All living things on Earth need water, because the chemical reactions that make life possible
happen in an aqueous solution.
Which means us land animals have to drink it to keep it inside of us all the time.
But…
what about fish?
It seems like since they’re always in the aqueous solution of whatever body of water
they’re swimming around in, they wouldn’t need to actually drink the stuff.
But it turns out that some fish actually do need to drink.
And others do not.
It depends on the fish, and what kind of neighborhood it lives in.
Water, you should know first of all, is really into finding a balance.
If you have a membrane that water can pass through, and there are different concentrations
of salt on each side, then the water will always flow toward the saltier side of the
membrane, until the concentrations on both sides are the same.
That means that freshwater fish have it easy.
Since the concentration of salt in their bodies is higher than the concentration in the water,
that fresh water just flows right in, mainly through their gills, and into their bloodstream.
They also swallow some water when they eat -- I mean, it’s kind of unavoidable -- but
they don’t need to actively gulp down water.
But, in order to retain that balance of concentrations, the fish’s tissues still need to have some
salt in them.
And that’s where chloride cells come in.
These are special cells in the gills that produce large amounts of an enzyme that controls
the flow of dissolved salts -- like sodium and potassium -- across cell membranes.
So, in freshwater fish, these chloride cells work hard to bring just the right amount of
salt into the fish’s bloodstream.
Now, saltwater fish have the opposite problem.
For most of them, the concentration of salt in the water is higher than the concentration
in their blood.
That means that as they pass water over their gills to breathe, the higher concentration
of salt outside of their body is constantly sucking water out of them.
So these fish have to drink a lot, which they just gulp down by the mouthful.
But since the only water around is salty, they have to filter out the salt to make it
safe for them to absorb.
A lot of that salt is filtered out in the fish’s kidneys.
But these fish have chloride cells in their gills, too.
It’s just that, in their case, those cells are constantly pumping salts out of the bloodstream
and into the saltier water.
So they may be surrounded by water, but no matter where they live, fish are always looking
to find that balance.
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