Have you ever wondered if fish drink water? It may seem like a somewhat absurd question, but the answer is very interesting since this directly depends on the environment where they live. In this post, we explain why sea fish drink water and river fish don’t.
Sea fish
The marine fish if drink water and also do continuously. Seawater, as everyone knows, is salty (for every liter there are approximately 33 grams of salt).
In the ocean, there are numerous species such as sponges, bivalves, anemones, or sea urchins, among many others, which have the same concentration of salts in their bodies as seawater.
However, a large number of species, especially marine teleosts – the infraclass to which most fish belong – have a lower concentration of salts than the marine environment in which they live.
Osmosis is the phenomenon of equalization of various concentrations separated by a semipermeable membrane, and the water flows in the bodies of salt concentrations to less salty.
For this reason, as its salt concentration is lower than the environment in which it lives, the body of the sea fish loses water, filtering to the outside through its skin, and undergoes progressive dehydration.
To counteract this and maintain the balance of their internal body fluids, sea fish are forced to drink a lot of water. Although when they do this they also ingest salts, these are not absorbed because they do not need them, at least not most of them, and they are expelled through the gills and urine, which is very concentrated and with a little amount of water.
River fish
The opposite is true for freshwater fish. Unlike seafarers who run the risk of expelling too much water and, therefore, dehydration, river fish, having a higher concentration of salts in their body fluids than freshwater, run the risk of getting too much water Or, in other words, they die from excess fluids.
To avoid this they use two very simple mechanisms. The first consists of providing a body structure that is impervious to water, and the second, in simply not drinking water.
Although the fish we observe in the aquarium open and close their mouths, what they are actually doing is breathing. Therefore, fish drink from the sea, but not from the river.
Even so, these mechanisms are limited and some freshwater enters the body through the phenomenon of osmosis, through the skin and gills. Therefore, to maintain internal balance it is necessary to eliminate water through urine, which, unlike marine fish, is very abundant and not very concentrated, since they need to retain salts.
The salmon is a very special case that fits throughout his life to both types of media. Some of the species of these fish are born in rivers, and after traveling their course they migrate to the sea.
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In fact, some return to the place where they were born to procreate. At first, in freshwater, they avoid drinking water and losing salts, but when they reach the ocean they do not stop drinking and eliminating them.
