Great white sharks move by swimming. These powerful predators attain top speeds of over 15 MPH, but are only able to sustain such speeds for brief bursts. Great whites swim constantly because they rely on a breathing technique called ram ventilation. They swim with their mouths slightly open to force water through their jaws and over their gills.
Great white sharks cannot swim backward because doing so does not force water through their mouths and gills. They also suffocate within minutes if towed backward by research vessels or during aquarium transport. Mako sharks and whale sharks also suffocate under these conditions.
Great white sharks have semi-rigid fins that prevent them from stopping abruptly. Sharks' fins also make it physically impossible for them to propel themselves in reverse. Sharks that must move backward to escape shipwrecks or other undersea structures must rely on gravity to help them fall or slide out. To accomplish this, the shark becomes completely still, sinking down and out of the tight spot. This works because great whites are not neutrally buoyant, and they rely on swimming to stay above the ocean floor. As soon as a trapped shark falls to freedom, it resumes swimming and recovers from the oxygen deficit. Sharks unable to free themselves in this manner drown.