Are the fish thirsty? A question that may seem quite obvious at first glance, but it is not, we explain.
Fish are not thirsty like you and me. They are not aware of drinking, it is a reflex, and water is always available to them. Drinking for a fish is as natural as breathing for a Human Being.
First of all, it is necessary to know if it is a freshwater or a saltwater fish. The salinity of the water and the skin of the fish are two code words. Saltwater fishes drink constantly. Water enters through their mouths and gills, but the sea salt is blocked.
Table of Contents
Can fish get thirsty?
The answer is not obvious actually, it depends on the fish. Freshwater fish don’t get thirsty while saltwater fish do.
In fact, it’s all about osmotic pressure. Osmotic pressure is the difference in pressure exerted on either side of a semi-permeable membrane by two liquids of different concentration. A semi-permeable membrane is one that allows water to pass through it, but remains impermeable to salt. Take, for example, a salty liquid and on the other side of the membrane a non-salty liquid.
By osmotic pressure, the fresh water will tend to go towards the salt water to soften it until the pressure on both sides of the membrane are balanced. This is what happens with fish. The membrane is their body.
Take marine fish. In the sea water, there is more salt than in the body of the fish. Because of the osmotic pressure, the fish tends to lose its water. The water in the fish goes away to soften the sea water. The fish must therefore drink a lot through its mouth. Moreover, its gills expel salt because the unfortunate fish drinks too much salt!
However, in fresh water, it is the opposite! Freshwater fish are saltier than the water they swim in. They tend to swell up with fresh water and spend their time eliminating it by peeing to restore their natural balance. In addition, their gills pump out the little salt available in fresh water since they tend to lose it.
When they are thirsty, do they drink sea water?
The short answer is yes, some fish drink sea water, but not all. Fish are amazing animals, and have interesting solutions for living in water. Naturally, different types of fish have developed various solutions.
Bony fish that live in the sea – like cod, herring or tuna – have different ways of getting water in and out of their bodies. In addition to swallowing and peeing, like us, these fish can pass water through their skin and gills.
To understand how this works, you first need to know that bony fish have a different concentration of salt in their body than the water they swim in: they are more or less salty.
The bodies of marine fish (which live in the sea) are less salty than the water they swim in, while the bodies of freshwater fish (which live in rivers and lakes) are more salty than the water.
Marine fish, like freshwater fish, must control the amount of water and salt in their bodies to stay healthy and well hydrated.
Are all fish thirsty?
There are some rare exceptions, such as the shark, which does not function like most fish. They have developed a completely different system. Their bodies have a slightly higher salt concentration than sea water. This means they don’t have the problem of bony fish, which lose water through their skin all the time.
Sharks have high levels of chemical wastes in their bodies – called urea and trimethylamine N-oxide – that other animals usually get rid of. Sharks keep these in their bodies, which keeps them “salty”.
Sharks absorb small amounts of water through their gills (by osmosis – as they are slightly saltier than the sea), which means they don’t really need to drink.
Sharks also have a salt gland to get rid of any excess salt they may have.
The problem of drinking seawater is not just for fish. Some seabirds – albatrosses, for example – must also drink seawater. Like sharks, these seabirds have a salt gland to get rid of excess salt.
Can a fish die of thirst?
While a sea fish drinks 10 to 30 ml/day, a freshwater fish drinks little or nothing, from 1 to 5ml/day. In addition to the water they swallow while feeding, marine fish regularly open their mouths to drink.
If fish stop moving, they die of asphyxiation, especially in environments naturally poor in oxygen. To breathe they manage to extract oxygen from the water thanks to a respiratory system adapted to the aquatic world: the gills. However, for some fish, this type of breathing is insufficient.
This is particularly the case for species of the Labyrinthidae family, of which the most famous representative is the Siamese fighter generally called (Betta Splendens). Indeed, the warmer the water, the less dissolved gas it can contain and therefore the less oxygen necessary for the survival of the fish.
In this case, the fish finds itself in an uncomfortable situation: it does not have enough oxygen in its water to breathe properly. However, as its name indicates, the labyrinthine fish has a particular organ: the labyrinth. It is an organ whose functioning is similar in some aspects to that of a lung.