Do Fish Drink the Water They Live In?
Fish consume the water they inhabit if it agrees with their system. Since the bodies of freshwater fish are much saltier than the water they live in, they don’t drink. They are in danger of too much water entering their bodies, diluting their body fluids. They urinate often to get rid of any extra water that soaks into their bodies.
Saltwater fish, however, drink large amounts of water, since the salty sea water absorbs moisture from their bodies, causing dehydration. These fish eliminate excess salt through their gills and very concentrated urine. Water doesn’t move easily through a fish’s skin, like it can through cell membrane, but it can move through the linings of the fish’s gills with great ease.
A fish’s gill must be extremely permeable so that oxygen from the water can enter the capillaries in the gills and provide the fish with the oxygen.
When saltwater from the ocean flows over the gills of a fish, the water is hypertonic to the blood in the capillaries in the gills, causing water to move out of the gill tissue where it is lost. This means that saltwater fish are constantly losing water out of their bloodstream through osmosis.
When freshwater from lakes and streams flows over the gills of a fish, the water is hypotonic to the blood in the capillaries in the gills, causing water to move into the gill tissue. This means that freshwater fish are constantly gaining water into their bloodstream through osmosis.
Both types of fish take in water through their mouths, exchanging the dissolved oxygen and carbon dioxide in the water by means of the gills. In this case fish are not drinking, they’re breathing.