Because fish have gills instead of lungs, they must get their oxygen by taking it out of the water as it passes over their gills. If you have a fish tank at home, you know that bony fish, or members of the superclass Osteichthyes, are able to remain still or rest on the bottom of the tank. A bony plate covering their gills, called an operculum, is what allows Osteichthyes to achieve this water movement without swimming.
In a process called buccal pumping, water is drawn in through the mouth, or buccal cavity, while the operculum is closed. Then, the mouth closes and operculum opens, creating a one-way flow of water over the gills, allowing the fish to extract oxygen from the water without body motion.
Members of the class Chondricthyes, including sharks, skates and rays, have skeletons made of cartilage, and therefore do not have a bony operculum. Instead, they have 5-7 exposed gill slits. This does not mean, however, that all Chondricthyes must swim constantly to move water over their gills. But, some do! Large, pelagic sharks such as the great white shark, mako, whale shark, and blue shark are ram ventilators. This means that they move water over their gills by swimming and “ramming” the water into their mouths and over their gills. Some sharks, such as the sand tiger shark, can switch between modified buccal pumping and ram ventilation.
Benthic Chondricthyes, such as a horn shark, round ray, or shovelnose guitarfish, live on the sea floor, with mouths located on the bottom of their heads to get food from the sand below them. Many of these animals also camouflage and avoid predation or sneak up on prey by burying themselves in the sand. In order to breathe without buccal pumping or ram ventilation, they use specialized holes behind their eyes called spiracles. Spiracles act like a straw or snorkel sticking out of the sand, drawing water over their gills and out the gill slits. This allows these animals to remain motionless and below the sand while still being able to get oxygen. If you look closely at a shovelnose guitarfish hiding in the sand on Catalina Island, you can actually see the spiracles opening and closing behind its eyes!