, Treaty of) that paved the way for settlement of the Alabama claims and other long-standing disputes with Great Britain. This was accomplished amid great difficulties, especially those offered by the vigorously anti-British chairman of the Senate foreign relations committee, Charles Sumner.

The period was one of constant trouble with Spain, arising out of the Ten Years War, and Fish was hard pressed to persuade Grant not to recognize the belligerency of Cuba. Under Fish's vigilant eye filibustering expeditions from the United States to Cuba were kept to a minimum, but the Virginius affair in 1873 nearly brought the nation, long sympathetic to the Cuban cause, to war with Spain. To secure Grant's support of other policies Fish supported without enthusiasm the President's unsuccessful project to annex the Dominican Republic.

Nicholas Fish, 1758-1833, b. New York City. He studied law before serving ably as a major in a New York regiment throughout the American Revolution. A New York City alderman (1806-17), he was a leading Federalist and a close friend of Alexander Hamilton. He also served (1824-32) as chairman of the board of trustees of Columbia College, a post later held by his son, Hamilton Fish (1808-93), the most illustrious member of the clan (see separate articles for Hamilton Fish, 1808-93, and for his youngest son, Stuyvesant Fish). Nicholas Fish, 1848-1902, b. New York City, was Hamilton's eldest son. He entered (1871) the U.S. diplomatic service and was minister to Belgium (1882-86).

A third son, Hamilton Fish, 1849-1936, b. Albany, N.Y., studied law and was admitted to the bar in 1873. He was a member of the New York state assembly (1874-96), serving as speaker in 1895-96, and was long Republican boss of Putnam co. On appointment by President Theodore Roosevelt, he was Assistant Treasurer of the United States in New York City (1903-8). He also served (1909-11) as a U.S. Representative.

The family's third Hamilton Fish, 1888-1991, son of the foregoing, b. Garrison, N.Y., was a football player at Harvard. A lawyer, Fish served in the New York state assembly (1914-16), distinguished himself in World War I as captain of an African-American infantry company, and from 1920 to 1945 was a U.S. Representative. A leading isolationist and vigorous anti-Communist, once accused of having connections with the Bundists and with other Axis supporters, he was opposed for renomination in 1944 by Gov. Thomas E. Dewey and other Republican leaders. Fish nevertheless won the primary but was defeated for reelection in November.

His son Hamilton Fish, 1926-96, b. Washington, D.C., continued the family's involvement in Republican politics. Admitted to the bar in 1957, he was elected to the U.S. House of Representatives from New York in 1968, where he consistently supported civil-rights legislation. He retired from Congress in 1995.

Any member of two unrelated groups of fishes: freshwater species in the genus Brachydanio (family Cyprinidae) and saltwater species in the genus Pterois (family Scorpaenidae). The zebra danio (Brachydanio rerio), a popular freshwater aquarium fish originally from Asia, grows to about 1.5 in. (4 cm) long and has dark blue and silvery longitudinal stripes. The distinctive saltwater zebra fishes (Pterois), used in marine aquariums, have extremely large pectoral fins, numerous extremely poisonous spines, and colourful vertical stripes. Some species are more commonly known as lionfish and turkeyfish.

Learn more about zebra fish with a free trial on Britannica.com.

Learn more about electric ray with a free trial on Britannica.com.

Learn more about scorpion fish with a free trial on Britannica.com.

Widely distributed species (Naucrates ductor, family Carangidae) of carnivorous fish that inhabits warm and tropical open seas. It is slender and has a forked tail, a lengthwise keel on each side of the tail base, and a few small spines in front of the dorsal and anal fins. It may grow to 2 ft (60 cm) but is usually about 14 in. (35 cm) long. Five to seven distinctive vertical dark bands mark the bluish body. Pilot fishes follow sharks and ships, apparently to feed on parasites and leftover scraps. It was formerly thought that they were leading, or “piloting,” the larger fishes to food.

Learn more about pilot fish with a free trial on Britannica.com.

Learn more about parrot fish with a free trial on Britannica.com.

Any of several species of showy Indo-Pacific fish of the scorpion-fish family (Scorpaenidae), noted for their venomous fin spines, which can inflict painful, though rarely fatal, puncture wounds. Lionfish have enlarged pectoral fins and elongated dorsal fin spines, and each species bears a particular pattern of bold stripes. When disturbed, the fish spread and display their fins, and, if further pressed, present and attack with the dorsal spines. Pterois volitans, sometimes kept by fish fanciers, is striped with red, brown, and white and grows to about 12 in. (30 cm) long. Several smaller Indo-Pacific species of the genus Dendrochirus are also known as lionfish.

Learn more about lionfish with a free trial on Britannica.com.

Any of about 40 species of oceanic fishes (family Exocoetidae). They are found worldwide in warm waters and are noted for their ability to “fly.” All species are less than 18 in. (45 cm) long and have winglike, rigid fins and an unevenly forked tail. Two-winged species have only the pectoral fins enlarged; four-winged species have both the pectoral and the pelvic fins enlarged. Rather than flying, they actually glide after jumping from the water. They can make several consecutive glides; the strongest fliers can travel as much as 600 ft (180 m) in a single glide, and compound glides may cover 1,300 ft (400 m). The behaviour is primarily a means of escaping predators.

Learn more about flying fish with a free trial on Britannica.com.

Species (Pandion haliaetus) of long-winged hawk found along seacoasts and large interior waterways. Ospreys are about 26 in. (65 cm) long and brown above and white below, with some white on the head. An osprey flies over the water, hovers above its prey, and then plunges feet first, seizing the fish in its long, curved talons. Ospreys breed on all continents except South America, where they live only in winter. They usually nest, singly or in colonies, high in trees or on cliffs. Bioaccumulation of pesticides caused populations to dwindle in the 20th century, but they are now recovering.

Learn more about osprey with a free trial on Britannica.com.

Rearing of fish, shellfish, and some aquatic plants to supplement the natural supply. Fish are reared in controlled conditions worldwide. Though most aquaculture supplies the commercial food market, many governmental agencies engage in it to stock lakes and rivers for sport fishing. It also supplies goldfish and other decorative fish for home aquariums and bait fish for sport and commercial fishing. Carp, trout, catfish, tilapia, scallops, mussels, lobsters, and oysters are well-known species raised through aquaculture.

Learn more about aquaculture with a free trial on Britannica.com.

Learn more about fish with a free trial on Britannica.com.

Any member of the vertebrate class Osteichthyes, including the great majority of living fishes and all the world's sport and commercial fishes. Also called Pisces, the class excludes jawless fishes (hagfishes and lampreys) and cartilaginous fishes (sharks, skates, and rays). There are more than 20,000 species worldwide, all with a skeleton at least partly composed of true bone. Other features include, in most species, a swim bladder (an air-filled sac to give buoyancy), gill covers over the gill chamber, bony platelike scales, a skull with sutures, and external fertilization of eggs. Bony fishes occur in all freshwater and ocean environments.

Learn more about bony fish with a free trial on Britannica.com.

Marine game fish (Albula vulpes) that inhabits coastal and island waters in tropical seas and is admired by anglers for its speed and strength. Its maximum length is about 30 in. (76 cm), and its maximum weight is 14 lbs (6.4 kg). The bonefish has a deeply notched caudal fin (near the tail) and a small mouth beneath a pointed, piglike snout. It grubs on the bottom for worms and other food.

Learn more about bonefish with a free trial on Britannica.com.

Any of five species (family Toxotidae) of Indo-Pacific fishes noted for their ability to knock their insect prey off overhanging vegetation by shooting it with drops of water expelled from their mouth. Archer fishes are elongated and have a relatively deep body that is almost flat from the dorsal fin forward. The head is pointed, the mouth is large, and the dorsal and anal fins are placed toward the back of the body. Different species are spotted or vertically banded with black. Archer fishes live in both fresh and salt water, usually remaining near the surface. One well-known species (Toxotes jaculator, or jaculatrix) grows to about 7 in. (18 cm) long.

Learn more about archer fish with a free trial on Britannica.com.

Learn more about Siamese fighting fish with a free trial on Britannica.com.

Learn more about Fish, Hamilton with a free trial on Britannica.com.