Even in oxygen-rich water, a net will impair a shark's movement so it can't breathe.

Brian Skerry/National Geographic/ Getty Images

Shark Sleeping

It might seem tiring to us humans to think about any sort of perpetual movement as a way to survive; we all like crashing on the couch every now and then. But as it turns out, it's more work for these sharks to remain still than it is to swim. In a study of lemon sharks, which switch between breathing methods, juveniles breathed 6 percent more efficiently when moving than when resting, even when resting so that the current allowed the water to flow directly into their mouths [source: Morrissey and Gruber].

This fact may help to explain what happens when sharks face the danger of hypoxia, or a deficiency of available oxygen. Sharks that breathe by buccal pumping increase the force of the pumping to try to bring in more oxygen while reducing their other activity to conserve energy. Obligate ram breathers, however, increase their energy, swimming faster and opening their mouth wider [source: Carlson and Parsons]. It may seem counterintuitive to speed up when faced with less oxygen, but that just may be more energy efficient for these sharks.

That's not to say these sharks don't catch a break every now and then. For obvious reasons, it can be hard to keep track of a shark that's constantly swimming, so it's difficult for scientists to know how or when they rest. An experiment with a small shark, the spiny dogfish, indicated that swimming is coordinated by the spinal cord, not by the brain, so sharks may be able to shut down their brain and rest while still swimming [source: Martin].

Sharks in need of some rest may also take advantage of the factors that affect the amount of oxygen in the water, such as salinity, temperature and even time of day. In the 1970s, scientists investigated what came to be known as the Caves of the Sleeping Sharks in Isla Mujeres, Mexico. Inside the caves were motionless reef sharks, which are normally obligate ram ventilators. The scientists determined that the water in the caves had an extremely high amount of oxygen and reduced salinity. These conditions likely made it easier for even these sharks to breathe without moving.

They may not have been asleep like humans (their eyes were open, for one thing), but it does appear that sharks can get some rest. Many other reef sharks also have been observed motionless at the bottom, even outside of caves [source: Martin]. Scientists still aren't exactly sure how they can do this.

There are some unfortunate incidences where the shark's coping mechanisms just won't work, however, and they're usually caused by man. Illegal finning, which occurs when a shark's fin is cut off and the shark is thrown back to sea, sometimes still alive, usually results in the shark's eventual drowning. Getting caught in fishing nets can cause death. Sometimes just being transported to an aquarium spells doom for obligate ram ventilators.

To learn more about sharks and how they work, take a look at the links on the next page.