Predators in perpetual motion. Sleepless in our seas. If that's your image of sharks, you're not alone. And for good reason: sharks must swim to breathe (or so we were told). The science of how sharks sleep and breathe is linked, and while all sharks use gills to breathe, there are two ways that they move oxygen-rich sea water over those gills. Some sharks, called obligate ram ventilators, 'ram' oxygen-rich sea water over their gills and need to keep moving to do so. Other species, called buccal pumpers, actively pump sea water over their gills while stationary.
