The oldest sharks, the modern sharks' ancestors, didn't have to constantly swim to breathe. Rather, they all pumped water through their mouth and over their gills. This method is known as buccal pumping, named for the buccal, or cheek, muscles that pull the water into the mouth and over the gills. Many sharks retain this method today, such as nurse sharks, angel sharks and carpet sharks, also known as wobbegongs. Skates and rays, the shark's cousins, also breathe this way. These species tend to spend most of their time lying on the bottom of the ocean floor.
In addition to an inactive lifestyle, there are some additional bodily differences that allow these sharks to breathe by buccal pumping. For example, many of the sharks that practice this method are dorsoventrally flattened (or squashed along the length of its back), like the angel shark. They have stronger muscles in the face. These sharks might also have a more prominent spiracle, which is a tube behind the eyes. When a shark is buried at the bottom of the ocean floor and can't breathe through its mouth, the spiracle acts like a mouth by pulling in water. The water then exits through the gill slits.
As sharks evolved and became more active, however, this method of pumping became secondary. It was simply more energy efficient to take in water while swimming, in effect "ramming" the water into the mouth and letting it flow out through the gills slits. This method of breathing is known as ram ventilation. Most sharks can alternate between buccal pumping and ram ventilation, depending on what they're doing. When they start swimming fast enough to force the water in more quickly than they could pump it, then they stop pumping. The sand tiger shark is an example of a shark that switches back and forth.
Some sharks, however, have completely lost the ability to breathe by buccal pumping, and these are the sharks that will indeed drown if they stop swimming and ramming water. These sharks are known as obligate ram breathers (or obligate ram ventilators); only about two dozen of the 400 identified shark species are required to maintain this forward swimming motion [source: Bennetta]. These include the great white shark, the mako shark, the salmon shark and the whale shark.
Do these obligate ram breathers ever get a break? Aren't they tired? Read on to see if they can catch a nap and what can sink them on the next page.