The anglerfish, with its bulbous head, beady eyes and sinister gape, is basically the posterchild for freakish, deep-sea adaptations. Rarely observed in their natural habitat, deep-dwelling anglerfish have proven tricky to study and much of what we know about the creatures is deduced from dead specimens hauled up in fishing nets. However, a new video filmed near Portugal’s Azores islands gives us some remarkable insights into the lives of these deep-sea denizens.

The footage shows a female anglerfish drifting in a deep-cyan ocean - an array of whisker-like projections shooting from her body and forming a kind of swaying forcefield around the fish. A close-up reveals that she's not alone: a much smaller male can be seen clinging to her underbelly, fused in a reproductive ritual biologists call "sexual parasitism".

Many species of anglerfish are known to form permanent pair bonds in which the male of the species bites into his mate and eventually fuses with her tissue. Once attached, the male receives protection as well as vital nutrients via her circulatory system, while the female gets a steady supply of sperm courtesy of her devoted cling-on. Although scientists were already aware of this quirky reproductive strategy – some dead specimens were found with mini-males merged to the bodies of females – the new video is the first to feature a sexually united pair alive in the wild.

“I’ve been studying these [animals] for most of my life and I’ve never seen anything like it,” Ted Pietsch, a deep-sea fish researcher at the University of Washington in Seattle, told Science Mag about the new video. “It was really a shocker for me."

A screenshot from the new video shows the male fused to the underbelly of the female anglerfish. Image: Kirsten and Joachim Jakobsen/Science Mag

The footage was captured by deep-sea explorers Kirsten and Joachim Jakobsen back in 2016 via a specially designed submersible. The husband-and-wife duo had already spent some time filming a deep-sea wall on the south side of São Jorge Island about 800 metres below the surface when the odd creature attracted their attention. They followed the anglerfish for about 25 minutes, maneuvering the craft as best they could to capture footage of the 16-centimetre fish.

The species was later identified by Pietsch as Caulophryne jordani, or the fanfin angler. Similar to other deep-sea anglerfish, the female sports a bioluminescent "lure" attached to the front of her head that's thought to attract unsuspecting prey. In addition to the alluring headpiece, fanfin anglers are also enveloped in a slew of spaghetti-like structures called filaments and fin rays. “Any prey item touching one of those would cause the angler to turn and gobble up that particular animal,” explains Pietsch. “They can’t afford to let a meal go by because there’s so little to eat down there.”

In another first for science, the video also seems to show bioluminescence at the tips and along the lengths of some of the fin rays and filaments. Although, it's unclear whether the structures are simply reflecting light from the submersible, Pietsch suspects that the glow may be produced by the creature itself, marking the first time that this behaviour has been recorded.

Also surprising for deep-sea ecologists was the flexibilty of the male cling-on, which seemed to be able to move around freely despite being firmly affixed to his female mate. “There’s no way I would have ever guessed that from a [museum] specimen,” Bruce Robison, a Monterey Bay Aquarium Research Institute ecologist, told Science Mag.

While the new video does helps shed some light on the little-studied habits of the mysterious anglerfish, there is still much we do not know. As advances in submersible technology continue to improve, it's likely that we'll be seeing a whole lot more of these deep-sea dwellers in the not-too-distant future.


Header Image: Kirsten and Joachim Jakobsen/Science Mag