A newborn pygmy spiny-tailed shark with a yolk sac still attached: The smalleye pygmy shark uses its glowing belly to hide from larger predators.

Jeffrey Rotman/Corbis

he shark is already the ocean's most feared predator. But two lesser-known (and relatively unintimidating) tiny species of the hunter have an extra "superpower," says Jeanna Bryner at LiveScience. The hand-sized smalleye pygmy shark and its slightly larger relative, the lantern shark, possess the astounding ability to make their bellies glow. And now, new research published in the Journal of Experimental Biology confirms why the animals evolved the trick. Here's a glimpse at the ocean's tiny, glow-in-the-dark predators:

Why do they glow?
It's long been speculated that these relatively vulnerable sharks use their ability to glow primarily as a camouflage strategy and the new study, conducted by biologists from the Catholic University of Louvain in Belgium, tested that theory. Whenever these small sharks swim close to the surface, they put themselves at risk of being spotted by larger predators below them. The glowing bellies are a form of "counter-illumination," says BBC News. "Without it, anything looking upwards for a meal would easily see the sharks' bodies silhouetted against the bright sky above."

How do they glow?
Both species have photophores dotting their undersides, which use different hormone triggers to emit a blue glow. 

What did researchers do?
The team captured a few hand-sized pygmies off the coasts of Taiwan. Researchers scraped off samples of the creature's glowing skin, and injected a number of substances (such as neurotransmitters and hormones) to see what triggered the biological glow. As with lantern sharks, the hormone melatonin made the smalleye pygmy shark's skin glow continuously. But another hormone called prolactin — which makes lantern sharks light up in 30-minute flashes — had a surprise effect on the pygmies: The glow dimmed instead.

What does that mean?
The pygmy only has one type of glow (spurred by melatonin), which is only used for camouflage. The lantern shark, on the other hand, has more control over its ability to glow and can use consistent light for camouflage or intermittent flashes (spurred by prolactin) to communicate. This distinction is important, says researchers, because it means the lantern shark is unique. 

Sources: BBC News, LiveScience, Red Orbit, PhysOrg