We see them wallowing blissfully and undisturbed in the aquariums, we give them to eat but what about thirst? Do fish drink or not? Everything you need to know.
We always see them immersed in water whether it is a bowl or an aquarium. But do we know if the fish drink or not? It is not such a foregone question when we consider that living beings need more to drink than to eat. Since one can survive longer without food but not without drinking water. The answer, however, is not so obvious, so it will be necessary to go deeper into the question and understand what are the mechanisms that allow the fish to breathe while drinking and how to survive in one type of water rather than another. So let’s see if the fish drink and how they do it.
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Do fish drink or not: a question of osmosis
If the fish is not eating enough, we can easily notice this. But how do we know if a fish, constantly immersed in water, drinks? For every organism, water is a primary good, indispensable for its survival. It has a double function: as a reactant in biosynthesis reactions and as a fundamental element for thermoregulation. Water serves to keep the parameters and chemical values inside the organism stable and in balance, while the external environment changes outside. This process is also called homeostasis.
What are the parameters that a fish’s organism must keep stable for its survival? Certainly the concentration of mineral salts. The cells of their organism are capable of giving life to the osmosis process: the membranes of their body are permeable and allow to obtain the same concentration of water (i.e. the density of water) equal in both environments.
The fish, therefore, absorb and eliminate the water: they drink when they are in saltwater, because it has a higher mineral content than that of freshwater. In the latter, the content of mineral salts inside the fish is greater than those of water and this allows them to produce a lot of urine.
What changes for fish according to the type of water: seawater
The body of the fish, or its membrane, acts between the external medium, that is the water of its habitat, and the internal medium, that is the body fluids of the fish. And here two different ‘options’ start.
If the fish is immersed in saltwater, its body will be less rich in mineral salts than the outside (already salty in itself): this means that its body will be subject to a constant ‘loss’ of water. In fact, for the osmosis process, the fish will have a greater need to ingest water, which will then be filtered and made salt (desalinated) first by the kidneys. Said in elementary terms, the fish will drink salt water, which will be expelled (purified by salt) in its urine, produced precisely by the kidneys. Your urine will be very thick. Saltwater is that of our seas, so we can say that marine fish drink.
Do fish drink in freshwater or not?
If, on the other hand, the fish is immersed in fresh water, the water of our house to be clear, its body fluids will have a higher salt concentration than the external medium. In this case, the water will enter your body and the fish will not need to make up for the lack of water in its body. He will have enough thanks to the continuous absorption, indeed perhaps too much: in fact, it is not by chance that he will have to urinate a lot and his urine will have a low concentration of salts.
The kidneys of fish that have always been used to living in freshwater are very developed, as they are used to filtering a large amount of water, retaining salts, and keeping a certain balance of them within the body. In fact, their conservation action of values is continuous, since the entry of external fluids and the dilution effect of internal salts are constant. But it is not a job entrusted only to the kidneys. But also to the gills, which obviously hold the oxygen, they need to breathe.
What happens if we put a marine fish in fresh water and vice versa
When we force the fish to live in conditions of life exactly opposite to its ideal conditions of life, it will have neither long nor easy life. Let’s see why in detail: if a saltwater fish is immersed in freshwater, it will absorb an amount of water that is not tolerable for its body. This would be the so-called case of osmotic shock or stress. His body would not be ‘accustomed’ to the expulsion of the water and therefore would keep it inside, causing a sort of explosion that would cause them to die.
If, on the other hand, a freshwater fish finds itself in the sea it would not take the amount of water it was used to; on the contrary, he would find himself losing it and expelling it through the urine. In a short time, it will have a very high salt concentration inside the body and a minimum quantity of water: this would cause an implosion due to the absence of water.
In conclusion, we can say that saltwater fish drink and produce little urine but with a high concentration of salinity; those of freshwater do not drink and produce a higher amount of urine and a low concentration of salinity.
The exceptions among fish
There are some fish, such as sharks and rays, which have learned to retain much of their urea (a chemical compound produced by the kidneys and liver) and produce little urine. In this way, the mineral salts inside their body remain at a slightly higher level than outside. They will therefore not be forced to drink salt water and then filter it. And purify it through the kidneys and then expel it in the urine.
Other fish such as eel, salmon, or mullet can safely live by independently regulating the concentration of salts inside them regardless of the type of external water. They are osmoregulatory, thanks also to the thick layer of skin that covers them and which is almost not waterproof at all.
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