The mesopelagic or “twilight zone” of the ocean just had a glow up, literally. Three bioluminescent shark species were found off the eastern coast of New Zealand by shark researchers—including one that is now the largest-known luminous vertebrate. Their study was published in the journal Frontiers in Marine Science.
The mesopelagic zone ranges from a depth of from 660 to 3,300 feet, where only the tiniest amount of sunlight can reach. Bioluminescence—the production of visible light through a chemical reaction of luciferin with oxygen by living organisms—is a widespread phenomenon among marine animals like jellyfish, lanternfish, and squids. But this is the first time it has been documented and analysed in the kitefin shark, the blackbelly lanternshark, and the southern lanternshark.
Jérôme Mallefet, a marine biologist, research associate at the Université Catholique de Louvain in Belgium and lead author of the new study, described being “overwhelmed” when he saw the kitefin shark (or the Dalatias licha) light up in the research vessel. “I nearly cried when I saw it ... it was so exciting,” he told National Geographic in an interview. “A lot of people know that sharks can bite thanks to Jaws but few know that they can glow in the dark.”
One of the sharks, the kitefin shark can grow up to nearly six feet in length, making it the largest known bioluminescent vertebrate. The other sharks—the blackbelly lanternshark and the southern lanternshark—are much smaller than the kitefin and sometimes accidentally caught by fishers. They are not vulnerable to extinction. While specimens had previously shown that the kitefin shark should be capable of producing light, they are “really difficult to observe” because of how deep into the ocean they live, Mallefet said.
Mallefet and a team of scientists from Belgium’s Université Catholique de Louvain and New Zealand’s National Institute of Water and Atmospheric Research spent a month aboard a deep-sea trawler in January 2020. The team would catch sharks in a net and then place them in seawater tanks in a dark room where they would observe them for signs of bioluminescence. Only a handful of kitefin sharks, blackbelly lanternshark and southern lanternshark showed signs of bioluminescence. Samples of their skin were later dissected and analysed so that the researchers could know more on what makes their flashlight-like organs glow.
Scientists are still unsure why exactly the sharks glow. Animals in the mesopelagic zone glow to attract a mate, lure prey, camouflage or to form schools. Scientists suspect the sharks glow to camouflage themselves. Since the bioluminescent patterns are found only on their bellies and undersides, the sharks are virtually invisible to predators from certain angles. From these angles, their blue-green stripes would blend into the blue sea and sky; this phenomenon is known as counterillumination. But their bioluminescence also leaves a danger. The sharks could be exposed to predators since they appear backlit from below to the predators in the mesopelagic zone.
Mallefet confirmed to The New York Times that the sharks don’t use their nervous system to light up, in contrast with other bioluminescent animals. Instead they use the hormone melatonin. “It makes us fall asleep,” Mallefet said, “but it’s lighting up the shark.”