A phrase to send shivers along the spine: Sharks with light sabers.
These are not Jedi sharks, of course, but, rather, E. spinax, the “velvet belly lanternshark”. As Rebecca Morelle explains for the BBC:
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.
Bioluminescence, in and of itself, is hardly a new phenomenon, nor one recently discovered. As Julien M. Claes et al. assert in Scientific Reports for Nature:
We report the discovery of light organs (photophores) adjacent to the dorsal defensive spines of a small deep-sea lanternshark (Etmopterus spinax). Using a visual modeling based on in vivo luminescence recordings we show that this unusual light display would be detectable by the shark’s potential predators from several meters away. We also demonstrate that the luminescence from the spine-associated photophores (SAPs) can be seen through the mineralized spines, which are partially translucent. These results suggest that the SAPs function, either by mimicking the spines’ shape or by shining through them, as a unique visual deterrent for predators. This conspicuous dorsal warning display is a surprising complement to the ventral luminous camouflage (counterillumination) of the shark.
To put that back into colloquy:
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” ….
…. “It’s surprising that these two apparently opposite behaviours can occur in a single organism at the same time. It is really paradoxical.”