How do animals that live in saltwater stay hydrated?
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My understanding is that most of the water required by their bodies is derived from the foods they eat.
One very interesting example is that there are some sea snakes that drink the rain water that floats on top of the surface of the ocean after a shower before it’s able to mix into the salt water.
Some aquatic reptiles and birds have salt glands that enable them to excrete excess salt out, sort of like tear ducts.
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.
Their insides are just ss salty as the water outside. We dehydrate because saltwater in our gut causes water to go from our cells into our gut, to balance the saltiness. Drinking saltwater gives us diarrhea that way, but a saltwater animal has saltier cells that are balanced with the salt content of the water. Putting them in freshwater does the opposite – too much water goes into their cells and they burst.
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