Why Sharks Have to Keep Swimming

Some sharks would struggle to breathe if they stopped swimming. Others take naps on the sea floor. What’s going on?

To understand this one, you have to know how a shark breathes. Water has to pass over its gills for it to get oxygen — and a shark can get that water moving in two different ways.

Method one: swim forward and let the water rush in. This is called ram ventilation, and it’s a bit like breathing by running. Method two: pump water over the gills on purpose by opening and closing the mouth. This is called buccal pumping, and it’s more like breathing deliberately.

Some sharks lean hard on method one. Great whites, makos, whale sharks and basking sharks are the famous “keep moving” sharks — they mostly rely on swimming to push water across their gills, so stopping isn’t really an option.

Other sharks are perfectly happy to sit still. Nurse sharks, wobbegongs and many bottom-dwelling species pump water the whole time, so they can rest on the sea floor, and some of them do it for hours. Our own Dozy considers this the single most important shark fact ever discovered.

The famous rule isn’t quite right

You’ve probably heard “if a shark stops swimming, it dies.” It’s true for some sharks, but it’s too neat for all of them. Scientists have found that at least a few “must-keep-moving” sharks can pump water a little, too, and the line between the two styles isn’t as sharp as the slogan suggests. Nature rarely files itself under one tidy rule.

There’s a cost, as well. Breathing by swimming means a shark is working while it breathes, which is one reason big, active sharks need to eat so much.

Did you know? A resting nurse shark can sit on the bottom and pump water over its gills without moving a fin. It looks like the laziest animal in the ocean, and it’s really just breathing.