Shark researchers working off the eastern coast of New Zealand have made an illuminating discovery. In a new study, published in the journal Frontiers in Marine Science, the scientists found that three species of deep-sea shark are bioluminescent, producing a soft blue-green light with specialized cells in their skin.
One of the species, the kitefin shark, grows to a length of nearly six feet, making it the largest known bioluminescent vertebrate. Giant squid, which get much bigger, are also known to produce light.
Bioluminescence had previously been documented in only around a dozen shark species, so this discovery significantly adds to our knowledge of how prevalent the phenomenon is in these and other marine animals, says Jérôme Mallefet, a research associate at the Université Catholique de Louvain in Belgium and lead author of the new study.
The expedition involved hauling up a small number of blackbelly lanternsharks, southern lanternsharks, and kitefin sharks from the ocean’s twilight zone, a scantly illuminated region that extends from a depth of 660 to 3,300 feet.
When Mallefet obtained a kitefin shark and saw it light up aboard the research vessel, he was overwhelmed. “I nearly cried when I saw it ... it was so exciting,” he says.
The other two species are somewhat smaller than the kitefin, and all are occasionally caught as unintended bycatch by fishers. None are considered vulnerable to extinction, but little is known about their lifestyles and biology.
Diva Amon, a deep-sea biologist and National Geographic emerging explorer who was not involved in the study, says she was “awestruck” by the discovery. “These findings remind us about how much we still have to discover and understand about the deep ocean and its inhabitants,” she says.
A glow in the dark
In January 2020, Mallefet and a team of scientists from the Université Catholique de Louvain and New Zealand’s National Institute of Water and Atmospheric Research spent a month aboard a deep-sea trawler. After catching live sharks in nets, the researchers placed the animals in a seawater tank, took them to a dark room, and watched for signs of bioluminescence.
As a result of stress or possibly shyness, only a handful of kitefin sharks, blackbelly lantern sharks, and southern lantern sharks flaunted their blue-green glow for the researchers, and those that did had their skin sampled for analysis.
Most bioluminescent organisms including fireflies have specialized cells or organs that contain certain chemicals, including a compound called luciferin, that interact with oxygen to produce light. Some luminous creatures, such as the deep-sea angler fish, get their glow by serving as host to colonies of bioluminescent bacteria. [Learn more about how bioluminescence works in nature.]
Bioluminescent sharks emit light from specialized cells on their skin known as photocytes, but exactly how they do this has long been a mystery. When Mallefet and his team analyzed the skin samples they had collected, they did not find any traces of luciferin or bioluminescent bacteria.
They did, however, confirm that these three species control their light emissions in part by using hormones. For example, the chemical melatonin, a hormone that helps govern diurnal cycles and induce sleep in mammals, induces luminescence in photocytes.
Why light up?
In the deep sea, where scientists estimate three-quarters of all creatures are bioluminescent, having the ability to create light can be extremely advantageous. Deep-sea animals use bioluminescence to do everything from attracting prey to deterring predators. Being bioluminescent can even help deep-sea animals camouflage themselves.
In the ocean’s twilight zone, which receives minimal amounts of sunlight, bioluminescent animals can hide their silhouette from predators lurking below by producing enough light to match their surroundings—a trick known as counter-illumination. All three species examined in this study had large concentrations of photocytes on their undersides, which suggests that these sharks may hide from predators—thought to include primarily larger sharks—in just this way.
However, the sharks also had photocytes on other parts of their bodies, so counter-illumination may be just one of many ways these animals use their bioluminescence. For example, the researchers found dense concentrations of photocytes on the dorsal fins of the kitefin sharks. The reason for this is unknown, but Mallefet speculates that it might help kitefin sharks communicate with one another.
The discovery that these three species produce light “is not surprising,” says David Ebert, director of the Pacific Shark Research Center at Moss Landing Marine Laboratories in California, but it is still exciting.
That’s because researchers think many more species of sharks are likely capable of producing light—Mallefet estimates that perhaps 10 percent of the 540 known species of shark are bioluminescent. But Ebert thinks even this is a conservative guess. As the field of deep-sea shark research advances, “I think that number will go even higher,” he says.
Both Ebert and Mallefet hope that more attention will be paid to deep-sea sharks in the future, as the creatures and their habitat are understudied and under threat.
“A lot of people know that sharks can bite, thanks to Jaws,” says Mallefet, “but few people know that they can glow in the dark.”