With well over 24,000 extant species currently known, the world's fishes comprise by far the largest and most diverse of all vertebrate groups. Occupying almost every conceivable aquatic habitat from high elevation mountain springs more than 5,000 meters above, to the ocean abyss some 7,000 meters below the surface of the sea, ichthyological variety in lifestyle, anatomy, physiology and behavior is unsurpassed among vertebrates.
A diorama of a coral reef, from the American Museum of Natural History's Hall of Ocean Life. ©AMNH Fishes live in water, the original medium for life on Earth. Permanent gills, median fins supported by rays of cartilage or bone, and often paired fins, are all characteristics of fishes that allow them to live successfully in water. No fishes have limbs with digits. The diversity of forms among fishes provide evolutionary biologists with some of the best examples of natural selection, adaptation, divergence, speciation, and historical development of fauna on scales from regional to continental. Far from being the "dead-end" that we land-dwelling creatures tend to assume, fishes are extraordinarily diverse and their watery habitats provide a vast array of places in which to live and thrive. Fish species range in size from the smallest known living vertebrate, Trimmatom nanus, a goby, which is mature at a mere 8 millimeters, to the giant whale shark, Rhincodon typus, which can grow as large as 12 meters. There are species of fish living at 5,200 meters above sea level in Tibetan hot springs and fishes that live in a depth of eight thousand meters below the ocean surface — this is an incredible span, over 13 kilometers of vertical distance. No other vertebrate group occupies such a wide band of habitable space. Marine habitats include the deep sea, the mid-oceans, shrimp burrows, coral heads and sponges, and even the insides of sea cucumbers. Freshwater habitats include streams, rivers, lakes, even 500-meter-deep underground caves, and...
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