A deepwater fish with 'lightsabers'--dorsal spine-associated luminescence in a counterilluminating lanternshark.

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.