This strange sea serpent brings death from above

Bec Crew is a Sydney-based science communicator with a passion for weird and wonderful animals. From strange behaviours and special adaptations to newly discovered species and the researchers who find them, her stories celebrate how strange and yet relatable many of the creatures living among us can be.

Image credit: William Flaxington

Meet the yellow-bellied sea snake.

YOU KNOW YOU’RE DOING SOMETHING RIGHT when you’re the most widely distributed snake in the world, with a range spanning the entire tropical Indo-Pacific, all the way to Costa Rica, southern California, and northern Peru.

And what about being the only sea snake in the world to have reached the Hawaiian Islands, and one of only two sea snakes to have reached New Zealand?

You also know you’re doing something right when you manage to survive the cold waters off the coast of Tasmania, when El Niño events or severe storms keep you away from the more comfortable temperatures of Australia’s east and west coasts.

And did we mention it’s the most aquatic snake in the world, having never developed the ability to survive on land?

Meet the yellow-bellied sea snake (Hydrophis platurus), an absolutely powerful reptile that you don’t want to mess with.

Males can grow up to 720 mm long and females 880 mm long, and have a beautiful dandelion-yellow belly, contrasting with a deep black, but almost blue, back.

Their paddle-shaped tails, which they use to propel themselves through the ocean, combine these two colors in a rather cheerful polka dot pattern:

(Image credit: Carpenter0/Wikimedia)

If these creatures look intimidating, that’s because they are: Yellow-bellied sea snakes have no natural predators and are incredibly venomous, with toxins that can damage skeletal muscle, cause paralysis, cause acute kidney failure, and can be fatal if left untreated.

The good news is that there has never been a recorded death in Australia due to a bite from a yellow-bellied sea snake, even though they are quite common in our coastal waters. In fact, this species is the most common sea snake on Perth’s beaches.

Fortunately, these skilled predators save their venom for hunting: a defensive bite won’t have the same toxic impact as an aggressive bite, which is great for us humans and bad news for the yellow-bellied sea snake’s prey.

Yellow-bellied sea snakes spend their entire lives underwater (eating, sleeping and breeding in the ocean) and feed primarily on fish, which they immobilize with a venomous bite.

But a newly discovered subspecies of the yellow-bellied sea snake (pictured below) does it even better. At night, it hangs upside down in a coiled position, just below the water’s surface, and attacks any unsuspecting fish that venture below.

(Image Credit: Brooke Bessesen)

This hunting technique has never been seen before in another aquatic species, and is unique to this strangely yellow subspecies, called Hydrophis platurus xanthos, from Costa Rica.

While yellow-bellied sea snakes are not interested in getting close to humans, they do sometimes wash up on the beach and that is when they are most dangerous to us.

Because even a weak or dead-looking snake can attack if you try to move it, so never approach a sea snake that has washed up on the shore. Call your local wildlife service so they can safely return it to the water.

Here’s a yellow-bellied sea snake in all its glory, hunting, swimming, and tying knots: