Something fishy is happening thousands of feet under the surface of the North Atlantic Ocean.
In a rare video, captured by the science-focused Rebikoff-Niggeler Foundation and first released as a Science Magazine exclusive, a female fanfin sea devil slowly pitches back and forth in the inky, cold waters 2,600 feet under the surface of the North Atlantic. If you look closely, you can see a tiny male hanging from her belly as a "sexual parasite." (Read: "Anglerfish, Taking Romantic Attachment to a Whole New Level")
Because of the intense underwater environment anglerfishes live in, the mysterious deep-sea dwellers have rarely been seen alive in their natural habitat. After wildlife photographers Kirsten and Joachim Jakobsen captured footage of this fish in August 2016 at the tail end of a five-hour excursion on a deep-sea submersible near Portugal's São Jorge Island, they showed the clip to Ted Pietsch, a University of Washington professor emeritus of the School of Aquatic and Fishery Sciences. More than a year later, the rare footage has been released to the public.
The video, which is part of 25 minutes of total footage, shows a floating fist-size anglerfish surrounded by a wispy cloud of whisker-like fin-rays, or filaments, each emitting bioluminescent light. Identified as Caulophryne jordani, there are 14 female specimens of this fish preserved in natural history collections worldwide, but a live male has never been seen.
"This is a unique and never-before-seen thing," Pietsch says in a press release. "It's so wonderful to have a clear window on something only imagined before this."
About 162 known species of deep-sea anglerfishes can be found in the world's oceans, but the intense environment they live in makes them hard to study. Pietsch says this is only the third time deep-sea anglerfish behavior has been caught on film. (Watch: "Rare Black Sea Devil Caught on Film for the First Time")
Female anglerfishes use a bioluminescent lure, called the esca, that dangles from their snouts to entice prey—about 980 to 16,400 feet underwater. Using their toothy mouths and expandable stomachs, the fishes can swallow prey larger than themselves in a single gulp.
The esca's glow is produced by symbiotic bacteria, and this fishing apparatus can only be found on females. Male anglerfish, which are only a fraction of the size of their mates, use their large, sensitive eyes and nostrils to help them home in on a chemical emitted by females. Once a male finds a female, he will bite down and latch onto her body, where his tissues and circulatory systems will fuse with hers. In exchange for nutrients from the female's blood, the male loses his eyes, fins, teeth, and most internal organs, only serving as a sperm bank for when the female is ready to spawn. (Related: "5 Gross and Amazing Ways Animals Deliver Sperm")
"Once he finds her, he bites on and their tissues fuse," Bruce Robison, a deep-sea ecologist at the Monterey Bay Aquarium Research Institute, told National Geographic in 2014. "I don't think of [anglerfish] as being ugly at all," he adds, responding to their Internet reputation.
Scientists know of this bizarre mating behavior from finding dead males attached to dead females. Deep-dwelling organisms like anglerfishes can't survive in labs, because the massive pressure change and increase in temperature can kill them.
In addition to capturing bizarre mating behaviors, this video also shows body structures that have never been seen on any fish. Whereas membrane-connected filaments in other fish species move as a single unit, the glowing ones on the fanfin sea devil appear to function independently, each equipped with its own set of muscles and nerves. (Related: "Meet the Deep-Sea Devil Fish's New Snaggletoothed Cousin")
"Any prey item touching one of those would cause the angler to turn and gobble up that particular animal," Pietsch tells Science Magazine. "They can't afford to let a meal go by because there's so little to eat down there."
Pietsch says the "light-show" the female emits could be a method to attract prey, or to trick would-be predators into thinking the anglerfish is much bigger than it is. He adds the technique could also be a way of mimicking venomous jellyfish to avoid being eaten.
The submersible the Jakobsens used to film the anglerfish has been in operation since 2013. With such sparse research on this curious species, there's more to discover in the future.
A previous version of this story incorrectly stated there were more than 200 species of deep-sea anglerfishes. There are more than 200 anglerfish species, but only 162 live in the deep sea. This story has been updated.