Do Fish Experience Thirst?

Fish live in water, but, just as our land-dwelling bodies have to carefully regulate our water and salt levels, so do fish bodies. Learn how fish stay hydrated in this episode of BrainStuff. 

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio. Hey
brain Stuff, Lauren vocal Bomb here with today's question, do
fish get thirsty? And? Okay, the short answer is that
we have no idea because thirsty is a human experience
of wanting a drink, and any ichthyologist would tell you
that we can't know what fish or any other non

(00:22):
human animals are thinking and feeling. You know, exactly when
you're thirsty, you might experience a range of symptoms, perhaps
dry mouth, fatigue, reduced to your an output, lightheadedness, and weakness,
among others. And when you're really really thirsty, like lost
in the desert without a canteen type of thirsty, you
simply crave water, sometimes to a degree that you can

(00:44):
hardly think about anything else. That powerful urge is what
keeps our bodies from becoming dangerously dehydrated. But because we
can't put ourselves in a fish's shoes or fins, there's
no way for us to know how fish perceive thirst
or if they do it all. But we spoke via
email with Boston College Assistant biology professor Christopher Kennelly. He said,

(01:06):
I think of first as a cognitive response to hydration,
and it's hard to get inside of fish's brain. However,
we do know quite a bit about how fishes regulate
water balance. Regardless of their thirst, drive or black thereof,
these creatures absolutely need hydration to stay alive. They regulate
water balance via a process called osmo regulation, which is

(01:27):
common to many other vertebrates, including humans. Ultimately, says Kennelly,
osmo regulation maintains the appropriate amount of salts and water
in the body, and two main organs facilitate this process. First,
the kidneys kick into gear, helping to maintain salt levels,
and second, the gills have special cells that exchange water

(01:48):
and salt with the environment. Notably, the process varies quite
a lot depending on where the fish lives, in freshwater
or in salt water. Freshwater fish don't stively drink water
because it would dilute their blood and bodily fluids. Kennely
said the challenge for a freshwater fish is different than
a marine fish. For fresh water fishes, the blood and

(02:10):
tissues are much saltier than the external environment, and thus
water follows this osmotic gradient. That is, the body is
a salty sponge. So the challenge in this case is
to keep water from diluting the body. To counter this,
the kidney of a freshwater fish expels a lot of
water from the blood and creates very dilute urine. He

(02:31):
noted that freshwater fish are almost always peeing out this
diluted urine, while their gills are constantly pumping salts back
into the body using these specialized salt cells. On the
other hand, saltwater or marine species often drink water through
their mouths to keep hydrated. The challenge there is to
avoid losing water to the much saltier environment and to

(02:52):
keep excess salt out. And Kennely explained their kidneys remove
salt and conserve water, while the salt sells in their
sills pump salt into the water. Using these different directions
of passing salt and water, the bodies of marine and
freshwater fish are equally hydrated and salty. But what about
a nadronous fishes like salmon that swim in both freshwater

(03:14):
and saltwater. We also spoke via email with Rebecca ash An,
assistant professor of Fisheries biology at East Carolina University. She
explained that salmon take the change in stages sort of
like an airlock quote an adult salmon migrated into fresh
water in order to reproduce. There's often a staging area
where the salmon hang out before completing their migration. This

(03:36):
staging area is located so that the fish are exposed
to some brackish or fresher water so that they can
gradually gain osmotic competency before migrating into their freshwater spotting grounds.
As climate change rapidly warms the world's waters, fish like
salmon may experience fast changing conditions regarding temperature stability in
water columns around the globe. This can dramatically all turn

(04:00):
the way fish adjust to the water's characteristics. Ash says
that in some cases where warming causes sea ice to
break off and float freely in a salmon staging area, quote,
there will not be a lot of mixing between fresh
and saltwater because the sea ice produces a large amount
of fresh water as it melts, and this serves as
a barrier preventing mixing. In that scenario, this barrier prevents

(04:23):
salmon from being exposed to brackish water, which delays their
ability to adapt to varying salt content in their environment.
If this all sounds fishy to you, it shouldn't our
own bodies share similarities with fish, Kennelly said, you can
even make the case that we are a type of fish,
after all, we descend from them. Thus we have retained

(04:44):
many of the same osmo regulatory mechanisms are fishy ancestors
had and modern fishes still use. That doesn't mean you
should put your fishy credentials to the test, especially when
it comes to saltwater. All animals can ingest a bit
of salt water, which is about three point five per
and salt by weight, but it won't say your thirst. Instead,
you'll get thirstier by the minute as your body uses

(05:06):
water to eliminate excess salt from your bloodstream. It's critical
that fish use osmosis to regulate salt in their bodies.
Without their sophisticated diffusion processes, saltwater fish would literally shrivel
up into slimy raisins and freshwater fish that take on
water until they burst. Today's episode was written by Nathan

(05:29):
Chandler and produced by Tyler Clang. Brain Stuff is production
of iHeart Radio's has Stuff Works. For more in this
and lots of other not so fishy topics, visit our
home planet has Stuff works dot com and for more
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or wherever you listen to your favorite shows.