Have you noticed that fish open and close their mouths while swimming? Have you ever wondered if they are breathing or drinking?
It is no silly question and you are not the only one who has asked it. What’s more, there are already those who have found the answer.
How do fish breathe underwater
When fish open and close their mouths it is to drink water, but not to hydrate, but to breathe.
As the water passes through the gills constantly, they generate the oxygen they need to survive.
This is typical of most fish, with the exception of so-called “lungfish”.
Can the sea dry out?
The fish that inhabit the sea need more water to breathe because, if not, they would dry up (as you read) within the sea itself.
This occurs because ocean water has greater amounts of salt than the fluid that fish have inside their body, since their cells can only absorb a small amount of salt.
These cells are covered by a membrane that only allows water to pass through, isolating the salt, like a filter.
As the water molecules pass through the area of the membrane where there is always a greater amount of accumulated salt, they lose a lot of liquid, especially thanks to the activity of the mucous membranes and gills.
What remains, in the end, is very little water inside the body and a high probability of dying.
Saltwater fish need to drink water (and lots of it)
The calculation is inevitable: the greater the amount of water ingested, the greater the amount of salt.
That is when the bronchi come into play, which are responsible for eliminating the unnecessary remnant.
Therefore, unlike most animals, marine fish do not urinate to relieve this burden, as doing so would further limit the water they need to survive.
Freshwater fish, quite the opposite
In the case of freshwater fish, the water enters their body spontaneously through the mucous membranes in their mouths and gills.
As the water does not contain salt, not only do they not need to drink it, but they must expel the excess and (in this case, they do) they do so through urine.
If they didn’t, they would end up exploding like a balloon.
Research carried out in this regard indicates that freshwater fish urinate three hundred millimeters per kilo of weight per day.
Taking this into account, a ten kilo carp would expel no less than three liters of urine per day. A measure that could perfectly compete with that of the human being.