Eel

No. Despite the name and the long body, the electric eel of South America is a knifefish, more closely related to catfish and carp than to true eels. It ended up eel-shaped by convergent evolution, because a long, flexible body suits a fish that lives in tangled, oxygen-poor swamp water and needs to surface regularly to breathe air. True eels belong to the order Anguilliformes and are defined by the transparent leptocephalus larva and the fused, continuous fin. The wolf eel, the spiny eels, the swamp eels, and several others are also eel-shaped fish from unrelated groups, so eel is a body form that has evolved many times as well as the name of one specific order.

The freshwater eels have a life cycle called catadromy, the reverse of a salmon's. An eel spends most of its life, often ten to thirty years or more, growing in fresh water, moving upstream as a young glass eel and elver, sometimes wriggling overland through wet grass to reach isolated ponds. When it is ready to breed, usually as an animal called a silver eel, it stops feeding, its body fattens, its eyes enlarge and its skin darkens, and it heads downriver to the sea. It then swims thousands of kilometres to the ancestral spawning area, the Sargasso Sea for Atlantic eels, spawns once, and dies. The tiny larvae drift back on the currents, taking months to years to reach the coast, where they transform into glass eels and enter the rivers.

Its numbers have collapsed. The count of young glass eels arriving on European coasts each year has fallen by more than 90 per cent since the 1980s, and by even more against the 1970s. The causes stack up across the eel's enormous range and long life. Dams, weirs, pumping stations, and hydropower turbines block or kill eels moving up and down rivers. Wetlands where eels grow have been drained. A parasitic swimbladder worm introduced from Asia weakens migrating adults. Pollutants build up in the eel's fatty body and may impair the once-in-a-lifetime ocean swim. Changes in ocean currents may be carrying larvae off course. And there is a large, partly illegal trade in glass eels, netted in Europe and flown to farms in Asia. No single fix exists, and recovery, if it comes, will be slow.

Freshwater eels are shy and not aggressive, and although a large one can give a firm nip if grabbed or handled roughly, they pose no real danger to people. Moray eels are a different matter around reefs. A moray will not chase or hunt a diver, but it defends its crevice, and it has poor eyesight and a strong sense of smell, so it may bite a hand that reaches into its hole, mistaking fingers for food, and its backward-hooked teeth make such bites deep and prone to infection. The sensible rule when diving is to keep hands out of holes and not to feed morays. Conger eels, which grow very large, can also bite hard if provoked.