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

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

Because freshwater fish don't drink!

Saltwater fish on the other hand?

They're constantly drinking.

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

an obvious question: Do fish pee?

What… you've never wondered that?!

Water is essential to life as we know it.

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

water in their bodies.

You'd think that fish have it made when it comes to staying hydrated, but it's not so easy.

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

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

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

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

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

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

One is covered in pure H2O.

And one goes into sugar syrup.

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

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

Life is all about balance.

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

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

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

And most of them do that a *lot*.

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

in the gills that pump out the extra.

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

salty as the pool they pee in.

Freshwater fish have the opposite problem.

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

through their skin and gills.

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

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

and the egg swells up!

Plus, they get even more water from their food.

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

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

which means they pee *a lot*.

They're pretty much peeing all the time.

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

20 times more pee than I usually make.

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

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

that plants crave.

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

ecosystems healthy.

Fish are little swimming, urinating, nutrient recycling plants.

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

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

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

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

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

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

It's also why their meat smells like pee.

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

I smell great.

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

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

the same concentration as the ancient oceans where life began.

You even have evidence of this in your body.

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

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

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

fish, who use it to balance their salty bodies.

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

Tinkle tinkle, little fish Homeostasis is what you wish

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

Urea-ly have mastered your niche.

Stay curious.