There are about 550 species of shark in the oceans. Around twelve percent of them glow.
These luminous fish belong to two groups: the kitefin sharks and the lanternsharks. They are little creatures, no bigger than 50 centimeters long, and they feed on small fish, squid, and crustaceans. They also live in the deep ocean, which means that we know very little about how they live. But whatever they’re doing, they’re clearly doing it well. “They’re some of the most successful groups of sharks,” says Julien Claes from the Catholic University of Louvain. One genus alone—the Etmopterus lanternsharks—includes 38 distinct species. “And we discover new ones every couple of years.”
Claes has been studying these elusive fish for a decade, and has been slowly piecing together the purpose of their glow. The light comes from many small organs called photophores, which dot their bellies and sometimes their flanks. No one knows how these structures produce light, but Claes and his team are getting close to knowing why they do so.
First, they showed that the sharks use their light for camouflage. Even though they live at extremely dark depths, any predators watching them from below would still be able to make out their silhouette against the faint remnants of sunlight welling down from above. But the glow from their bellies perfectly matches this downwelling light, and cancels out their outlines. In technical terms, the sharks use counter-illumination. In simple terms, they cast no shadows.
Next, the team showed that at least one species, the velvet belly lanternshark, uses its light as a warning. It has two wicked spines behind its back fins, which are illuminated by a row of unusually placed photophores—Claes called them light sabers. Together with vision expert Dan-Eric Nilsson, he showed the floodlit spines are visible at a distance of 3-4 meters—a perfect distance to ward off an approaching predator, without also advertising the shark’s presence.
Finally, the team thinks that some lanternsharks also talk to one another using light. Most species only have photophores on their bellies, and are probably using those solely to camouflage themselves. But the Etmopterus lanternsharks also have photophores on their flanks, and the patterns vary from one species to another. Perhaps they act as badges of identity, helping the sharks to find others of their own kind. (That is certainly a valuable skill, since several of these species are often found in the same area.)
Claes knew this idea had legs after watching captive velvet belly lanternsharks swimming in a tank. Suddenly, he noticed that the glow from their flanks seemed to turn on and off. “We thought, Oh my god, these sharks can flash!” he says. “Then we realised that it’s an optical illusion.” The photophores produce a very narrow beam that’s only visible from certain angles. And as these sharks swim, they rotate their bodies from left to right, turning what’s actually a steady stream of light into what looks like a strobe. “You really get the feeling that it’s flashing like a firefly,” says Claes. And that, he thought, looked like communication.
“Testing communication in deep-sea animals like glowing sharks is extremely challenging and almost impossible,” he admits. So, his team had to resort to other methods. Working with Nilsson again, they confirmed that the velvet bellies can clearly see each other’s flank markings from a reasonable distance.
Next, they showed that the Etmopterus lanternsharks, with their flank photophores, have diversified into new species far more quickly than those which only have belly photophores. That’s exactly what you’d predict if the glow was indeed acting as a badge of identity. By allowing the sharks to recognise their own species, these markings ensure that they don’t cross-breed. And without cross-breeding, newly diverging species will become even more firmly separated, quickly giving rise to a lush family tree full of new shining branches.
“The very first glowing sharks probably only used the light organs only for counter-illumination,” says Claes. “Then, some genetic change allowed some glowing organs to move a bit up to the side, and that allowed the owners to see members of their own species more easily. When you’re living in the permanent darkness of the deep sea, it’s a big advantage to be able to signal your presence to [others of your kind].”
Reference: Claes, Nilsson, Mallefet & Straube. 2015. The presence of lateral photophores correlates with increased speciation in deep-sea bioluminescent sharks. Royal Society Open Science http://dx.doi.org/10.1098/rsos.150219
More: The Barnacle That Eats Glowing Sharks
Related Topics
Go Further
Animals
- These 'trash fish' are among Earth's most primitive animalsThese 'trash fish' are among Earth's most primitive animals
- These photos are works of art—and the artists are bugsThese photos are works of art—and the artists are bugs
- The epic migration of a 6-foot long, 200-pound catfishThe epic migration of a 6-foot long, 200-pound catfish
- Frans de Waal, biologist who studied animal emotion, dies at 75Frans de Waal, biologist who studied animal emotion, dies at 75
Environment
- Are synthetic diamonds really better for the planet? The answer isn't clear-cut.Are synthetic diamonds really better for the planet? The answer isn't clear-cut.
- This year's cherry blossom peak bloom was a warning signThis year's cherry blossom peak bloom was a warning sign
- The U.S. just announced an asbestos ban. What took so long?The U.S. just announced an asbestos ban. What took so long?
- The most dangerous job? Inside the world of underwater weldersThe most dangerous job? Inside the world of underwater welders
- The harrowing flight that wild whooping cranes make to surviveThe harrowing flight that wild whooping cranes make to survive
History & Culture
- Why Swedish children celebrate Easter by dressing up as witchesWhy Swedish children celebrate Easter by dressing up as witches
- Meet the powerful yokai that inspired the demon king in ‘Demon Slayer’Meet the powerful yokai that inspired the demon king in ‘Demon Slayer’
- A surprising must-wear for European monarchs? Weasels.A surprising must-wear for European monarchs? Weasels.
- Meet the woman who made Polaroid into a cultural iconMeet the woman who made Polaroid into a cultural icon
Science
- LED light treatments for skin are trendy—but do they actually work?LED light treatments for skin are trendy—but do they actually work?
- NASA smashed an asteroid. The debris could hit Mars.NASA smashed an asteroid. The debris could hit Mars.
- Humans really can have superpowers—scientists are studying themHumans really can have superpowers—scientists are studying them
- Why engineers are concerned about aging infrastructureWhy engineers are concerned about aging infrastructure
Travel
- Why you should try beach-hopping by boat around Paxos, GreeceWhy you should try beach-hopping by boat around Paxos, Greece
- Mansion museums show visitors the gritty side of the Gilded AgeMansion museums show visitors the gritty side of the Gilded Age
- 2024 will be huge for astrotourism—here’s how to plan your trip2024 will be huge for astrotourism—here’s how to plan your trip