People often assume sharks need to “keep swimming or they die.” It’s not true for every shark - but for some, it really is. The reason comes down to how their gills work.
Most fish actively pump water through their gills using muscles, the way you breathe air. Some sharks have that ability too. But certain species - great whites, makos, whale sharks - rely on what’s called ram ventilation. They swim with their mouths open, and forward motion forces water through their gills. If they stop swimming, the water stops flowing, and they suffocate.
For these sharks, sleep looks very different from ours. They never fully stop. Half their brain rests while the other half keeps the body cruising, much like whales. They’re effectively in motion their entire lives - born swimming, dying still swimming.
