Introductionfish, limbless aquatic vertebrate animal with fins and internal gills. There are three living classes of fish: the primitive jawless fishes, or Agnatha; the cartilaginous (sharklike) fishes, or Chondrichthyes; and the bony fishes, or Osteichthyes. These groups, although quite different from one another anatomically, have certain common features related to their common evolutionary origins or to their aquatic way of life. Fish were the earliest vertebrates and presumably evolved from a group of aquatic lower chordates (see Chordata); the terrestrial vertebrates evolved from fishes.
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.
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