Deep-sea anglerfish are a strange and fascinating sight to behold. Their mouths are gaping and full of sharp fangs, their bodies are dotted with tendril-like filaments and fin-rays, and their heads are crowned with a dangling, bioluminescent lure that draws unsuspecting prey straight to their jaws. But because anglerfish swim in the cold, dark depths of the ocean, they are scarcely seen alive in their natural environment.
Fortunately, as Katie Langin reports in an exclusive for Science, a pair of deep-sea explorers captured rare footage of a female anglerfish floating through the ocean. Even more remarkably, she had a male anglerfish attached to her belly, marking the first time that the creature’s odd mating habits have been caught on film.
Explorers Kirsten and Joachim Jakobsen were cruising through the deep waters around Portugal’s Azores islands in a submersible when they spotted “something with a funny form,” Kristen tells Langin. They filmed the fish and her mate for 25 minutes. After the explorers surfaced, they sent their video to Ted Pietsch, a leading deep-sea fish expert at the University of Washington in Seattle. Pietsch identified the critters as belonging to the Caulophryne jordani species (also known as the fanfin seadevil). According to Elaina Zachos of National Geographic, there are 14 specimens of female C. jordani preserved in museum collections, but a live male had never been observed before.
“This is a unique and never-before-seen thing,” Pietsch says in a University of Washington statement. “It’s so wonderful to have a clear window on something only imagined before this.”
In Jakobsen’s video, the female anglerfish, which is about six inches long, drifts through the water. A much smaller male dangles beneath her. Though they are tiny, male anglerfish have relatively large eyes and nostrils, which help them detect a chemical attractant that females emit. When a male zeroes in on a partner, he bites down on her and will not let go. The pair’s circulatory systems and tissue fuse, and the male becomes a “sexual parasite.” He survives on nutrients in the female’s blood, and in exchange, he provides the female with sperm when she is ready to spawn.
Scientists already knew that anglerfish mated in this way; dead male anglerfish have been found attached to dead females. But the Jakobsens’ video allows researchers to observe anglerfish in their natural habitat. Scientists were able to see, for example, that C. jordani has an unusual body structure that has not been observed before. While the filaments and fin-rays of most other fish move as a single unit, those of C. jordani move independently, each one contains its own set of muscles and a long nerve.
“Any prey item touching one of those would cause the angler to turn and gobble up that particular animal,” Pietsch tells Langin of Science. “They can’t afford to let a meal go by because there’s so little to eat down there.”
In the video, the anglerfish’s filaments and fin-rays also appeared to be glowing. It is possible that the structures were reflecting the light of the Jakobsens’ submersible, but Pietsch believes the glow was bioluminescent, or emitted by the fish itself. In the University of Washington statement, he speculates that this “light show” might attract prey, or make the fish appear larger to predators. The glowing appendages might also mimic the stinging tentacles of a jellyfish, warning predators to keep away.
Anglerfish cannot survive in laboratory conditions; they are unable to adapt to pressures and temperatures that are so vastly different from their natural environment in the deep ocean. So the Jakobsens’ footage is crucial to scientists' understanding of these strange and elusive creatures.
“I have spent hundreds of hours staring into deep waters, but this is one of the most amazing video footage I have seen to date,” Antje Boetius, a biological oceanographer at the Alfred Wegener Institute in Germany, said in the statement. “It brilliantly shows the otherness of deep-sea life, and how important it is to observe these animals in their own realm, to understand their behavior and adaptation.”