Sharks are the most fierce animal in the ocean.they appear in the earth about 300,000,000 years before dinosaurs.The skin of sharks are always indurate and slippery,and it's very sensitive to the other objects in the ocean.Marine animals such as fishes,shrimps and crabs are elementary food for sharks.Even though,some kind of sharks still attack human beings,e.g.great white sharks.During these recent years,the total number of sharks has Significantly reduced,owning to their expensive fins.