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Sharks are a family of fish native to most of the world's oceans and seas. They are a salt water fish, although some can be found in freshwater rivers. Sharks vary in size from the dwarf shark at 7 inches to the whale shark at 40 feet in length. The sharks most recognizable feature is its dorsal fin that can be seen sticking out of the water when the shark swims near the surface. The popular movie "jaws" made this dorsal fin a very feared sight by humans swimming in the water. Unlike other fish, the shark doesn't have a swim blatter to keep it a float. The shark must keep swimming or it will sink to the water's bottom. Play the following videos to learn more about the shark.