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