Glowing Sharks Light up to Threaten Potential Predators

Velvet belly lanternsharks are known to have the ability to glow in the dark - a trait called bioluminescence. A new study has found that the sharks use it as a warning system against potential predators as well as to hide from prey.

Velvet belly lanternsharks (Etmopterus spinax) are small marine mammals that belong to the family of Dalatiidae. They are found in deep waters of the Atlantic Ocean, the Mediterranean and the Black sea. They feed on prey like squids and small fish like Mueller's pearlside. Earlier studies have already shown that the shark's belly is illuminated by light-producing cells called photophores.

Sharks use these cells to hide themselves from prey. But a new study by researchers from the Catholic University of Louvain in Belgium has found that the sharks have luminescence on their top side, which illuminates the two spines, each one located in front of each dorsal fin. The research team noticed that the sharks try to be invisible from below their body and become visible from the dorsal side. Photophores in the sharks' belly help them camouflage themselves, reports BBC.

"Imagine you are below the shark, the shark is swimming and you have the light from the sun coming down," lead author Julien Claes, a shark biologist from the Catholic University of Louvain in Belgium, explained to BBC.

"If you are just below the shark what you are going to see is a shadow. So imagine if the shark can actually produce a light, which is identical to the light produced by the sun. Then the shadow of the shark is going to disappear."

A small fish that swims below the sharks will not be able to see them coming. Based on visual modeling experiments, researchers found that the small prey could detect the light only from a distance of about 1.5m, which gives them less chance to escape.

However, big predators can detect the glow from the sharks at a longer distance. Sharks use the glow on the top side to illuminate their threatening spines so as to warn potential predators. "It's a way to say: 'Don't bite me, I'm dangerous, I have spines on my back. You could be hurt'," Claes told BBC.

According to the National Geographic, three hormones - melatonin, prolactin and alpha-MSH - control the light emission of velvet belly lanternsharks. Apart from their ability to glow continuously, these sharks can illuminate small patches of their skin in short bursts so that the male and female sharks can recognize each other in the dark environment.

The findings of the study, "A deepwater fish with "lightsabers" - dorsal spine-associated luminescence in a counter-illuminating lanternshark", are published in the journal Scientific Reports.