Do Fish Drink Water?
Yes, fish drink water. In fact, fish that live in the ocean drink considerably more water than freshwater fish. Ocean fish must gulp water constantly because the salty water they live in absorbs moisture from their bodies, causing them to dehydrate.
The problem is just the opposite for freshwater fish. Since their bodies are much saltier than the water they live in, they are in danger of too much water entering their bodies, diluting their body fluids and causing them to swell up. What little water freshwater fish take in is absorbed through the gills.
All the water that goes inside the fish through the mouth goes to its branchias and respiratory system, where the water exchange happens. This process functions differently in fresh and salty water fish. In the first case, the water goes normally through the fish’s body; the opposite happens in marine animals: they lose water to the environment.
This process is called osmosis, which is the balance of concentrations between a medium and another. The same happens in the sea: the fish “delivers” its water to the ocean because the sea has a larger salt concentration. After the liquid enters, the fish closes its mouth and some small bones obstruct the surface of the branchias.
The kidneys of fresh water fish are more developed, since it is necessary for them to filter a larger amount of water. The blood runs on the opposite direction of the water, enabling oxygen to be transferred to the blood and the water to capture the carbon dioxide.