Do fish drink water?

Do you think fish drink water? I can already see your smile. After all, as soon as a fish opens its mouth, it is full of water.

Do you think fish drink water? I can already see your smile. After all, as soon as a fish opens its mouth, it is full of water

And along with food, inevitably, whether the fish wants it or not, a certain amount of water enters the stomach. Is it enough? Are fish thirsty? Scientists have long found the answer to these questions.

Modern fish have mastered all natural reservoirs, but each species can live only in its usual conditions. Very few people can switch from fresh water to salt water and back without harm to their own health. Eels can be considered virtuosos in this area. They spend half their lives in salt water, and the other half in fresh water. What prevents fish from freely moving from one water to another? The skin, the integument of the mouth, gills and other parts of the body, as well as the membranes of individual cells of all organs and tissues of fish are permeable to water. It seeps freely through them, and these shells are impenetrable for salts and most other substances.

Where will the water seep: into the reservoir or from the reservoir? It does not depend at all on where it is more. The osmotic pressure of solutions, which is created by the substances dissolved in them, controls the diffusion process. The more of them, the higher the osmotic pressure and the stronger the solution sucks in water. In fresh water it is practically equal to a bullet, and in the blood and tissue fluids of fish there are many salts and protein substances that create an osmotic pressure of 6-10 atmospheres. With this force, the body of freshwater fish sucks water, which from the outside intensively enters their body. If they did not have the adaptations for quickly removing its excess from the body, the body would quickly swell and the animal would die. Thus, freshwater fish never need to drink water. They have enough worries about how to get rid of water penetrating from all sides.

Another thing is their relatives - sea bony fish. There are much more salts in seawater than in fish tissues. The osmotic pressure of ocean water is 32 atmospheres, while in the body of marine teleost fishes it reaches only 10-15. Therefore, the insatiable ocean greedily sucks water from their bodies. At first glance, a paradoxical phenomenon arises: sea water is able to dry out the fish swimming in it. No wonder they are always thirsty.

Not all marine fish drink water. The most ancient of them, sharks and rays, which, apparently, migrated to the ocean earlier than bony fish, otherwise adapted to life in salt water. They have learned to preserve a rather harmful substance in their blood - urea, from which all other animals rush to get rid of as soon as possible. To do this, they had to put on the gills in a special shell impermeable to urea. The osmotic pressure of the blood of sharks and rays is much higher than that of seawater. Their bodies, just like freshwater fish, suck water from the ocean, so sharks and rays are only concerned about how to get rid of it.

The same principle was borrowed from sharks and the crabeater frog, which is found in Southeast Asia. Of all the amphibians, she is the only one who has adapted to life in salt water. True, these frogs still spawn in fresh water, but when the frogs grow up, they go to the sea, where they feed on crabs. Like sharks, frogs preserve urea in the blood, but they only do it arbitrarily: before going into seawater, they stock up on urea, when they go into fresh water, they are freed from its excess. Therefore, wherever they live, these frogs, like the rest of their relatives, do not need to drink water.

Tools