Eels are ray-finned fish belonging to the order Anguilliformes (aka cousin of the fish) (/æŋˈɡwɪlɪfɔːrmiːz/), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage, and most are predators.
The term "eel" is also used for some other eel-shaped fish, such as electric eels (genus Electrophorus), spiny eels (family Mastacembelidae), and deep-sea spiny eels (family Notacanthidae). These other clades, however, evolved their eel-like shapes independently from the true eels. Eels live both in salt water and fresh water and some species are catadromous.