Sometime just over half a billion years ago, the first vertebrates appeared in the fossil record.

And they, like you, had blood that was colored red by the presence of an oxygen-transporting protein called hemoglobin. Now, a lot of stuff has happened in the last 500 or so million years, five mass extinctions, the rise and fall of the dinosaurs, and all of human evolution, but vertebrates have pretty much stuck with the hemoglobin-rich, red blood that they shared with their common ancestor the whole time.

And that’s not because it’s the only option out there. Invertebrates go in for a whole rainbow of blood colors: blue, purple, green. There are rare exceptions to the red-blood rule among vertebrates.

Several species of skinks have green blood, but it’s still full of hemoglobin, the red color is just overpowered by a green pigment called biliverdin. There’s also one family of fish that has blood with no color at all, and that means no hemoglobin, either. [♪ Intro]. Meet the family of crocodile icefish, also known as Antarctic icefish.

There are at least 15 species in this family, almost all of which live in the Southern Ocean around Antarctica. Most icefish species can grow to be more than 55 cm long, with the biggest ever found measuring75 cm in length. Which is a lot bigger than most Antarctic fish.

They’re also naked and for a fish, that means they just don’t have scales. And the Southern Ocean is a great place to live, if you’re an icefish. The water temperature off the Antarctic Peninsula ranges pretty consistently between about 1.5˚C in the summer and -1.8˚C in the winter, and if you’re one of the species that lives near the Ross Ice Shelf, it’s a brisk -1.9 C year-round.

That seawater is so cold that it is also almost completely saturated with oxygen, which is part of why icefish can survive there. See, cold water is better than warm water at dissolving oxygen because, in warmer water, both the oxygen and the water have more kinetic energy. And as the kinetic energy of the oxygen goes up, its molecules can more easily break away from the attraction of the water molecules and go back to being a gas.

So, thanks to the marvels of chemistry and physics, the freezing cold water of the Southern Ocean is full of dissolved oxygen, and being in such an oxygen-rich environment is pretty crucial to the icefish’s evolutionary story. Remember how I said they have clear blood, because they don’t have hemoglobin? Well, no hemoglobin means their blood’s ability to transport oxygen kinda sucks.

The oxygen carrying capacity of icefish blood is less than 10% as good as the red blood of closely-related fish. Without hemoglobin to ferry that oxygen around, it’s just dissolved in solution in their weird, translucent blood. And that system is only efficient enough to keep these fish going because of the high oxygen content of the frigid waters of the Southern Ocean, along with some physiological adaptations of the icefish themselves that help them compensate.

For example, icefish have really big hearts for fish of their body size, and their blood volume is up to four times as large as that of other fish in their infraclass. They’ve also got extra wide capillaries and, unsurprisingly, super low metabolic rates. Their blood is relatively thin and full of antifreeze proteins, and all these things help them circulate enough blood to oxygenate their tissues, and, like, you know, not die.

It sounds like icefish have a pretty sweet set-up. But, here’s the thing. At some point, the random genetic mutation that caused the ancestor of these fish to just lose all its hemoglobin should have absolutely meant *game over*.

The only reason it didn’t was because it happened to be in the right place at the right time, and, for a mutation like this one, the environment is the thing that determines if it’s good, bad, or neutral. The icefish isn’t the only bizarre beast out there that’s dodged this kind of evolutionary bullet, either. For example!

We humans, and our ape, monkey, and tarsier relatives, can’t make our own vitamin C, unlike most other mammals, because of a different random genetic mutation that happened in our common ancestor around 61 million years ago. But we got all the vitamin C we needed from the fruit in our diets, so things turned out okay. And, for the icefish, too, a mutation that could’ve been very bad ended up actually being pretty neutral, because of its environment.

Between 40 and 20 million years ago, Antarctica became fully separated from South America and Australia, and the cold Antarctic Circumpolar Current started to swirl around the continent. And this current, the largest wind-driven current in the world, combined with deep oceanic trenches to help isolate the marine life that lived in the Southern Ocean. As things got colder, sometime between 25 and 22 million years ago, the Antarctic ice sheet expanded, changing ecosystems and potentially causing a major crash in fish diversity in the area.

This would’ve opened up niches for the ancestors of the icefish, and reduced competition. Now, there is still some debate about when exactly the icefish family evolved, whether it was between 20 and 15 million years ago, closer in time to when the Antarctic Circumpolar Current really got going, or more like 5.5 to 2 million years ago, well after the Southern Ocean had become a very stable, cold, and oxygen-rich environment. But since all the members of this family lack hemoglobin, that means it’s very likely a trait that they inherited from a common ancestor at the base of the family tree.

Either way, the isolation created by the Current along with that earlier crash in fish diversity meant that the hemoglobin-less icefish, instead of dying out like you would expect, could hold their own against more efficient red-blooded Antarctic fish. The loss of hemoglobin wasn’t an advantage for the icefish, but the lack of competition meant that it wasn’t a disadvantage, either. If that same mutation had happened in warmer waters, well, we wouldn’t be doing this episode because that individual probably would have died.

Instead, they became the common ancestor of at least FIFTEEN DIFFERENT SUCCESSFUL SPECIES! So, yes! The icefish is incredibly bizarre in its own way, because its blood is so unlike that of any other animal that we know.

But it’s worth noting and remembering that the forces of evolution and natural selection have acted on them just like they do on us, and just like they did on those first red-blooded vertebrates over 500 million years ago. This month we’re extending the pin club subscription window so you can grab your pins in time for the holiday season! The icefish pin is just exceptionally good.

I love it a lot. I can’t wait to get mine. You can sign up anytime from November 6th to the 15th to get the cool icefish and all future Bizarre Beasts pins in your mailbox every month, or give a subscription as a gift to the animal nerd in your life.

For the holidays, we are also launching new merch including these awesome stickers and a tote bag. AND as a special holiday treat, from November 13th to the 15th, you get free shipping on orders over $75 at DFTBA.com. Be sure to check us out on Twitter @BizarreBeasts, and on Instagram and Facebook @BizarreBeastsShow for even more weird facts about the icefish and also all of our upcoming and past animals.

And, as always, profits from the pin club go to support our community’s efforts to decrease maternal mortality in Sierra Leone. [♪ Outro].