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A video still shows a female anglerfish with whiskery fin-rays glowing in the deep-sea dark. The rays may be bioluminescent, or they may be reflecting light from a submersible’s lamps. The male of the species can be seen hanging from the female’s belly. (Rebikoff-Niggeler Foundation Photo)

Scientists studying deep-sea anglerfish have long known about the bizarre mismatch between the species’ whiskered females and teeny-tiny males. But they’ve never captured video of live fish mating — until now.

A newly released video, captured by researchers Kirsten and Joachim Jakobsen during a five-hour dive in a submersible off the Azores in the mid-Atlantic, documents the sexual hookup for the first time.

Ted Pietsch, a University of Washington professor emeritus of aquatic and fishery sciences and curator emeritus of fishes at the Burke Museum of Natural History and Culture, was stunned by the footage.

“This is a unique and never-before-seen thing,” Pietsch said in a UW news release issued Thursday. “It’s so wonderful to have a clear window on something only imagined before this.”

Pietsch is one of the world’s foremost experts on anglerfish, having described and named more than 70 of the fish family’s species. So it’s natural that the Jakobsens asked him to identify the mated pair. (They’re Fanfin Seadevils, a.k.a. Caulophryne jordani.)

About 160 anglerfish species have been identified: The females are about the size of a fist, and festooned with fin-rays. The whiskery structures are dotted with points of light. One especially ornate structure dangles in front of the fish’s mouth, serving as a bioluminescent lure to attract prey. That’s why the creatures are known as anglerfish.

The male of the species is only a fraction of the size of the female. When it’s time to mate, he homes in on the scent of a female and bites into her belly. From then on, the male is a “sexual parasite,” permanently attached and totally dependent on the female. The female provides the male with nutrients through their fused tissues, and the male provides sperm in return.

The fish are rarely seen in their natural habitat. Scientists figured out how the sexual hookup works by studying dead anglerfish dredged up from the deep, but the Jakobsens’ 25-minute video provides the first live look at the arrangement.

“I have spent hundreds of hours staring into deep waters, but this is one of the most amazing [pieces of] video footage I have seen to date,” Antje Boetius, a biological oceanographer at the Alfred Wegener Institute in Germany, was quoted as saying. “It brilliantly shows the otherness of dee-sea life, and how important it is to observe these animals in their own realm, to understand their behavior and adaptation.”

The video was recorded at a depth of about 2,600 feet in August 2016 from the LULA1000, a research submersible operated by the Rebikoff-Niggeler Foundation. It first came to light in a report published online by the journal Science.

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