Sharks may be known as terrors of the sea, but in some cases they're more like night lights. That's because many deep-sea sharks, like the smalleye pygmy shark (Squaliolus aliae), can make their own light, glowing from tail to snout as a possible means of camouflage. Now, a new study shows how this predator, the world's smallest shark, powers its luminescence.
Smalleye pygmy sharks aren't just petite—they grow no more than 22 centimeters long—they're also hard to find, says study co-author Julien Claes, a shark biologist at the Catholic University of Louvain in Belgium. These fish swim hundreds of meters below the water's surface in the Indian and western Pacific oceans. When scientists do manage to pull one of these animals up, they sometimes catch an odd sight: a blue glow coming mostly from the shark's belly.
Claes co-authored a paper in 2009 that showed that a second group of luminescent sharks, called lantern sharks (Etmopterus spinax), trigger their own glow  using two hormones common in many animals: melatonin and prolactin. But it wasn't clear if smalleye pygmy sharks and their close relatives relied on the same molecules.
So Claes and his colleagues launched a second survey, collecting 27 pygmy sharks off the coast of Taiwan. To determine what controlled their unearthly glow, the researchers took patches of the fish's skin and soaked them in various chemicals known to cue luminescence in other species. They then recorded the resulting glow—often so faint that it was tricky to see at a distance even in a dark room—using a light detector. And sure enough, when Claes tried melatonin, which in people helps to control cycles of sleep and waking, voila! There was light, just like in lantern sharks, the group reports online today in The Journal of Experimental Biology. The hormone cues hundreds of thousands of tiny, pore-like organs dotting the sharks' skin to open , revealing cells below that shine brightly with their own, likely chemically produced, light.
But that's where the similarities between lantern and pygmy sharks end. Prolactin—which, among other actions, signals mammals to begin lactating—flips on the glow of lantern sharks. But in pygmy sharks, this hormone acts like an off switch, closing the animals' light-producing organs.
The differences between the fish likely stem from what they use their luminescence for, Claes suggests. When pygmy sharks float up from the ocean bottom to hunt jellyfish and smaller prey, he suspects that they turn on their glow to brighten their undersides. That would make their narrow silhouettes more difficult for predators cruising below to spot against a background of downwelling light—a nifty camouflage trick used by some bony fish. Claes says that pygmy sharks could, theoretically, secrete tiny amounts of prolactin into their bloodstream to slowly dim their glow, precisely matching the levels of natural light around them.
Lantern sharks, on the other hand, need to do double duty. They glow faintly, a feat controlled by melatonin, but can also pulse their lights quickly, possibly to communicate between fish. And they kick-start that strobe-like effect using prolactin. Claes suspects that light production in the first luminescent sharks was more similar to the simpler pattern of pygmy sharks. Then, when the ancestors of modern lantern sharks emerged, they repurposed prolactin, turning it from an off-switch into a second cue to light up. "The pygmy shark, in that sense, is the missing link in the evolution of shark luminescence," Claes says.
"This is a very nice and a very good way to gain information on these hard-to-study animals," says Nicolas Straube, a shark biologist at the College of Charleston in South Carolina. David Ebert, who studies sharks at Moss Landing Marine Laboratories in California, agrees, saying that deep-sea sharks often fail to get the same attention as their more deadly relatives, such as great white sharks. But a growing body of research suggests that these overlooked sharks may be just as interesting, he adds—and possibly more eerie, too.