True eels (Anguilliformes) are an order of fish, which consists of four suborders, 19 families, 110 genera and approximately 600 species. Most eels are predators. The term "eel" is also used for some other similarly shaped fish, such as electric eels and spiny eels, but these are not members of the Anguilliformes order.
True eels are elongated fishes, ranging in length from 5 centimetres (2.0 in) in the one-jawed eel (Monognathus ahlstromi) to 3.75 metres (12.3 ft) in the giant moray. They possess no pelvic fins, and many species also lack pectoral fins. The dorsal and anal fins are fused with the caudal or tail fin, to form a single ribbon running along much of the length of the animal. Most true eels prefer to dwell in shallow waters or hide at the bottom layer of the ocean, sometimes in holes. These holes are called eel pits. Only the Anguillidae family regularly lives in fresh water, and returning to the sea to breed. Some eels dwell in water as deep as 4,000 metres (13,000 ft), or are active swimmers (the family Nemichthyidae — to a depth of 500 metres (1,600 ft).
Eels possess a flat and transparent larva, called a leptocephalus. These drift in the surface waters of the sea feeding on dissolved nutrients, before developing first into glass eels and then into a young eel, referred to as an elver, and seeking out the adult habitat.