It is a fish with an elongated body that usually measures around 70 cm, although it can measure 2 meters. Females are usually larger than males. Their skin is covered with a mucous secretion that makes them very slippery, giving them their best-known characteristic, as well as protecting them, since they have microscopic scales. Males normally live 8 years and females 12.
Behind the head they have two small fins. The back is greenish brown, almost black; the belly is white and yellowish.