Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Under this broader definition, the earliest known sharks date back to more than 420 million years ago. Since then, sharks have diversified into over 500 species. They range in size from the small dwarf lanternshark, a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark, the largest fish in the world, which reaches approximately 12 metres (40 ft) in length. Well-known species such as the tiger shark, blue shark, great white shark, mako shark, thresher shark, and hammerhead shark are apex predators—organisms at the top of their underwater food chain. Sharks are found in all seas and are common to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can be found in both seawater and freshwater. Sharks are a common seafood in many places, including Japan and Australia. In the Australian state of Victoria, shark is the most commonly used fish in fish and chips. Shark fin soup is a status symbol in Asian countries and is erroneously considered healthy and full of nutrients. The rise in demand for shark products increases pressure on fisheries. Major declines in shark stocks have been recorded—some species have been depleted by over 90% over the past 20–30 years with population declines of 70% not unusual. A study by the International Union for Conservation of Nature suggests that one quarter of all known species of sharks and rays are threatened by extinction and 25 species were classified as critically endangered. -Copy and pasted from Wikipedia Courtesy of The Bellisseria Oceanographic Institute