Sharks are a type of fish. They respire with the use of five to seven gill slits. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue, but no bones. Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater, with a few exceptions. Sharks have diversified into 440 species, ranging in size from only 17 centimetres (7 in) in length, to approximately 12 metres (39 ft).