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.