EEL

any of more than 500 species of fish of the order Anguilliformes. True eels are elongated or even wormlike bony fishes and include the common freshwater eels as well as the voracious marine morays. Only the freshwater eels (family Anguillidae), which are in places abundant and greatly valued as food, are of major economic importance. Regardless of their final habitat, all eels probably pass through an extended larval phase (leptocephalus) in the open ocean. They undergo metamorphosis to a juvenile stage that is a smaller version of the adult. At maturity, eels range from 10 centimetres (four inches) (in the deep sea Cyema atrum) to 3.5 metres (111/2 feet) (in the moray Thyrsoidea macrura). They occur to considerable depths in most oceans and are greatly diverse in tropical seas. They range in colour from drab gray or black (in deep sea species) to colourful and patterned (in tropical reef species). any of more than 500 species of fish of the order Anguilliformes. Eels are slender, elongated, usually scaleless fish with long dorsal and anal fins that are continuous around the tail tip. Some, the snipe eels (families Nemichthyidae, Cyemidae, and Serrivomeridae), have long jaws; and others, the snake eels (family Ophichthidae), have pointed tails that enable them to burrow backward into sand or mud. Young eels are transparent, leaflike larvae (leptocephali) totally unlike the adults. Eels are found in all seas, from coastal regions to the mid-depths. One group, the freshwater eels of the family Anguillidae, live in freshwater but return to the sea to breed. The freshwater eels are active, predacious fish with small, embedded scales. They comprise about 16 species and are found almost worldwide. They grow to maturity in fresh water and, as adults, make a single spawning migration to the sea (the Sargasso Sea in the case of the European and American species). After spawning, they die. The young hatch in the sea and drift to the coast, where they metamorphose into slender forms known as elvers and make their way upstream. Freshwater eels are valued food fish. Species include the American eel (Anguilla rostrata), which grows about one metre (40 inches) long; the European eel (A. anguilla), a very similar species reaching a maximum length of about 1.4 metres; and the Indo-Pacific A. marmorata, which grows about 1.8 metres long. There are about 19 families of true eels, including the moray and conger eel (qq.v.) groups, families Muraenidae and Congridae, respectively. The gulpers (q.v.) of the deep sea are included in this order by some authorities and separated as the order Saccopharyngiformes by others. Unrelated eel-like fish include spiny eels and the electric eel. Additional reading L. Bertin, Eels: A Biological Study (1957; originally published in French, 1942), a comprehensive, readable account of the biology of freshwater eels; J. Schmidt, The Breeding Places of the Eel, Rep. Smithson. Instn. (1924), pp. 279316 (1925), a summary by the original researcher of the classical biological study of eels; A.F. Bruun, The Breeding of the North Atlantic Freshwater-Eels, Adv. Mar. Biol., 1:137170 (1963), an important review of the controversy regarding the eel life cycle; L. Bertin and C. Arambourg, Anguilliformes, in P.P. Grass (ed.), Trait de Zoologie, 13:23142327 (1958), a classical text in French with a good coverage of eels; P.H.J. Castle, The World of Eels, Tuatara, 16:8597 (1968), a general article of college-level difficulty, and An Index and Bibliography of Eel Larvae, Spec. Publs. Inst. Ichthyol. Rhodes Univ., 7:1125 (1969), a comprehensive bibliographic index to leptocephali; F.W. Tesch, The Eel: Biology and Management of Anguillid Eels (1977).