In what may sound like soggy logic, the smalleye pygmy shark hides in the dark by lighting up, a new study says.
The research is helping to illuminate one of sharkdom's biggest evolutionary puzzles: how some species came to shine in the first place.
"More than 10 percent of currently described shark species are luminous," lead study author Julien Claes said via email.
"However, bioluminescence remains one of the most mysterious areas of shark biology," added Claes, a biologist with Belgium's Catholic University of Louvain.
Growing to be just 6 inches (15 centimeters) long, the smalleye pygmy is among the world's smallest sharks. In its deep, open-ocean habitat, the species' shining blue belly acts as camouflage, the study says.
"If you're swimming in the deep-blue dark waters, seen from underneath, you create a shadow against the bluish light coming from the surface," said study co-author Jérôme Mallefet.
"If you produce the same bluish light on your belly, you disappear—you don't show your shadow anymore," added Mallefet, also a biologist at the Catholic University of Louvain.
Earlier studies had revealed that three hormones—melatonin, prolactin, and alpha-MSH—control light emission in another shark species, the velvet belly lantern shark.
In addition to being able to glow continuously, the roughly 2-foot (60-centimeter) lantern sharks can illuminate small patches of skin in short bursts.
For example, "males and females can recognize each other in the dark just by looking at their sexual parts," Mallefet said. For brief periods, "the male's claspers"—a pair of sperm-transferring organs—"are luminous."
Curious whether the lantern and pygmy sharks regulate their glows in the same way, the researchers injected the three hormones—as well as neurotransmitters that control bioluminescence in deep-sea bony fish—into pygmy shark skin samples.
In both species, the study found, melatonin induces a steady glow, and alpha-MSH turns off the glow. Likewise, neurotransmitters have no effect in either species.
But prolactin, which triggers bursts of light in lantern sharks, causes pygmy shark light organs to dim, study leader Claes said.
That means the pygmy shark can't glow in short bursts and must light up only for camouflage, the team concluded.
(See animal camouflage pictures from National Geographic magazine.)
Because lantern and pygmy sharks belong to the only two shark families known to glow, the discovery that both species use the same hormones hints that all shark bioluminescence sprang from a single act of evolution, the study says.
But the simplicity of the pygmy shark's system suggests that the tiny species is more closely related to the first glowing shark ancestor, the researchers say.
Lantern sharks must be related to a later ancestral species, one that took up residence in deeper, darker reaches and evolved more complex lighting control.
"These sharks used their luminescence not only to disappear," Mallefet said, "but also to communicate."
Despite the researchers' success at unraveling how bioluminescent sharks glow, it's still unclear what substance within the sharks is glowing—something the study team is now investigating.
So far, sharks' light-emitting organs don't appear to be powered by bacteria, as in many glowing animals, or by chemicals known to illuminate other creatures, said Mallefet.
"It's an enigma ... maybe something really new" in the animal kingdom, he said.
The glowing-shark study will be published May 15 in the Journal of Experimental Biologists.