It’s a widespread belief: If sharks stop swimming, they die. But is that actually true?
Let’s find out.
If you look at the nurse shark and the tiger shark, this belief is already proven false: these, and a few other shark species, can stop swimming whenever they want. They breathe by way of buccal pumping, actively “inhaling” water by using cheek muscles to draw it into the mouth and over the gills. This allows sharks to stop moving but continue breathing. They can rest on the ocean floor without worry and can even partially bury themselves in the sand, using respiratory openings behind the eyes, called spiracles, to pull water through their gills when their mouths are covered.
But some shark species don’t have the luxury of buccal pumping. For example, the great white shark, the whale shark, and the mako shark don’t have buccal muscles at all. Instead, these sharks rely on obligate ram ventilation, a way of breathing that requires sharks to swim with their mouths open. The faster they swim, the more water is pushed through their gills. If they stop swimming, they stop receiving oxygen. They move or die.
Other shark species, such as the reef shark, breathe using a combination of buccal pumping and obligate ram ventilation. When swimming slowly, they can use buccal pumping to supplement the amount of oxygen received from ventilation. And if they choose to stop moving for a few minutes, they won’t risk their life, though they generally aren’t as adept at stillness as sharks that breathe by buccal pumping alone.
Of these three ways that sharks breathe, the combination of buccal pumping and obligate ram ventilation is by far the most common. Most kinds of sharks, then, won’t die if they stop swimming.
So why do many people think they will?
Sharks are often compared to bony fish, a class of fishes that breathe through obligate ram ventilation. Since all bony fish are constant swimmers, many people assume that all sharks are too. Even people who don’t make that connection may ask, “Have you ever seen a sleeping fish?”
Before we go, here’s another myth to be busted: all fish other than sharks die if they stop swimming. In reality, breathing in nonshark fish species is just as varied as it is in shark species. Some fish sleep on the ocean floor just as some sharks do. Maybe people who think otherwise simply haven’t visited those fish at bedtime.