Thirst is a physiological signal of the body to restore water balance.
Marine life is constantly at risk of dehydration. The water they swim in is much saltier than their internal fluid. At the same time, water, according to the law, leaves the area with low salt concentration (fish body) to the area with high salt concentration (sea water). It’s a paradox, but marine fish are constantly losing fluid in the water.
To prevent dehydration, marine fish always absorb water. They get rid of excess salt thanks to the chloride cells in the gills. At the same time, such fish rarely relieve themselves.
For freshwater fish, the situation is just the opposite: They are constantly at risk of “flooding”. Their bodies contain more salt than the surrounding water. As a result, their bodies absorb minimal amounts of water and actively remove excess water in the urine. Chloride cells in such fish absorb salt from the environment.
Sharks also have a unique way of regulating salt balance in the body. Since their internal environment is close to sea water in terms of salinity, they do not need to drink water constantly. Additionally, in these fish, excess salt is excreted through a gland in the rectum.
Source: Ferra
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