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Speaker 1: Welcome to brain Stuff production of I Heart Radio. Hey

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Speaker 1: brain Stuff, Lauren vocal Bomb here with today's question, do

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Speaker 1: fish get thirsty? And? Okay, the short answer is that

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Speaker 1: we have no idea because thirsty is a human experience

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Speaker 1: of wanting a drink, and any ichthyologist would tell you

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Speaker 1: that we can't know what fish or any other non

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Speaker 1: human animals are thinking and feeling. You know, exactly when

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Speaker 1: you're thirsty, you might experience a range of symptoms, perhaps

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Speaker 1: dry mouth, fatigue, reduced to your an output, lightheadedness, and weakness,

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Speaker 1: among others. And when you're really really thirsty, like lost

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Speaker 1: in the desert without a canteen type of thirsty, you

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Speaker 1: simply crave water, sometimes to a degree that you can

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Speaker 1: hardly think about anything else. That powerful urge is what

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Speaker 1: keeps our bodies from becoming dangerously dehydrated. But because we

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Speaker 1: can't put ourselves in a fish's shoes or fins, there's

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Speaker 1: no way for us to know how fish perceive thirst

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Speaker 1: or if they do it all. But we spoke via

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Speaker 1: email with Boston College Assistant biology professor Christopher Kennelly. He said,

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Speaker 1: I think of first as a cognitive response to hydration,

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Speaker 1: and it's hard to get inside of fish's brain. However,

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Speaker 1: we do know quite a bit about how fishes regulate

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Speaker 1: water balance. Regardless of their thirst, drive or black thereof,

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Speaker 1: these creatures absolutely need hydration to stay alive. They regulate

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Speaker 1: water balance via a process called osmo regulation, which is

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Speaker 1: common to many other vertebrates, including humans. Ultimately, says Kennelly,

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Speaker 1: osmo regulation maintains the appropriate amount of salts and water

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Speaker 1: in the body, and two main organs facilitate this process. First,

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Speaker 1: the kidneys kick into gear, helping to maintain salt levels,

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Speaker 1: and second, the gills have special cells that exchange water

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Speaker 1: and salt with the environment. Notably, the process varies quite

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Speaker 1: a lot depending on where the fish lives, in freshwater

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Speaker 1: or in salt water. Freshwater fish don't stively drink water

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Speaker 1: because it would dilute their blood and bodily fluids. Kennely

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Speaker 1: said the challenge for a freshwater fish is different than

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Speaker 1: a marine fish. For fresh water fishes, the blood and

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Speaker 1: tissues are much saltier than the external environment, and thus

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Speaker 1: water follows this osmotic gradient. That is, the body is

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Speaker 1: a salty sponge. So the challenge in this case is

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Speaker 1: to keep water from diluting the body. To counter this,

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Speaker 1: the kidney of a freshwater fish expels a lot of

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Speaker 1: water from the blood and creates very dilute urine. He

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Speaker 1: noted that freshwater fish are almost always peeing out this

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Speaker 1: diluted urine, while their gills are constantly pumping salts back

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Speaker 1: into the body using these specialized salt cells. On the

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Speaker 1: other hand, saltwater or marine species often drink water through

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Speaker 1: their mouths to keep hydrated. The challenge there is to

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Speaker 1: avoid losing water to the much saltier environment and to

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Speaker 1: keep excess salt out. And Kennely explained their kidneys remove

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Speaker 1: salt and conserve water, while the salt sells in their

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Speaker 1: sills pump salt into the water. Using these different directions

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Speaker 1: of passing salt and water, the bodies of marine and

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Speaker 1: freshwater fish are equally hydrated and salty. But what about

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Speaker 1: a nadronous fishes like salmon that swim in both freshwater

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Speaker 1: and saltwater. We also spoke via email with Rebecca ash An,

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Speaker 1: assistant professor of Fisheries biology at East Carolina University. She

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Speaker 1: explained that salmon take the change in stages sort of

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Speaker 1: like an airlock quote an adult salmon migrated into fresh

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Speaker 1: water in order to reproduce. There's often a staging area

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Speaker 1: where the salmon hang out before completing their migration. This

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Speaker 1: staging area is located so that the fish are exposed

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Speaker 1: to some brackish or fresher water so that they can

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Speaker 1: gradually gain osmotic competency before migrating into their freshwater spotting grounds.

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Speaker 1: As climate change rapidly warms the world's waters, fish like

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Speaker 1: salmon may experience fast changing conditions regarding temperature stability in

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Speaker 1: water columns around the globe. This can dramatically all turn

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Speaker 1: the way fish adjust to the water's characteristics. Ash says

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Speaker 1: that in some cases where warming causes sea ice to

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Speaker 1: break off and float freely in a salmon staging area, quote,

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Speaker 1: there will not be a lot of mixing between fresh

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Speaker 1: and saltwater because the sea ice produces a large amount

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Speaker 1: of fresh water as it melts, and this serves as

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Speaker 1: a barrier preventing mixing. In that scenario, this barrier prevents

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Speaker 1: salmon from being exposed to brackish water, which delays their

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Speaker 1: ability to adapt to varying salt content in their environment.

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Speaker 1: If this all sounds fishy to you, it shouldn't our

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Speaker 1: own bodies share similarities with fish, Kennelly said, you can

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Speaker 1: even make the case that we are a type of fish,

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Speaker 1: after all, we descend from them. Thus we have retained

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Speaker 1: many of the same osmo regulatory mechanisms are fishy ancestors

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Speaker 1: had and modern fishes still use. That doesn't mean you

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Speaker 1: should put your fishy credentials to the test, especially when

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Speaker 1: it comes to saltwater. All animals can ingest a bit

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Speaker 1: of salt water, which is about three point five per

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Speaker 1: and salt by weight, but it won't say your thirst. Instead,

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Speaker 1: you'll get thirstier by the minute as your body uses

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Speaker 1: water to eliminate excess salt from your bloodstream. It's critical

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Speaker 1: that fish use osmosis to regulate salt in their bodies.

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Speaker 1: Without their sophisticated diffusion processes, saltwater fish would literally shrivel

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Speaker 1: up into slimy raisins and freshwater fish that take on

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Speaker 1: water until they burst. Today's episode was written by Nathan

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Speaker 1: Chandler and produced by Tyler Clang. Brain Stuff is production

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Speaker 1: of iHeart Radio's has Stuff Works. For more in this

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Speaker 1: and lots of other not so fishy topics, visit our

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Speaker 1: home planet has Stuff works dot com and for more

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Speaker 1: podcast for iHeart Radio, visit the iHeart Radio app, Apple Podcasts,

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Speaker 1: or wherever you listen to your favorite shows.