Glow-in-the-Dark Sharks in Deep Seas Hunt Using their Glowing Belly

According to a recent study, sharks that glow in the dark and live in deep waters use their glowing bellies to see the ocean floor when they are hunting for prey.

Although kitefin sharks, Dalatias licha, have been studied since the 18th century, scientists didn't observe one for the first time until January 2020. Although one in ten species of sharks can glow at night, kitefins are by far the largest species to date, growing to a maximum length of 1.8 meters.

Glow-in-the-Dark Kitefin Shark

Jérôme Mallefet, a biologist at Belgium's Université Catholique de Louvain, had been hoping for years to see a glowing kitefin shark. To see that massive shark glowing, he claimed, was one of his holy grails. They may have organs that emit light in their brown skin, according to studies from the 1980s, but nobody had ever seen one that was both lit up and alive.

His dream came true while aboard a fisheries research vessel that was surveying species that live 800 meters below the surface in the twilight zone while traveling 1,000 miles east of New Zealand. Kitefin sharks were frequently caught during earlier surveys, but Mallefet was the first one to carefully transport them into a dimly lit space. He noticed that the live kitefin sharks emit a blue-green glow, which covered their bodies when the lights were dim.

Other Glowing Sharks

Deep sea sharks of smaller species use their lights to communicate with each other and to stay alive. Some of them have streaks on their backs that glow like lightsabers, signaling intruders to stay away. Some males use a trick to light up their claspers, which are sharks' versions of penises, to entice females. Additionally, many sharks have glowing stomachs, which enable them to blend into the blue light coming from the surface and conceal their silhouettes from lurking predators beneath them in the deep.

However, because they are so large, kitefin sharks don't have to worry as much about trying to hide from other predators.

Kitefin Advantage

According to Mallefet, their glowing undersides may help them hunt by illuminating the seafloor and enabling them to approach prey covertly.

This would also explain why kitefins have been discovered with sharks in their stomachs that can swim much more quickly than they do. As one of the slowest shark species, kitefins would require well over 10 minutes to move through 100 meters at their regular cruising speed. These large, sluggish sharks are likely gliding through the shadows, making surprise attacks on their prey which is usually lying on the bottom. According to Mallefet, this is similar to a cat jumping on a mouse.

Photophores and Hormones

Sharks' skin is covered in tiny cup-shaped structures called photophores that generate light, as shown in the presentation published in Research Gate. Each cup contains light-emitting cells in the center, and a lens on top focuses the light outward. Shark bioluminescence is regulated by hormones.

According to Mallefet, sharks use at least two or three hormones to start or stop light emissions. Sharks can glow for an hour or two because it is a gradual process. Once lit, they are unable to change quickly.

How exactly sharks produce light is one aspect of their bioluminescence that is still unknown, including for kitefin sharks. Sharks have not been found to contain the symbiotic bacteria that glow alongside other glowing animals. Chemicals are produced by other glowing creatures that emit light.

Finding out what causes sharks to glow is proving to be a difficult task, according to Mallefet, and it is still a mystery, The Guardian reports.

The study by Mallefet and his colleagues was published in the journal Frontiers in Marine Science.