The myth of the mermaid has endured for centuries. In Ancient Syria, people worshipped Atargatis, a half-human, half-fish goddess who ruled over fertility. Then in Ancient Greece, sailors would set out to sea in fear and lust of the siren, who, if given the chance, would sing them to their deaths.

And of course, there is Hans Christian Andersen’s 19th-Century fairy tale The Little Mermaid, which has been adapted by Disney yet again, this time in live action. But how much truth is there in the myth? Does the idea of humanoid fish (or is it fishy humanoid?) have any legs? Or is it just fishful thinking?

“First of all, we know that it’s possible to move around in a mermaid-style fashion,” says marine biologist Dr Helen Scales.

“People impersonate mermaids, and I’ve done the equivalent, which is using a mono-fin. Unlike bi-fins, with a fin on each foot, you put your feet together in one big tail. It’s a really good way of propelling yourself around quickly.”

The problem, says Scales, is not necessarily with the fish part of the mermaid but with the human torso (at least, how it appears in The Little Mermaid), which has not evolved enough to survive for long periods of time underwater.

Read more:

Why real-life mermaids would struggle to breathe

“My concern is respiration,” says Scales. “If they need to swim to the surface to take a breath, they would need to have something more akin to a blowhole than a nose and mouth. This is why crocodiles have their nostrils higher on their heads, so it’s easier for them to breathe on the surface.”

If mermaids were to live a solely aquatic life, however, they would either need gills (which they don’t seem to have) or, according to Scales, to have evolved some way of supplying themselves with oxygen.

“The anatomies of marine mammals are adapted to surviving in the ocean for long stretches of time,” she says. “If a human diver goes underwater, their lungs halve in size every 10 metres from the pressure. Sperm whales have a special rib cage to protect their lungs when this happens.

More like this

"They also have a lot of myoglobin in their muscles and in their blood, which has a much higher affinity for oxygen. A mermaid would have to have something similar.”

There is also the fact that the human torso, with its gangly arms and intricate fingers, might serve us well on land (they do, after all, allow us to make tools and eat pizza), but would put mermaids at a disadvantage compared to their peers.

“Arms are not particularly good for swimming, hence why whales have evolved flippers,” says Scales. “When I used the mono-fin, you can really feel the drag of your arms.”

The classic teardrop shape of a fish is the most hydrodynamic, adds Scales. “If mermaids had a longer evolutionary history, I would say something would have to happen to their arms,” she says.

“Unless, of course, there is some evolutionary reason they have arms. There could be something about their environment that makes gathering food a priority over moving quickly!”

According to Scales, the mermaid’s mixture of human and marine anatomy ultimately offers the worst of both worlds, and it wouldn’t make much of a difference if they were reversed, with human legs on the bottom, and a fish head on top.

“The tail is a great way to propel around,” she says. “But if they wanted to breathe, a fish front-half with gills would be very handy.”

But even then, how would either of their exposed human torso or flailing human legs deal with the icy cold of a winter ocean?

“They would need quite thick blubber,” offers Scales. “Either that, or they could go down the sea otter route and use fur.”

Coming soon, to a cinema near you: The Hairy Mermaid.

About our expert, Dr Helen Scales

Dr Helen Scales is a marine biologist, broadcaster and science writer. She is the author of Spirals In Time and The Brilliant Abyss. Holding a Cambridge PhD in marine biology, Scales has logged more than 300 hours underwater as a scuba diver. She has appeared on several BBC Radio 4 shows including The Life Scientific and The Museum Of Curiosity

Read more:


Stephen Kelly is a freelance culture and science journalist. He oversees BBC Science Focus's Popcorn Science feature, where every month we get an expert to weigh in on the plausibility of a newly released TV show or film. Beyond BBC Science Focus, he has written for such publications as The Guardian, The Telegraph, The I, BBC Culture, Wired, Total Film, Radio Times and Entertainment Weekly. He is a big fan of Studio Ghibli movies, the apparent football team Tottenham Hotspur and writing short biographies in the third person.