A glow-in-the-dark shark scares off predators with "lightsaber-like" spines on its back, a study suggests.
The research was carried out on the velvet belly lanternshark, a small species found in the deep waters of the Atlantic Ocean and Mediterranean Sea.
The scientists believe that while the light-up spines can be seen by larger, potentially dangerous fish, they are harder for the shark's prey to spot.
The study is published in the Nature journal Scientific Reports.
This species of lanternshark (Etmopterus spinax) lives in the mesopelagic zone of the ocean, which has a range between 200m and 1,000m in depth.
It is a diminutive shark; the largest can measure up to about 60cm in length, but most are about 45cm long.
Until recently, little had been known about this species, apart from the fact that like many deep sea creatures it has the ability to glow - a trait called bioluminescence.
Previous research found that the shark has light-producing cells called photophores in its belly, and it uses this light to camouflage itself.
"Imagine you are below the shark, the shark is swimming and you have the light from the Sun coming down," explained Dr Julien Claes, a shark biologist from the Catholic University of Louvain in Belgium, and the lead author of the study.
"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."
Any prey lurking below, typically a small fish called Mueller's pearlside, will not see the shark coming.
However, this new study revealed that the shark is also luminescent on its top side.
Dr Claes said: "There are two spines, one in front of each dorsal fin, and just behind them you have two rows of photophores. They are like lightsabers - they illuminate the spine.
"It was surprising - why would you try to be invisible from below but visible from the dorsal side?"
Visual modelling experiments revealed that potential predators could see the light from several metres away.
The shark's prey, however, could only see the glow from a distance of about 1.5m, giving them less chance of making an escape.
The team concluded that the glowing spines were acting as a beacon, illuminating the shark's threatening spines.
Dr Claes: "It's a way to say: 'Don't bite me, I'm dangerous, I have spines on my back. You could be hurt.'
"When you live in this dark place, what you try to do is avoid is to be seen by other animals, because there are no places to hide.
"It can be very dangerous - you put yourself at risk when you produce light from your back, unless it acts as a warning system."
He said it was unusual to find an animal that was using light to both hide and advertise itself at the same time.
"It's surprising that these two apparently opposite behaviours can occur in a single organism at the same time. It is really paradoxical."