True Eels
True Eels
The true eels are elongate bony fish with a snakelike slimy body in the order Anguilliformes. There is one family of freshwater eels (the Anguillidae), and 14 families of exclusively marine eels. The freshwater eels must return to the oceans to spawn.
The general characteristics of eels include soft-rayed fins and elongate dorsal and anal fins which merge with the caudal fin. Eels lack pelvic fins and have small pectoral fins commonly situated immediately behind the head. The jaws of eels are relatively small, but are strong, with numerous small teeth. Most ocean-living eels do not have scales, although the freshwater eels have small, oval-shaped scales embedded in their skin.
All eels are predators, feeding on a wide range of prey, including small fish, crustaceans, mollusks, and worms.
Freshwater eels
The freshwater eels, comprising about 15 species, are the most familiar family of eels to most people. These fish have an unusual characteristic in their life history, known as catadromy, in which the fish spend most of their lives in fresh waters, but run to the ocean to spawn.
The common freshwater eel of North America is the American eel (Anguilla rostrata). The European eel (A. anguilla) of western Europe and the Japanese eel (Anguilla japonica) of the north Pacific coast are closely related to the American eel. Relatively large numbers of eel species, about 12, occur in the Indo-Pacific region.
The American eel can reach a length of about 4 ft (1.2 m). This species is abundant in fresh waters that link directly to the ocean. This range includes coastal rivers, and the larger drainage of the Saint Lawrence River, extending inland to Lake Ontario and to a lesser degree, the upper Great Lakes. There are no freshwater eels on the west coast of the America, or on the east coast of South America.
American eels spends almost all of their life in fresh waters. However, to breed, this species of eel runs to the sea and migrates to a warm-water region between the West Indies and Bermuda that is known as the Sargasso Sea. European eels also commonly spawn in the Sargasso Sea, following a migration of 4,000 or more miles (6,440 km). After spawning, the adult eels die. The baby eels, or elvers, are leaf-shaped and transparent, and migrate to fresh waters, where they transform into miniature but transparent replicas of the adult body form, sometimes known as ‘glass eels.’ These small fish then run up rivers, and take up residence in still waters of large rivers and lakes, where they live as adults for as long as 15 years.
This unusual breeding strategy was discovered only relatively recently. For many centuries, naturalists pondered the fact that they could never find spawning or larval eels, and the breeding habits of these fish were a mystery. It was not until 1922 that newly hatched eel larvae were observed, in the Sargasso Sea.
Freshwater eels are an economically important species of fish, and are particularly appreciated as a food in France, Belgium, the Netherlands, and Great Britain.
Other families of eels
The Moringuidae is a family of six species of marine eels found in tropical waters, which includes the genera Moringua and Stilbiscus. These fish typically occur in shallow, soft-bottomed or gravelly habitats, where they burrow in the substratum during the day. The eel Stilbiscus edwardsi is a common species of the western Atlantic, while Moringua macrochir occurs in coastal waters of the Hawaiian Islands.
The moray eels (Muraenidae) include about 200 species that live in shallow, tropical and subtropical waters. These impressive, snake like fish have a large mouth, well-armed with teeth and poison fangs. Morays are often brilliantly colored and marked, and are very attractive fish. Morays generally occur in rocky or coral habitats, where they hide during the day in crevices or in burrows in sediment. Many unsuspecting divers have received a nasty surprise in the form of a painful moray bite, when reaching into a rocky crevice after marine animals or other interesting things.
The zebra moray (Echidna zebra) is an attractive species with a brown-yellowish body and white stripes. The zebra moray occurs in the Indian and Pacific Oceans, and is sometimes seen in aquaria. The largest moray, and the largest species of living eel, is Thyrsoidea macrurus of coastal regions of the Indian and Pacific Oceans. This species can achieve a length greater than 9 ft (2.7 m). Morays are often caught as food fish, especially in Asiatic waters and, to a lesser degree, in the Mediterranean region.
The conger eels (Congeridae) also occur in tropical and subtropical waters. The best known species is the conger eel (Conger conger), with an almost worldwide distribution in suitable habitats (except for the eastern Pacific), and achieving a length of almost 9 ft (2.7 m). Some species occur in relatively deep waters, for example, Ariosoma balearica and Promyllantor latedorsalis, off the Azores of the tropical Atlantic Ocean.
KEY TERMS
Catadromous —Refers to fish that spawn at sea, but spend most of their life in fresh waters.
See also Spiny eels.
Resources
BOOKS
Aida, K., K. Tsukamoto, and K. Yamauchi. Eel Biology.
New York: Springer, 2004.
Nelson, Joseph S. Fishes of the World. 4th ed. New York: John Wiley & Sons, 2006.
Whiteman, Kate. World Encyclopedia of Fish and Shellfish.
New York: Lorenz Books, 2000.
Bill Freedman