Why Does a Shark Glow in the Dark?

glow in the dark shark

Sharks in the deep sea are not usually known to glow. But a team of researchers has just filmed the first-ever glowing shark, a species called the kitefin, living in the dark mesopelagic zone near New Zealand. While the discovery was hailed as “amazing,” it’s still not entirely clear why these creatures shine. Fortunately, researchers can look to other luminescent ocean animals for clues: jellyfish, anglerfish, coral, and some squid are among the marine organisms that use bioluminescence to paint their dim underwater world.

These ocean critters have been using their bioluminescence for at least a couple of hundred million years, scientists say. They’re all part of a group of organisms that generate light through a chemical reaction, or bioluminescence, that relies on proteins called luciferins and luciferases. The precise recipes for lighting up differ between organisms, but these two chemicals are common to all luminous organisms. It’s as if Nature has tinkered around with this process to come up with some incredibly diverse ways to produce light.

For these sharks, the glow is generated from a special type of cell in their skin. This cell creates green light by absorbing blue photons and releasing a brighter color. The sharks also produce a reddish-orange color that is likely produced by other cells. The sharks’ glow may help them camouflage themselves from predators looking down on them in the mesopelagic zone, where they can be as deep as 1,000 meters below the surface, the scientists say.

Kitefins, which can reach a length of 180 centimeters, are the largest luminous vertebrates ever discovered. But it’s not clear whether they’re using their glowing underbellies to camouflage themselves or to illuminate the ocean floor as they hunt for prey, 9News reports. The team from Universite Catholique de Louvain and University of Hawaii suspects that the sharks may be using their bioluminescence to signal other members of their species, as other luminescent deep-sea animals do.

The team’s findings were published in iScience this week. The authors of the study describe a process they call fluorescence that allows these sharks and other marine organisms to turn blue light into other colors. To figure out why these sharks glow, they analyzed the different chemicals in their skin. They found that the light-emitting metabolites in their light skin were based on brominated tryptophan and kynurenine, which are similar to the small molecules used by other luminous ocean animals to turn blue light into other colors.

Marine biologist David Gruber at City College of New York explains to CNet that he and his colleagues have built a camera that mimics how fluorescent sharks see each other. They used it to film swell sharks and chain catsharks in the wild. The shark-eye cameras are a great way to experience the world as these fish do, but the sharks’ glowing green underbellies still don’t explain why they glow. They might be using the light to communicate with other sharks, but it could be that their glowing just helps them blend into their surroundings.