Sharks play an essential rol in the health of our oceans


Do Sharks Sleep?

Resting whitetips a Roca Partida, picture by Guy Stevens

Humans love to crash on the coach every day, but sharks don’t. Some sharks, like the pelagic species, just have to keep swimming. For them, swimming is also easier: staying in one place would require move effort than moving! Like other fish, sharks extract oxygen from seawater as it passes over their gills.

And they even speed up, when they want to get in more oxygen. This may seem weird, but it could be more energy efficient for these sharks, they have lost the ability to pump water through their gills and must swim without rest, they are ‘obligate ram ventilators’: while the shark is moving, water passes through the mouth and over the gills. 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.

The spiracle behind the eye of a leopard shark / picture by Mark Strickland

Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. Bottom-dwelling sharks have a modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks (these spiracles are reduced or missing in the ‘ram ventilators’). While at rest, most of these sharks pump water over their gills to ensure a constant supply of oxygenated water. During the day, species like whitetip reef sharks and nurse sharks spend much of their time resting on the sea floor.

But some sharks that rely on ram ventilation can take a break, and can also take advantage of the factors that affect the amount of oxygen in the water, such as salinity, temperature and even time of day. Sleeping sharks can be found in Isla Mujeres, in Mexico. Scientists discovered that the oxygen content in the water of the caves was very high, while salinity was lower. These conditions allow reef sharks to breathe without moving.

‘Sleeping’ Caribbean reef shark in a Cancun Cave / picture credit unknown



When caught in a net or on a line, the shark’s coping mechanisms just won’t work. This can cause death, and sometimes being transported to an aquarium spells doom for obligate ram ventilators.

A pelagic shark in a net will drown… / picture credit: unknown

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