Scientists explain why the water in the sea is salty, but the fish in it are not

Some fish usually live in fresh water, while others live in sea water. This is because one or the other environment provides fish with opportunities that contribute to their survival. The obvious difference between the two habitats is the salt concentration.

Freshwater fish maintain physiological mechanisms that allow them to concentrate salts in their bodies in a salt-deficient environment; marine fish, on the other hand, excrete excess salts in a hypertonic environment. Fish living in both environments retain both mechanisms, Scientific American writes.

Scientists explained why fish living in salt water are not saturated with salts and do not have a salty taste.

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It is noted that chloride cells in the gills of marine fish produce an enzyme called gill sodium-potassium ATPase, which allows them to rid the plasma of excess salt that accumulates when they drink seawater.

Fish use the enzyme to pump sodium out of their gills for energy. In addition, their kidneys selectively filter out divalent ions, which they then excrete.

An alternative set of physiological mechanisms allows freshwater fish to concentrate salts to compensate for low salinity environments. They produce very dilute, copious urine (up to one-third of their body weight per day) to get rid of excess water, while actively absorbing ions with their gills.

Cartilaginous fish are isosmotic, but the salt concentration in their bodies is much lower than in the environment. The equalization of osmotic pressure is achieved due to the increased content of urea and trimethylamine oxide in the blood. The low salt concentration in the body of cartilaginous fish is maintained by the excretion of salts by the kidneys, as well as by a specialized rectal gland that connects to the digestive tract.

In the course of evolution, bony fish have developed mechanisms that allow for the excretion or retention of ions. Marine bony fish with a low concentration of ions in the body constantly lose water, which is released from the tissues under the influence of osmotic pressure. These losses are compensated for by drinking and filtering salt water. Sodium cations and chloride ions are excreted from the blood through the gill membranes, while magnesium cations and sulfate anions are excreted by the kidneys.

Earlier, scientists managed to film the feeding process of a whale shark at the bottom of the ocean .

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