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Drinking is not quite the term, as they drink virtually no liquid. The small amount that enters through the mouth goes to the gills, respiratory organs where water exchanges with the environment also take place. In freshwater fish, liquid naturally enters the body by osmosis. This happens due to the concentration of salts being higher in the body of the fish than in the surrounding water. As they absorb a lot of water, they have a well-developed kidney, capable of eliminating excesses. In marine fish, however, the trend is the opposite: it is the animal that loses water to the environment and the kidneys are poorly developed (precisely to avoid greater loss of liquid). Excess salts are eliminated through special glands located in the gills. To carry out all these functions, it is essential to maintain good water circulation.
Therefore, after the entry of the liquid, the fish closes its mouth and small bones called opercula obstruct the surface of the gills, also known as gills. “With these holes closed, a pressure is created that propels the water towards the branchial filaments, responsible for removing oxygen”, explains biologist Naércio Aquino Menezes, from USP’s Zoology Museum. Blood flows in the capillaries located in the gills against the direction of the water. This counter-current causes oxygen to pass into the blood, while water absorbs carbon dioxide. After this process, which lasts a few seconds, the fish opens the caps, eliminating the water. Because they live in a liquid environment, fish do not need to drink water to moisturize their skin, unlike land animals.