Do fish drink water?

"Do fish drink water? How do they drink ANYTHING at all??" - Anonymous

We, as humans, can't imagine a life without water... because we'd be dead for the most part. Every land mammal we know needs water to live - so what about fish? 

Funnily enough, despite living in water, some fish need to drink! To put it bluntly, saltwater fish need to drink, while freshwater don't, and there's one reason for this: Osmoregulation,  the balance of the water/salt concentrations.

The gills of the fish allow an exchange between the water surrounding the fish and it's capillaries, with the difference of salt causing the interchange. Therefore, because freshwater fish are saltier than the water around them, they don't need to drink - they do inevitably when they eat, but they don't physically need to. 

Freshwater fish have specific cells in their gills that produce enzymes to help maintain and regulate the flow of salts in and out of the body.

Sharks and rays pass their urea (a nitrogen-containing substance that's cleaned out of the blood by the kidneys and passed into urine) back into their bloodstream, and therefore raise their salt concentration to match with the waters in their environment.
Fish are always trying to find waters with the same amount of salts. 

Saltwater fishies don't have it so easy. Because of their surroundings being higher in salt, water is constantly being sucked out of them, and this means some of them to have to drink a lot it. They also have to filter the water because it's so salty, and those filters are found in the gills and the kidneys.

Thank you to anonymous for this question of the week!

By Kristina Krejza