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Fish, diverse group of animals that live and breathe in water. All fishes are vertebrates (animals with backbones) with gills for breathing. Most fish have fins for swimming, scales for protection, and a streamlined body for moving easily through the water.

Fishes may be divided into two distinct groups, jawless fish and jawed fish. The jawless fish are represented by two families of distantly related eel-like fish, the hagfish and the lampreys. Both fishes have tongues equipped with numerous small teeth and lack paired fins and a bony skeleton. Although these two families include only a handful of living species, the fossil record shows they were once a highly diverse group that also included fish whose head and trunk were covered with a hard bony shell. Hagfish are the vultures of the abyss, feeding on carcasses of dead fish and other animals. Lampreys, in contrast, feed on live fish by attaching their sucking disk to their host and rasping away tissue with their toothed tongue.

The jawed fish may also be separated into two major groups: bony fish, which have skeletons made of rigid bone, and cartilaginous fish, which have skeletons made of elastic cartilage. There are nearly 1000 species of cartilaginous fish, including sharks, rays, and chimaeras, or ratfish. Sharks and rays live in relatively shallow ocean waters and occasionally freshwater, while chimaeras are found only in the ocean, mostly in deep water. Sharks have an age-old reputation for savagery, but only a few of the approximately 370 species deserve this reputation. Most sharks, like the spiny dogfish, are predators of small fish and invertebrates, while the largest, such as megamouths, whale sharks, and basking sharks, feed by filtering tiny invertebrates from the water. The nearly 200 species of rays are essentially sharks flattened like a pancake that have adapted to life on the ocean floor.

The bony fishes encompass by far the largest diversity of fish, with about 24,000 species inhabiting nearly every body of water on the earth. They are divided into two groups—the lobe-finned fish and the ray-finned fish. Lobe-finned fishes include the lungfish, a small group of primitive air-breathing fish, and the coelacanth, the single living species of a group long thought to be extinct.

The ray-finned fishes are divided into two major groups, the primitive sturgeons and paddlefish, and the more evolved new-finned fishes. Most of the common and well-known fish species are new-finned fish, including the herrings, which support one of the largest fisheries in the world, and the eels, which are found in nearly all marine habitats. Other new-finned fishes include the ostariophysans—minnows, characins, and catfish—which inhabit the freshwaters of the tropics and surrounding areas. Salmon have adapted to the coasts of northern oceans by living part of their lives in freshwater and part in the ocean. There are over 9000 species of perch, including tunas, jacks, billfishes, sunfishes, and darters, making it the largest vertebrate order. Perches and their relatives are the dominant fishes in tropical marine waters. Closely related to the perches are the flatfish, which look and swim like normal fish when young, only to lay on one side of their body as adults after one eye migrates to the “top” side.

Scientific classification: Fishes are classified differently by different zoologists. Some of the classifications are very complex and divide fishes into more than 100 orders and suborders. In the most generally used system, the subphylum Vertebrata is divided into two superclasses: Agnatha, which includes the lamprey and other fishes without jaws; and Gnathostomata, which includes the fishes with hinged jaws. The latter are further divided into the class Chondrichthyes, the cartilaginous fishes such as the sharks, rays, and chimaeras, and the class Osteichthyes, the bony fishes.

The bony fishes are made up of the subclass Sarcopterygii, lobe-finned fishes, and the subclass Actinopterygii, ray-finned (or spiny-finned) fishes. The recent ray fins consist of two groups, the Chrondrostei and the Neopterygii, which includes the large division Teleostei or modern bony fishes.