Why do sharks have to continuously move to breathe but fish can breathe just fine while sitting still?

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What else separates sharks from the normal fish? I know they aren’t in the same genus(?) / phylogeny (?)

In: Biology

Actually, only a small number of sharks (a couple dozen out of about 400 known species, though those dozens include some of the most famous ones, like the Great White, so popular knowledge projects this on to sharks in general) need to keep moving. These are called “obligate ram ventilators”.

The reason is basically that they lack cheek muscles (known as the “buccal muscles”, they lost them at some point in their evolution, as ram ventilation worked fine for them, as they were already in constant motion for other reasons), which prevents them from sucking in water, so they have to keep moving to push water into their mouths and through their gills to get enough oxygen.

Most sharks (and all fish) don’t need to keep moving, as they still have those muscles, so they can suck in water like you sucking up soda through a straw.

[This article](https://animals.howstuffworks.com/fish/sharks/shark-drown1.htm) has some more information.