Why do most sharks need to swim constantly?
Sharks lack a swim bladder, the organ found in most other fishes to help them keep afloat. If a shark stops swimming, it will start to sink to the bottom. Once a shark has landed on the bottom, it is no longer pumping water through its gills. Without this water, the shark cannot get the oxygen it needs for respiration. Interestingly, a shark gets much of its buoyancy from its liver. There are some sharks, however, that do not need to keep swimming. The nurse shark is an example. It can actively pump water through its gills so it can rest at the bottom of the water and still breathe.
