common name for any fish in the order Anguilliformes, and characterized by a long snakelike body covered with minute scales embedded in the skin. Eels lack the hind pair of fins, adapting them for wriggling in the mud and through the crevices of reefs and rocky shores. Most species are marine; the largest and most diverse group is the family Ophichthidae, the snake eels. Other large families are the conger eels, family Congridae, and the moray eels, family Muraenidae. Sharp-toothed and vicious, moray eels have a highly developed second set of jaws (pharyngeal jaws) that hold and pull prey into the throat after the main jaws snare it. The only freshwater eels are those of the family Anguillidae. The freshwater European eel, Anguilla anguilla, is found in the Atlantic coastal regions of Europe and the Mediterranean area; A. rostrata, the American eel, in North America E of the Rockies. Several other freshwater species are native to Asia, Australia, New Zealand, and the Pacific.
The mature European eel migrates 3,000 to 4,000 mi (4,828–6,437 km) to its spawning ground in the deep sea SW of Bermuda, a journey lasting several months; they use ocean currents to help them swim there, where they reproduce and then die. The young hatch as transparent ribbonlike larvae (called glass eels) that drift north and east on ocean currents for three years before entering a river; they then develop into elvers, tiny versions of the adult eel. The American eel follows the same pattern, except that the young require only one year to reach freshwater.
Once in freshwater, the developing elvers feed voraciously on dead and living animals, even traveling over short stretches of land in search of frogs and lizards. They hunt at night and rest by day. The male, which attains a length of 2 ft (61 cm), remains at the river's mouth, while the female (4 ft/122 cm) swims upstream, staying there from 5 to 20 years. When the eels are sexually mature their enormous appetite wanes, and they do not eat during migration to the spawning ground. Their oily flesh is regarded by some as a delicacy; the skin was formerly used as leather.
Eels are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Anguilliformes.