• Some fish - but not all - need to drink seawater, according to our partner The Conversation.

  • The problem of seawater consumption is not just for fish.

    Some sea birds - albatrosses, for example - must also drink it.

  • The analysis of this phenomenon was carried out by Claire Lacey, PhD candidate in Marine Biology at the University of St Andrews (Scotland).

That's a great question, Torben, thank you very much for sending it.

The short answer is yes, some fish drink salt water, but not all.

Fish are amazing animals, and have some really cool solutions for living in water.

Of course, different types of fish have developed different 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.

VIDEO: 

Are fish thirsty?

(RTS Kids)

To understand how it works, you must first know that bony fish have a different concentration of salt in their body than the water in which they swim: they are more or less salty.

The bodies of marine fish (which live in the sea) are less salty than the water in which they swim, while the bodies of freshwater fish (which live in rivers and lakes) are more saline than the water.

Marine fish, like freshwater fish, need to control the amount of water and salt in their bodies to stay healthy and well hydrated.

Not easy to stay hydrated

Bony marine fish are constantly losing water from their bodies, through a process called "osmosis".

During osmosis, water moves through a membrane (like skin), from areas of low concentration to areas of higher concentration.

Remember that a marine fish's body is less salty than the seawater it swims in, which means it has a lower salt concentration.

These fish therefore lose water by osmosis: it passes from their body, through their skin and their gills, into the sea.

Can a fish drink from the cup?

© Sebastian Pena Lambarri / Unsplash

As they are constantly losing water, these fish need to drink a lot of seawater to stay hydrated.

The opposite occurs in freshwater fish.

Water enters their body by osmosis, instead of leaving it.

This means that they usually don't need to drink - but they do have to pee a lot.

We all know that too much salt is bad for us.

So, of course, an animal that drinks seawater must have a way to get rid of the excess salt.

Marine fish have kidneys that pump excess salt into their pee to get it out of their bodies.

They also have special cells in their gills that push excess salt into the sea. Together, these two systems keep marine fishes hydrated.

What about sharks?

Sharks have developed a completely different system.

Their body has a slightly higher salt concentration than that of seawater. This means that 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 - which other animals typically dispose of.

Sharks keep them in their bodies, which keeps them "salty".

Despite my tall size, I don't drink © David Clode / Unsplash

Sharks take in small amounts of water through their gills (by osmosis - since they're 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.

Our "FISH" file

The problem of seawater consumption 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.

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This review was written in English by Claire Lacey, PhD candidate in Marine Biology at the University of St Andrews (Scotland).


The original article was translated and published on The Conversation website.

Declaration of interests

Claire Lacey does not work, advise, own shares, receive funds from any organization that could benefit from this article, and has not declared any affiliation other than her research organization.

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