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The earliest known sharks date from more than 420 million years ago. The whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 meters (39 ft). Sharks are found in all seas and are common down to depths of 2,000 meters (6,600 ft). Most sharks need to constantly swim in order to breathe and cannot sleep very long without sinking (if at all). However, certain species, like the nurse shark, are capable of pumping water across their gills, allowing them to rest on the ocean bottom. Sharks may have up to 3,000 teeth at one time. Shark specialists estimate that 100 million sharks are killed for their fins, annually.One pound of dried shark fin can retail for $300 or more. It's a multi-billion dollar industry.