There’s nothing wrong with this upside down anglerfish. That’s just how they swim

More than two decades ago, researchers captured the first confirmed footage of a deepsea anglerfish. But something about it seemed off. Or, more specifically, upside down.

The female whipnose anglerfish was bobbing around belly-up just above the ocean's floor.

"What wasn't known was: Is this abnormal behaviour?" Andrew Stewart, curator of fishes at the Museum of New Zealand, told As It Happens host Nil Köksal.

Now, after reviewing footage of whipnose anglerfish from around the globe, Stewart and his colleagues can say with certainty that this is more than "one wonky fish doing something unusual."

"There's something more here," he said. "This is the normal behaviour for these fishes."

Their findings have been published in the Journal of Fish Biology.

University of California marine biologist Milton Love has been studying and writing about Pacific Coast fish for decades, and he says he's never seen anything like the videos in this study.

But the deep sea remains vastly unexplored, and advancements in underwater technology are just now starting to shed light on what was once a dark and mysterious ecosystem.

"The more we look at the world of fishes, particularly in parts of the world that have not been well explored, the more cool stuff we will find," Love, who was not involved in the study, told CBC in an email.

"The term 'fishes' encompasses such a massive, perhaps not even closely related, group of animals, that trying to find characteristics that all fishes share, now even including swimming right-side up, is a bit of a challenge."

Stewart and his colleagues reviewed eight instances of female whipnose anglerfish captured swimming upside down in oceans around the world.

And it doesn't appear to be accidental. In one case, an anglerfish that was knocked right-side up by the propellers of a submersible immediately readjusted herself back to her previous, belly-up position.

Asked to explain this odd behaviour, Love was baffled.

"As to theories, shoot, I have none — other than to suggest that this is the kind of behaviour that [Greek goddess] Gaia created after consuming maybe one more cannabis gummy than she should have while working," he said.

A hunting technique?

Stewart and his colleagues also can't say for sure why they do it — but they suspect it has to do with hunting.

Female anglerfish, which grow to about 40 centimetres in length, are fearsome predators who use bioluminescent lures on the ends of their long, dangling snouts to attract curious prey, then gobble them up.

"We believe that the articulation point for that lure, if they were swimming right up and lunged at prey, there's a very good chance the lure would bend back around and into the mouth, and it would be something of an own-goal," Stewart said.

If it is a hunting technique, it seems to be a pretty effective one. Stewart recalls dissecting a whipnose anglerfish specimen and finding an entire gonatid squid in her belly.

"And it wasn't just a little wee baby squid. This prey item was a good half the body length of the anglerfish," he said, adding this species of squid is known for its speedy getaways.

"For an anglerfish, just to sort of be able to sneak up and then lunge quickly enough to grab it. It shows that it's a very successful strategy."

What about the males?

So far, scientists have only observed this behaviour in female anglerfish.

"We don't know what the males do. We think they probably swim probably the, quote, right-side up — maybe," Stewart said.

Male whipnose anglerfish, he says, are much smaller, don't have lures and "are characterized by looking like tadpoles with very large eyes."

They live higher up in the water than females, and only venture into the dark depths when it's time to seek a mate, searching them out by their bright and deadly lures.

"Having found her, they then have to — sort of like a spider — approach and not become a meal because she's probably not going to be too selective as to what's around," Stewart said.

Even if they avoid becoming dinner, the mating process doesn't generally end well for males.

"In some cases, the males become parasitically attached to the females. So they become a lifestyle gigolo. Their blood vessels fuse and they synchronize their maturation and spawning," Steward said.

"Then from all accounts, they just shrivel up and drop off like warts, leaving behind some embedded teeth in the skin as a kind of souvenir of boyfriends past."

While these upside down huntresses may seem grisly, Steward says has a soft spot for them.

"They've been characterized as grotesque and ugly and monsters," he said. "I refer to them as my lovely girls. Because I say they're so ugly, they're beautiful again."