Curious Kids: why do frilled sharks look more like sea snakes?

Why do frilled sharks look more like sea snakes? Hi Will. Thanks for this question! Frilled sharks are very strange looking sharks. You’re right – they look much more like an eel or a sea snake than a shark.

Both are quite long and skinny, with adults growing to about two metres. You’ll find frilled sharks have their fins a long way back towards their tail, which adds to their weird shape. They’re certainly not very sharky looking! A ‘living fossil’ Frilled sharks are considered “living fossils”, because they haven’t changed for about 80 million years! They get their name from the frilly gills on their throats, which look a bit like lace. They have six pairs of gills which they use to breathe under water.

There are two species of frilled shark. Both might look like sea snakes, but they’re actually very different up close. For one, frilled sharks have gills to breathe under water, while sea snakes have to come to the surface to breath air into their (one) lung – but they’re amazing at holding their breath.

Also, frilled sharks have fins, and snakes have no arms or legs at all. And snakes have a bony skeleton, whereas shark skeletons are made of cartilage (like what you’ve got in your nose).

And while both have loads of sharp and pointy teeth, sea snakes are highly venomous – and frilled sharks are not.

Life as a frilled shark

Frilled sharks are rarely seen in the wild, so we don’t know that much about them. Although they are occasionally caught in fishing nets since they like to live in places with lots of fish.

During the day frilled sharks rest on the bottom of the ocean, but as night approaches they swim close to the surface to chase prey such as octopus, squid and fishes. While swimming, they bend their body like an eel.

Their mouth is full of needle-like teeth that they use to grab their prey, which they swallow whole! Baby frill sharks hatch in an egg inside their mother’s tummy and keep growing until they are ready to be born. This takes about three and a half years, which is more than four times longer than a human baby takes, and possibly the longest of any animal.

A large female can have up to 15 babies, or “pups”, which are about 50cm long when they are born. Scientists think frilled sharks live for about 25 years, but no one knows for sure.

What about sea snakes?

Sea snakes are found in warm, shallow waters around coral reefs near Australia and New Zealand. They’re closely related to venomous land snakes in Australia.

Snakes have a funny history if you look a long way back in time, because their ancestors originally lived on the land and looked a bit like a goanna. On the other hand, the ancestors of frilled sharks were always in the ocean.

Snakes’ ancestors then started to live in the water, where they got their snakey shape: they lost their legs and arms and began to swim like eels.

Eventually, they came back to life on land, and to this day most snakes still move on land the same way they used to in the water – slithering from side to side.

But at some point, sea snakes decided to go back to live in the water again, where they still slither around today. I guess they couldn’t make up their minds!

(By Culum Brown, Macquarie University Sydney. The Conversation)