Curiosities: Why do sharks have to swim constantly?

For two reasons, says James Kitchell, professor of zoology at UW–Madison. First, sharks lack the swim bladder that most fish use to adjust their buoyancy. Swimming creates lift that prevents sharks from sinking, using much the same principle that a wing uses to lift an airplane.

Second, and more important, sharks, like other marine animals, get their oxygen from the water. Bony fishes use muscles to pump water through their gills, where oxygen is extracted. Sharks, however, use a ramjet ventilation system that requires them to swim to force water to the gills. 

However, nurse sharks do have mechanisms to move water through their gills, and thus do not need to swim constantly to “breathe,” Kitchell says.