do-fish-pee

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Hey smart people, Joe here.


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Have you ever heard the phrase “to drink
like a fish?”


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Well, unless you’re a ocean dwelling species,
that probably means staying sober.


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Because freshwater fish don’t drink!


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Saltwater fish on the other hand?


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They’re constantly drinking.


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But if some fish drink all the time and other
fish don’t drink at all, we’re left with


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an obvious question: Do fish pee?


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What… you’ve never wondered that?!


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Water is essential to life as we know it.


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To keep their chemistry running smoothly,
living things need just the right amount of


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water in their bodies.


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You’d think that fish have it made when
it comes to staying hydrated, but it’s not


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so easy.


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Fish gills lets oxygen flow into their bodies,
but they’re also letting water constantly


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escape.


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Ocean water is *saltier* than the insides
of fish.


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So water wants to move from the inside out
to balance that out.


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That’s a problem if you’re a fish who
wants to stay alive.


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It’s a process called osmosis, and we can
demonstrate that with these eggs.


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I’ve dissolved the shells away, leaving
a membrane that’ll allow water to pass through.


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One is covered in pure H2O.


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And one goes into sugar syrup.


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The syrup contains lots of sugar molecules,
and not much water, so water passes out of


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the egg to balance it out, and the egg shrinks!


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Life is all about balance.


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Now, we keep the salts and dissolved chemicals
in our blood balanced by flushing the extra


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through our kidneys, and drinking fresh water
when we’re thirsty.


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But ocean fish only have one option: drink
salt water in order to get the H2O they need


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to stay alive.


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And most of them do that a *lot*.


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But they need to get rid of the salt without
dehydrating, which they do using special cells


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in the gills that pump out the extra.


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Ocean fish only make a tiny amount of urine,
and what little pee they make is almost as


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salty as the pool they pee in.


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Freshwater fish have the opposite problem.


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Their insides are saltier than where they
live, so water is constantly leaking *in*


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through their skin and gills.


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It’s just like the naked egg we put in pure
water.


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The inside of the egg is saltier than the water around it, so water flows in


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and the egg swells up!


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Plus, they get even more water from their
food.


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So even in their wet world, they never have
to take a drink.


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Freshwater fishes have to *get rid* of water
constantly to keep their cells from bursting,


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which means they pee *a lot*.


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They’re pretty much peeing all the time.


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If I peed as much as a freshwater fish, I’d
release up to 28 liters a day, which is about


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20 times more pee than I usually make.


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For fish, peeing in the yard is a big help
to their neighbors.


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Fish pee is full of ammonia, a nitrogen containing
compound, and phosphorous, both nutrients


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that plants crave.


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These also help feed algae and coral reefs,
like a liquid fertilizer that helps keep aquatic


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ecosystems healthy.


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Fish are little swimming, urinating, nutrient
recycling plants.


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Some fish get crazy with this salt-and-water
balancing act.


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Some are able to live in water so salty it’s
hard for them to stay below the surface.


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Others, like salmon, can tweak their body
chemistry to move between salty and fresh


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water.


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Sharks and their relatives are especially
weird when it comes to salt balance.


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Using a special enzyme, they stuff urea–another
nitrogen containing compound–into their


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tissues.


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By loading their insides with another ion,
they keep water from wanting to rush out of


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their bodies.


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It’s also why their meat smells like pee.


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Thankfully, we (sniff sniff armpit) don’t
do that.


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I smell great.


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But we do owe our ability to regulate water
and salt to our fishy ancestors.


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Scientists think that a healthy salt level
for the bodies of most living animals is about


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the same concentration as the ancient oceans
where life began.


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You even have evidence of this in your body.


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Before we’re born, human embryos develop
three different types of kidneys.


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Two get absorbed, and one eventually becomes
the final pair.


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The extra organs are an evolutionary leftovers:
early kidneys that we also see in primitive


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fish, who use it to balance their salty bodies.


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It reminds me of the famous poem by I.P.


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Freely:


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Tinkle tinkle, little fish
Homeostasis is what you wish


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The ocean’s your toilet, and even though
you soil it


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Urea-ly have mastered
your niche.


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Stay curious.