How do animals that live in saltwater stay hydrated?
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In fish, salt water fish drink constantly and pee out nearly straight crystals, so they can replenish their fluids (which are getting sucked out of them by the ocean being saltier than they are) without getting too much salt in their bodies.
This is the opposite of fish who live in fresh water, who never drink and pee really dilute fluids with as little mineral content coming out of them as possible, to counter the water getting absorbed through their skin.
Makes for an easy visual way to tell which is which in a photo – if it lives in salty water the mouth is pretty well always open, while if it lives in fresh water it’s pretty well always closed. But it’s also why outside of a couple exceptions a fish will die almost immediately if you put it in the wrong type of water.
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