Will Sharks Die If They Stop Moving?

immense liftoff or propulsive downward thrust. Obviously, a shark has to swim to catch its prey, meet its mates, and avoid its predators. But does a shark have to swim just to stay alive. You may have heard that a shark will drown if it stops moving, an idea that's been sited everywhere from biology textbooks to Ripley's Believe it

or not. This theory came about by comparing sharks, which are classified as cartilaginous fish, to bony fish, which have many more muscles around their breathing apparatus is the gills. To understand why this distinction is important, let's take a look at how sharks and other fish breathe. To breathe, sharks must remove oxygen from the water around them. The water enters the shark's mouth, its nose being used exclusively for smell, and then the water flows over the gills.

Inside those gills, there are hundreds of feathery gill filaments. Each filament, in turn has thousands of leaf like lamellie or flaps, which contain blood vessels. The blood absorbs the oxygen from the incoming water, and the excess water flows back out of the shark's body through gill slits. Sharks have five to seven pairs of gill slits, depending on the species. Using this method, sharks can extract about eighty per cent of oxygen out of the meager one percent

of free oxygen that's present in the water. To compare, the air that we breathe is made up of about available oxygen, but human lungs only extract about of that anyway, to maintain a steady flow, the shark constantly needs to be taking in water. But does it have to constantly swim to take in this water? A scientist's thought so, because other fish seemed to have the equipment to actively pump the water through their mouth and over their gill slits,

whereas sharks looked less developed. But that doesn't account for sharks that just kind of chill, like angel sharks and nurse sharks. It turns out that not all sharks have to stay moving to breathe. The oldest sharks, the ancestors of more modern sharks, didn't have to constantly swim in order to breathe. Rather, they pumped water through their mouth and over their gills. This method is known as buckle pumping, named for the buckle or cheek muscles that pull the

water into the mouth and over the gills. Many sharks retain this method today, such as the aforementioned nurse and angel sharks, as well as carpet sharks, eskates, and rays. The sharks cousins also breathe this way. These species tend to spend most of their time laying on the bottom of the ocean floor. Many of these sharks are dorso ventrally flattened, that is, sort of squashed looking along the length of their Backlike the angel shark, they have stronger

muscles in the face. These sharks might also have a more prominent spircle, 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 spircle can pull in water instead. As sharks evolved and became more active, however, buckle 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 through the gill slits. This method of breathing is known as a ram ventilation. Most sharks can alternate between buckle 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 buckle pumping, and these are the sharks that will indeed drown if they stop swimming and thus ramming water. These sharks are known as obligate ram breathers or obligate ram ventilators, and only about two dozen of the four hundred identified shark species are required to maintain this forward swimming motion. These include the great white shark, the mako shark, the salmon shark,

and the whale shark. So do these obligate ram breathers ever get a break? Aren't they tired? As it turns out, it's more work for these sharks to remain still than it is to swim. In a study of lemon shark switch switch between breathing methods, juveniles breathed six percent more efficiently when moving than when resting, even when resting so that the current allowed the water to flow directly into their mouths. That's not to say that 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 the brain, so sharks may be able to shut down their brain and rest while still swimming.

Sharks in needisome rest may also take advantage of the factors that affect the amount of oxygen in the water, such as salinity, temperature, and even the time of day. In the nineteen seventies, scientists investigated what came to be known as the Caves of the Sleeping Sharks in Mexico. Inside the caves were motionless reef sharks, which are normally obligate 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, and many other reef sharks have also been observed motionless at the bottom even outside of caves. Scientists still aren't exactly sure how they can do this. Today's episode is based on the article will a shark drown if it stops moving?

On how stuffworks dot Com, written by Molly Edmonds. Brain Stuff is production of I Heart Radio in partnership with how stuff works dot Com and is produced by Tyler Klang. Four more podcasts my heart Radio, visit the i heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.