Why Don't River Fish Drink The Water, While Sea Fish Do?

Fish have no problem with water deficit. They live in water environment, after all. Some fish live in freshwater like rivers, lakes, or ponds and some live in seas, oceans or salt lakes. Life in water has some negative sides.

Is proven that fish that live in freshwaters don’t drink the water, unlike the sea fish that do drink it. In the world of physics there is one process called osmosis. That happens when water flows from a place with low concentration of a given compound to environment with large concentration of the same compound. In this case, the compound is salt.

In order to understand this better, imagine separating one container full of water with a net. On the one part of the container we put a solution with high concentration of salt and on the other half a solution with low concentration of salt. The water will flow towards the solution with high concentration of salt until the concentration of salt in the whole container is equal. This is a passive process and it is happen in our bodies, too, but it is controlled.

The body of a fish that live in freshwaters contains a lot of different elements and compounds. The water from its environment enters the body through the skin. Because of that all freshwater fish don’t drink water (because the water passively enters their bodies).

The situation is reverse with sea fish. The water goes from the inside body into the sea water. That’s the reason why all sea fish drink the water. The proof for this are the nasal glands for discharging salt that are found in these types of fish. Life in the water has given many more difficulties to fish, but evolution has helped them overcome most of them.
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