Why do we call the different parts of the ocean by different names, when really, all that water is just one big, salty bathtub that the continents are hanging out in?

There are certainly variations in how warm or cold or salty or fresh certain parts of our global ocean are, but ultimately, it’s all a single body of water that eventually circulates around the world. So why can’t animals that live in the ocean just go everywhere?

You’d think that the ones adapted to life in the open ocean especially would ignore arbitrary distinctions like ‘Pacific’ and ‘Atlantic.’ But they don’t. Or, at least, this month’s Bizarre Beasts, the sea snakes, don’t… Because, despite being well-adapted for life at sea, there are no sea snakes in the Atlantic Ocean. [♪♪ INTRO ♪♪] First, we need to talk about what sea snakes are. Because we have talked about a number of critters on this channel that live in the ocean and are snake-y looking, but are definitely not snakes, like eels and hagfish.

Taxonomically, sea snakes are actual snakes, they’re part of the family that includes coral snakes and cobras, called the Elapidae. Within that family, there are two subfamilies that evolved their ocean-going ways independently: true sea snakes and sea kraits. They range in maximum length from about half a meter to almost three meters long.

There are around 60 species of true sea snakes, which already feels like too many snakes in the ocean, to be honest. Don’t get me wrong, I like snakes just fine, I just don’t expect to see them in the ocean, which probably says more about my North America-adjacent-ocean bias than anything else. And it’s worth pointing out that at least three species of true sea snakes and one sea krait have also been found in freshwater, so, ‘sea’ is a bit of a misnomer there.

Anyway, the only place you’re going to find the true sea snakes living is in the water. They can’t really go on land, their bodies are kinda flattened from side to side, like an eel’s, and the ends of their tails are even flatter, forming a paddle. And some species have completely lost the large scales on their bellies that help terrestrial snakes get around.

True sea snakes also give birth to live young, which is another adaptation to their fully marine life and one of the major ways they’re different from sea kraits. Sea kraits can get around well on land, even climbing into crevices in limestone caves, and, in fact, have to leave the ocean to lay their eggs. Some species also digest their prey on land.

They still have the large belly scales that allow them to crawl, and their bodies are less flattened and more cylindrical than true sea snakes, though they also have paddle-shaped tails. And while there are a lot of species of sea snakes, there are only eight species of sea kraits. Something all of these marine snakes have in common, though, is that they’re predators, as you might expect a snake to be.

They also all have pretty potent venom, with one exception, which might be because it’s one of two genera that specializes on a diet of fish eggs, hardly the kind of prey that a snake needs venom to immobilize. The other sea snakes and kraits eat mollusks, fish, crustaceans, and eels, with at least 10 species of true sea snakes going after burrowing eels in particular, with some pretty hilarious results. See, in order to prey on burrowing eels, you have to be able to follow them back into their burrows or, at a minimum, make it enough of the way in to get the eel out.

And that means your head and some amount of your body have to be narrow enough to fit. Except it’s also advantageous for sea snakes to have bigger bodies. A bigger body means you can dive deeper, go after larger prey, and give birth to a greater number of young at once.

So how did these sea snakes deal with these two competing pressures? Tiny heads. Over time, what ended up evolving was skinny heads and front-ends attached to much chunkier back-ends, leaving them looking like two mismatched snakes sewn together in the middle.

How that happens in practice is, as the snakes grow up and develop, their back-ends grow faster than their front ends. And this goofy anatomy convergently evolved in a number of different species, most of which didn’t start out with small-headed ancestors. Now, if you want to see any of these silly-looking sea snakes, or any of the other sea snakes in the wild, you’re going to have to go to the shallow coastal waters of the Indian and western Pacific Oceans, particularly the region often referred to as the Coral Triangle.

That’s the only place they live, with one exception: the yellow-bellied sea snake. These guys are pelagic, which means they live in the open ocean, carried along by the currents. They have the distinction of being the widest-ranging snake in the world.

And they can be found in a lot of places. Like, throughout the tropical parts of the Indian and Pacific Oceans, from the eastern coast of Africa up to the Arabian Peninsula. And all through the islands of Southeast Asia and north to the Korean Peninsula, and even to the western coast of the Americas.

But not in the Atlantic Ocean. Now, the yellow-bellied sea snake, along with the other members of its genus, evolved sometime between 1 and 8 million years ago, somewhere in the Coral Triangle. And on the eastern Pacific side of its range, it’s blocked from entering the Atlantic Ocean by the Isthmus of Panama.

There’s a lot of controversy around when exactly the Isthmus formed, but it has linked the Americas since at least 3 million years ago and maybe as much as 16 or 23 million years ago. So it’s reasonable to say, given that we have no sea snakes in the Atlantic today or evidence that members of modern groups were there in the past, that it has always been a barrier to the yellow-bellied sea snake’s world domination. They don’t seem to make it through the Panama Canal, either.

Or, at least, not often enough to establish a population in the Atlantic. On the Indian Ocean side, these sea snakes run into Africa. But it’s not the continent itself that really stops them, theoretically, they could just drift around the tip of South Africa.

It’s two things and both of them are water. The first one 200-to-300-kilometer-wide current just off the southwestern coast of Africa that causes upwelling of cold water from the deep ocean. This water is about 13 to 18 degrees Celsius, which is just too cold for the sea snakes to survive for very long, they prefer water that’s between about 23 and 30 degrees Celsius.

And the second thing is that sea snakes must drink fresh water or they’ll dehydrate. Which sounds bonkers, considering true sea snakes literally can’t leave the ocean… so where are they supposed to get fresh water? Weirdly enough, it’s from thin lenses of fresh water floating on the surface of the ocean, like tiny fresh oases in a vast salty desert.

These form when it rains heavily and can stick around for hours to days. And we have physics to thank for them: rain water is less dense than sea water, because it doesn’t have salt and this difference in density allows it to float. And, along with very cold water, the southwestern coast of Africa also gets almost no rain, due to a permanent high pressure zone.

No rain, no sea snakes in the Atlantic Ocean. It’s kind of fitting for a Bizarre Beast to be both highly adapted for life in the ocean and also to have water be one of its main obstacles to having a global distribution. It reminds us that while we might see the ocean as just one big connected body of water, the life that’s evolved to thrive in it doesn’t always see it that way.

The Bizarre Beasts pin club subscription window is open from now until the end of August 14th! Join the club to get this super rad sea snake pin and help us keep making the show. The pins this month are so great!

One of them does glow in the dark, and they all are really cool. And we have something extra special for you this month. A sea snake patch!

This is going to look so flippin' cool on a backpack or jacket. Or, heck, tell people you lettered in sea snakes! We’ll see you next month for more Bizarre Beasts. [♪♪ OUTRO ♪♪]