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Sea snakes are the most common group of marine reptiles in the entire ocean, with almost seventy individual species.
But despite their impressive diversity, none of them live in the Atlantic Ocean.
Which is weird, and also nice! There’s a whole ocean with no snakes in it!
But also there’s an entire ocean of snake food and living space out there, so what’s going on?
The answer lies in their evolution, their biology, and a weirdly “dry” part of the ocean.
The first reason is deceptively simple: modern sea snakes didn’t evolve in the Atlantic.
All sea snakes alive today evolved in a part of the Pacific known as the Coral Triangle and nearby regions,
with most species evolving in the last two and a half million years.
So they might not have started in the Atlantic, but they’ve had millions of years to theoretically move there and establish new populations.
But they haven’t, in part because it’s a pretty big ocean for such little reptiles.
Only one species of sea snake, the yellow-bellied sea snake, lives in the open ocean.
Most of the others only live in near-shore shallows.
The yellow-belly is the only species that would have much chance of making it to the Atlantic.
But each of the different ways it could even hypothetically reach the Atlantic has its problems.
Like, going around the tip of South America or Africa might be too cold.
Yellow-bellied sea snakes need temperatures above 20 degrees Celsius in order to be toasty enough to breed.
And too far below 18 and they’ll flat out die.
Given an annual mean ocean temperature of less than 17 degrees off the coast of South Africa, that’s a no-go.
Yellow-bellied sea snakes actually can live directly south of Africa, but it might be that the ocean is too cold just west of there,
so they still couldn’t make the whole trip.
Oddly enough, going around Africa might also be too dry.
Sea snakes can’t drink salt water.
Instead, they drink a thin layer of rainwater that floats on the surface of the ocean after storms.
Yes, this is a thing. That’s wild.
But the southwest coast of Africa includes the Namib Desert with its infamous Skeleton Coast.
This region may go years without rain, even out at sea.
Sea snakes going this way could become dehydrated and die.
And they might not have any more luck trying to go the other way, where North and South America are the obstacle.
Millions of years ago, there was an open corridor between the Pacific and the Atlantic in what is now Panama—
a much warmer route than rounding the southern tips of Africa or South America.
Trouble is, the isthmus of Panama formed long before sea snakes evolved,
blocking them from simply swimming from one ocean to the other.
We humans have opened that passage back up in the form of the Panama canal.
And there have been reports of individual sea snakes on the Caribbean coast of Colombia—which presumably made the trip through the canal.
Even so, scientists think it’s unlikely they’ll establish an actual breeding population via this route.
So in the end, even though it might seem like the Atlantic could be a nice home for the sea snakes with warm water and tasty prey,
their biology and evolutionary history put that possibility pretty much out of reach.
If you’re the kind of person who wonders why certain reptiles never moved to certain parts of the ocean—well, you are as curious as we are.
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