Kitefin sharks (Dalatias licha) have been known since the 18th century, but it wasn’t until January 2020 that scientists first saw them glow in the dark. They’re not the only bioluminescent sharks – around one in 10 species have this ability – but up to 1.8 meters, kites are by far the largest that have been found.
Jérôme Mallefet, a biologist at the Catholic University of Louvain, Belgium, had hoped for years to see a shiny kite shark. “It was one of my holy grails, seeing that big shark shine,” he says. Studies in the 1980s suggested they had light-emitting organs in their brown skin, but no one had seen one alive and illuminated.
His dream came true 1,000 miles east of New Zealand aboard a fisheries research vessel that used a massive commercial-scale trawl to study species inhabiting the twilight zone, about 800 meters deep. Previous studies of the area have consistently captured wing sharks, but Mallefet was the first person to carefully transfer them to a darkened room. With the lights dimmed, he saw that the live kite sharks had a blue-green glow on their bodies.
The smaller species of deep-sea sharks use their lights to survive and to communicate with each other. Some have spines on their backs that light up like lightsabers, warning intruders to leave them alone. Some males light up their clasps, the shark equivalent of a penis, a trick that presumably helps attract mates. And many sharks have glowing bellies, which help them hide their silhouette from predators lurking beneath them in the depths, blending in with the blue light from the surface.
But kite sharks are so big that they don’t have to worry about hiding from other predators.
Mallefet believes their shiny undersides can light up the seabed to help them hunt, while letting them sneak up on their prey unseen.
It would also explain how kitefins have been found with much faster swimming sharks in their stomachs. Kites are among the slowest of all sharks; at their normal cruising speed, it would take them over 10 minutes to travel 100 meters. These large, slow-moving sharks likely glide around the twilight zone and launch surprise attacks on bottom-laying prey. “Like a cat jumping on a mouse,” says Mallefet.
To produce light, sharks have tiny cup-shaped structures, called photophores, dotted across their skin. In the center of each cup are light-emitting cells and a lens on top directs light out. Bioluminescence in sharks is controlled by hormones.
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“They use at least two or three hormones to trigger or slow down the emission of light,” Mallefet explains. It’s a slow process and the sharks can glow for an hour or two. “Once they light up, they can’t change quickly,” he says.
One aspect of shark bioluminescence that remains enigmatic, including for kitefin sharks, is how exactly they produce light. Many glowing animals harbor symbiotic bacteria in their bodies that act as glowing partners, but none have been found in sharks. Other luminous creatures produce chemicals that perform light-emitting reactions.
In sharks, these also prove difficult to track. “It’s still a mystery,” said Mallefet.