Eels are elongated fishes, ranging in length from 5 centimetres (2.0 in) in the one-jawed eel (Monognathus ahlstromi) to 4 metres (13 ft) in the slender giant moray. Adults range in weight from 30 grams to well 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, among rocks, or in cracks found in coral reefs. 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 also live in deeper water on the continental shelves and over the slopes deep as 4,000 metres (13,000 ft). Only members of the Anguillidae family regularly inhabit fresh water, but they too return to the sea to breed.