Eel - biology.

Eel - biology. Eel, elongated, serpentlike fish of the eel order, comprising nearly 600 diverse species. These species, including the conger eel and moray, are grouped into about 20 families. They inhabit shallow coastal waters throughout the world. Most eels have no scales and are protected by a layer of slippery mucus. Their dorsal and anal fins, which run from close to the head to the often nonexistent tail fin, provide much of the thrust for these lithe swimmers. Most species are less than 1 m (3 ft) long. One conger eel, however, is known to grow up to 3 m (9 ft) long and is found as deep as 250 m (820 ft) in the ocean. Freshwater eels return to the ocean to spawn. These eels, which are the most important as food and are often sold live in markets, have dense capillary systems close to the skin that can absorb oxygen directly from air or water. The eels hatch from eggs as leptocephali, which are transparent, very thin, leaflike larvae bearing little resemblance to the adults. They drift about the ocean surface for as long as three years, feeding on plankton, then metamorphose into round-bodied young eels called elvers, or grass eels, and feed on fish, crabs, and other invertebrates until they reach full size. The migration and reproduction of freshwater eels remained a mystery until the 20th century, when their spawning beds were discovered in the Sargasso Sea between Bermuda and Puerto Rico. When the very similar European eel and the American eel reach maturity in freshwater lakes and rivers, they take watercourses, sometimes slithering overland through dewy grass, to reach the ocean, where they swim or drift with currents for as long as a year until they reach the sluggish, weed-filled Sargasso. Here the eels spawn in deep water, and, before dying, the female produces as many as 20 million free-floating eggs. The leptocephali drift with the Gulf Stream, taking one year to reach North America and three years to reach Europe. By this time they have become elvers and accumulate at the mouths of rivers in great masses. The yellow elvers swim upstream and feed on lake- and river-bottom animals until they become black-and-silver-bodied adults, completing the cycle. Scientific classification: Eels make up the order Anguilliformes. Freshwater eels make up the family Anguillidae. The conger eel is classified as Conger conger, the European eel is classified as Anguilla anguilla, and the American eel is classified as Anguilla rostrata. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.