Without water, there is not life; and this is an un-escapable fact for all organisms on earth. For animals, droughts can have a devastating effect causing mass extinctions and population crashes. Water is taken in via drinking it in through the mouth, a simple method that is second nature to us.

But what about species that live in water?

Do fish drink water?

Why water is important for life

Water makes up a large part of our bodies, as it does with all animals. In people this figure is around the 60%-70% mark. Now luckily we are not walking puddles, and the majority of this fluid is contained in and around our cells. It allows all our biological processes to function, giving them a medium to work in. However we also lose water regularly, via sweat, urine and breathing.

Do fish need water too?

Fish have an apparent advantage in that they live in an aqueous environment. Yet how much does this affect them?

They may live in the sea but fish are again made up of a collection of cells that need water to function. However fresh water fish and salt water fish are able to obtain this water very differently. In short, fresh water fish do not drink water; but salt water fish do.

Why the difference?

Well, it all comes down to salt. Salt is a solute, which means that it is a substance that can be dissolved in a solvent; which in this case is water. Now most aquatic vertebrae animals are osmoregulators, which means that the concentration of solutes (salt) in their cells is different from the water solution that they are inhabiting; be it the fresh or salt water. After all, some salt is naturally required in the body for certain biological processes.

When two solvents come into contact with each other, a process called diffusion occurs if they are at different concentration levels. Here, the solutes move from the area where they are in a higher concentration to the area where they are in a lower concentration. As this process is dealing with water in this case, it is then further characterised as osmosis.

Salt or sea water is of course, much higher in salt than fresh water. This makes it hypertonic, meaning it has a higher concentration of solutes that the other solution which in this case is the fish. By comparison, fresh water is hypotonic meaning it has a lower concentration of solutes than the fish. This therefore, regulates the movement of water in and out of the body.

So how does this happen?

Of course, fish can’t simply take water in by swimming around in it. The water cannot penetrate through their scales, the only way it can enter is via the gills or mouth opening.

This then has a knock on effect on whether or not the fish needs to drink. In fresh water fish, the salt in their bodies is unable to leave as it cannot move through the scales and only small amounts are directed out through the gills. At the same time however, a lot of water from the environment moves into the fish again using the openings in the gills due to the hypertonic nature of the environment.

Once taken in, the water is taken up by the capillaries and then it is able to move into the rest of the body. Of course, the fish can’t keep continuously taking on water , there isn’t enough room! To counteract this, fresh water fish have to urinate a lot. The kidneys work overtime to flush this excess water from their system. As freshwater fish are passively taking in water through the gills in large amounts, they therefore have no need to drink.

In salt water fish, the situation is reversed. Now drinking salt water is known to increase dehydration in people, as the salt taken into the stomach actually removes water from the blood stream. In fish, the water from their bodies faces a constant battle with the extreme difference to their salty surroundings. There is constant pressure to move out through the gills, and the only way they can deal with this is by drinking in the salty sea water through their mouths to rectify this balance.

This then leaves them with too much salt in their bodies. To rectify this and expel the unwanted excess salt, they then produce highly concentrated urine that contains high amounts of salt and low amounts of water. This is in direct comparison to fresh water fish who produce large amounts of diluted urine. Once again, the kidneys have to work overtime to achieve this!

Did you know?

Salmon spend their early days living in fresh water, before migrating out to sea and only returning to rivers to breed. To prepare themselves for life in the salty ocean water, the young salmon start to drink excessively. They then reduce the amount of urine that is produced, and alter the amount of sodium in their bodies by pumping it out instead of in. Once they have achieved this, they can survive in the salt water conditions. When they enter freshwater to breed, these processes have to be changed accordingly or the fish would die.

One exception to the regulated salt levels in the blood rule is the shark, who’s blood is naturally very salty. There is little concentration gradient between the salt levels in the sea and their blood, meaning that osmosis does not occur. Instead, by pumping urea (i.e. urine) around their blood they are able to maintain this homeostasis. This means they have no need to drink.

Conclusion

Water is essential for life, and that fact is undisputed. For fish living in water, the need to drink is regulated by passive biological processes that vary depending on whether they live in fresh or salty water; and to a certain extent, their species. On the whole however, fresh water fish do not need to drink while their salt water cousins have to spend all of their lives gulping it down!

References

http://www.amnh.org/exhibitions/water-h2o-life/life-in-water/surviving-in-salt-water

http://www.macmillanlearning.com/catalog/static/whf/phelanpreview/doc/01_fish.pdf

http://www.evolutionnews.org/2015/08/how_salmon_adju098821.html