Eel

Eels are elongated fish, ranging in length from five centimetres to four metres (13 ft). Adults range in weight from 30 grams to over 25 kilograms. 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, forming a single ribbon running along much of the length of the animal. Most eels live in the shallow waters of the ocean and burrow into sand, mud, or amongst rocks. A majority of eel species are nocturnal and thus are rarely seen. Sometimes, they are seen living together in holes, or "eel pits". Some species of eels live in deeper water on the continental shelves and over the slopes deep as 4,000 metres (13,000 ft). Only members of the family Anguillidae regularly inhabit fresh water, but they too return to the sea to breed. Eel blood is poisonous to humans and other mammals, but both cooking and the digestive process destroy the toxic protein. The toxin derived from eel blood serum was used by Charles Richet in his Nobel Prize-winning research, in which Richet discovered anaphylaxis by injecting it into dogs and observing the effect.

Source: Wikipedia