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More than three quarters of Earth's surface is covered by salt water; in addition, large areas are inundated with freshwater in the form of lakes, rivers, canals, swamps, and marshes. It is therefore not surprising that animals and plants have undergone a wide radiation in such habitats. One of the most successful groups of animals that have evolved to fill all of these habitats is the fish. Today it is possible to find different sorts of fish at all depths of the oceans and lakes-from the shoreline to the base of the deepest ocean trenches.

There are two types of fish on Earth: those that have a skeleton comprised of cartilage and those with a bony skeleton. The former include the sharks, dogfish, skates, and rays. The remainder, and by far the most abundant in terms of numbers and species, are known as the bony fishes. More than 25,000 species have been described. The majority of these are streamlined to reduce water resistance, with specialized fins that provide propulsion. Fins are basically of two types: vertical, or unpaired fins, and paired fins. The former include a dorsal fin in the midline of the back, an anal fin along the underside and a caudal fin at the rear end of the fish. The paired fins are known as pectoral and pelvic fins; they correspond to the limbs of terrestrial vertebrates.

In the majority of species, there is no neck, and all external appendages, with the exception of the fins, have been reduced. The body is covered with tiny, smooth scales that offer no resistance to the water. The form, size, and number of fins varies considerably according to the individual's habitat and requirements. In fast-swimming species such as tuna or mackerel, the dorsal and anal fins form sharp thin keels that offer little resistance to water flow. Departures from this body shape, however, are very common. Puffer or porcupine fish, for example, have short, round bodies with greatly reduced fins that are more effective in brief, sculling movements than rapid movement. Yet other species such as eels have lost almost all traces of external fins and swim instead by rhythmic movements of their muscular bodies.

In exploiting the aquatic and marine habitats, fish have evolved a number of unique features. One of these is the manner in which they breathe. The respiratory surface of fish forms special gills which are highly convoluted and well supplied with blood. Water is passed over the gills as the body moves through the water. As it does, the highly dissolved oxygen in the water meets the respiratory surface, diffuses across the membrane and into the blood where it is taken up by hemoglobin pigment in the blood cells.

Another important adaptation which has meant that fish have been able to thrive in the rich waters of the seas and rivers has been the development of the swim bladder- a special organ which has arisen from an outgrowth of the alimentary canal. This gas-filled chamber fulfills several functions, but one of the most important is in providing buoyancy, a feature that enables bony fish to remain at the same level in the water column without expending any energy. Sharks and rays do not possess a swim bladder.

In conquering the water environment, fish have developed a wide range of behavioral specializations that include feeding adaptations, courtship, and breeding behaviors, and defensive and attacking postures. Many of these are assisted or augmented through special sensory organs, most of which have evolved independently in many of these species. Altogether, they combine to provide the fishes at all stages of their lives with a wide range of specialized adaptations that enable them to live and reproduce so successfully on Earth.

See also Cartilaginous fish.

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