True eels span thumb-length fish to four-meter giants
True eels, the order Anguilliformes, number about 1,000 species of ray-finned fish in 164 genera, and nearly all hunt other animals. They range from the 5-centimetre one-jawed eel to the 4-metre slender giant moray, and each begins life as a flat, see-through larva.
Eels have no pelvic fins, and many lack pectoral fins as well. Their dorsal, anal and tail fins merge into a single ribbon along the body, and they swim by sending waves down their length, reversing the wave to back up. Adults weigh anywhere from 30 grams to well over 25 kilograms. The heaviest is the European conger, reported at up to 3 metres and 110 kilograms, while the longer slender giant moray is much lighter.
Most are marine and nocturnal, burrowing into sand, mud or rock in shallow seas, sometimes crowding together in eel pits; others live on continental shelves and slopes as deep as 4,000 metres. Only Anguilla eels regularly live in fresh water, and they too return to the sea to breed. Larvae called leptocephali drift near the surface eating marine snow, then turn into glass eels and elvers. Many elvers swim upriver and must climb weirs, dams and waterfalls; Gertrude Elizabeth Blood found that loosely plaited grass ladders hung over barriers greatly improved the eel fishery at Ballisodare.
The name is also borrowed loosely. Electric eels, swamp eels, slime eels and deep-sea spiny eels are not true eels, and all but the spiny eels, a sister group, evolved their snaky shape independently. Classification schemes differ too: FishBase lists 20 families, while ITIS adds more, from spaghetti eels and gulper eels to garden eels.
The earliest fossil eels come from Late Cretaceous rocks in Lebanon and still had pelvic fins. Ear-stone fossils show modern families were diversifying before the extinction that ended the Cretaceous, and one mud-dwelling species appears to have flourished just after it. Genomic studies point to a single lineage that arose among deep-sea eels.
Source: Eel
