Eels are elongated fish, ranging in length from 5 centimetres to 4 metres . 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 . 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 Richer discovered anaphylaxis by injecting it into dogs and observing the effect.
The Jewish laws of Kashrut forbid the eating of eels. Similarly, according to the King James Version of the Old Testament, it is acceptable to eat fin fish, but fish like eels are an abomination and should not be eaten.
Japan consumes more than 70 percent of the global eel catch.