June 19, 2007 6:14pm CST
Some say a shark never sleeps. It is unclear how sharks sleep. Some sharks can lie on the bottom while actively pumping water over their gills, but their eyes remain open and actively follow divers. When a shark is resting, they do not use their nares, but rather their spiracles. If a shark tried to use their nares while resting on the ocean floor, they would be sucking up sand rather than water. Many scientists believe this is one of the reasons sharks have spiracles. The spiny dogfish's spinal cord, rather than its brain, coordinates swimming, so it is possible for a spiny dogfish to continue to swim while sleeping. It is also possible that a shark can sleep with only parts of its brain in a manner similar to dolphins.