In the waters surrounding Antarctica, the water temperature never rises much above freezing.

At minus 1.9 degrees Celsius, any normal fish would be a fish-sicle.

But...this fish seems happy enough.

How?

It has a type of antifreeze in its blood.

It’s a notothenioid, one of a group of fishes that have evolved a whole bag of tricks for surviving in super-cold temperatures, only one of which is their biochemical antifreeze.

And they need ‘em, because it is COLD.

The Southern Ocean isn’t totally frozen over, but it is covered by sea ice for much of the year.

In the coldest waters near the coastline, ice crystals form in the water column and mats of ice blanket the ocean floor.

For the fish, there’s nowhere to hide.

At the risk of stating the obvious, freezing is really bad for most living things.

Ice crystals are sharp, and as they form and expand, they can lead to serious tissue damage by basically causing cells to burst and die.

But these Antarctic fish are...cool with it.

They’re often found, alive and happy, hiding and looking for food right there in the ice.

Having ice outside is one thing, but their insides are full of tiny ice crystals too, swallowed from the seawater.

And while the freezing point of seawater is -1.9 degrees Celsius, the freezing point of the fluid inside a fish’s body is in the neighborhood of a degree warmer.

That’s because the amount of stuff dissolved in water lowers its freezing point, which in chemistry we call freezing point depression.

Dissolved salts -- like what’s in the ocean -- are quite effective at lowering the freezing point of water.

But fish are only about one-half as salty as the sea, so they risk freezing in places where the seawater gets cold enough to freeze.

So once that ice get into the fish, it should freeze from the inside out.

Normal freezing point depression isn’t enough here.

That’s where the antifreeze comes in.

Specifically, these fish have antifreeze proteins in their bodies.

The proteins glom onto the surface of invading ice crystals, completely covering them.

Researchers believe the protein actually fits in with the structure of the ice crystals, so that it can bind to them in place of more water molecules and prevent the ice crystals from growing any bigger.

Then the ice crystals hang out in microscopic form in the fish’s body and eventually end up in its spleen, unable to grow bigger and cause the fish any major harm.

This antifreeze action is so powerful, it effectively creates a gap in the freezing point of the fish’s body fluids, between where they should freeze and where they actually do.

They lower the fish’s internal freezing point by more than a 1 degree Celsius, to as low as -2.7.

Since the seawater can’t get much colder than its freezing point of -1.9 , the fish literally can’t freeze.

Antifreeze proteins aren’t unique to Antarctic fish, either.

Fish like THESE and even some insects have evolved their own varieties of antifreeze proteins.

But while they explain how fish can survive in far-off cold oceans, antifreeze proteins might be closer to home than you realize.

They prevent ice crystals from growing too large, and that’s an appealing consumer application.

Like, ever had ice cream grow gritty in your freezer?

That happens because the small ice crystals in it have started to grow.. To prevent this, some manufacturers actually do add lab-grown fish antifreeze proteins to ice cream to stop ice crystals from growing larger and help ice cream retain its satisfying smoothness.

Even more importantly, and yeah, I know, what’s more important than ice cream, research is looking for ways to leverage these natural antifreezes to preserve other types of biological material than the fish that evolved them.

Like, keeping organs for transplants at lower temperatures, so they can last longer without being damaged by ice.

But that’s still a work in progress.

The fish, meanwhile, have managed to use some very clever biochemistry developed over millions of years of evolution to survive one of the harshest habitats on Earth.

You go, little guys.

Thanks to Henry Kaiser and Paul Cziko for all the great fish footage, without which we definitely could not have made this video.

Thanks for watching, and if you wanna see more wannabe nature documentary vids that are still totally about chemistry, remember to share, subscribe, and turn on notifications so you never miss a week.