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Fish, cold-blooded aquatic vertebrate that breathes by means of gills. Typically, a fish's body is streamlined and covered by a layer of scales. Fish swim by means of fins, especially a vertical tail fin. All fish possess a 2-chambered heart. Fish are found wherever there is natural water, unless it is poisoned. Some fish, such as the African lungfish, spend some time out of water, breathing by means of a lunglike air bladder. Modern fish can be divided into 2 main groups: the cartilaginous fish (sharks, skates, and rays) and the bony fish. There are over 20,000 species of bony fish, ranging in size from the 1/2-in (1.3-cm) freshwater goby of the Philippines to the 20-ft (6-m) sturgeon of the former USSR. The largest of all fish, the 50-ft (15-m) whaleshark, is a cartilaginous fish. Most fish reproduce by shedding eggs into the water at the same time as the sperm and allowing fertilization to take place in the water. Perhaps 1 or 2 eggs in 10,000 grow to maturity. The female mouthbreeder, however, keeps the eggs and later the young in her mouth, and the discus, after carefully tending the eggs, secretes a special mucus in its skin to feed the young. These fishes do not need to lay thousands of eggs, as chances of survival improve with care. There must be, overall, a delicate balance between the number of eggs laid and the amount of care taken. Fish's eyes are adapted to underwater vision. Some, like the archer fish, which catches flying insects, can see well in air. The four-eyed fish can see out of the water and under water at the same time as it cruises just under the surface. The upper part of the eye focuses light in air and the lower half focuses in water. The sense of smell is developed variously, and some fish can taste as well as feel with barbels or other fleshy protuberances that trail in the water or along the bottom to detect food. Fish's ears are poorly developed, but a special sense organ, the lateral line system, enables fish to detect and identify many kinds of vibrations in the water. Some fish survive if blinded, but have great difficulty both in catching food and in escaping enemies without their lateral line system. Fish have bodies denser than water and would have to swim constantly to keep from sinking if it were not for the swimbladder, a sac that contains air or some gas secreted by the fish. The fish regulates the amount of gas by secreting more or by expelling some of it and thus keeps itself at its preferred depth. A few species of fish are able to generate electricity. Specially developed muscles build up a charge of electricity that is emitted to repel possible enemies. The South American electric eel is the most powerful, emitting up to 550 volts, enough to kill fish and even possibly a human.

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