The discovery of an unusual bioluminescent display in a small deep-water shark is described in Scientific Reports this week. The velvet belly lanternshark has dorsal glowing organs, or photophores, adjacent to its defensive mineralized spines, which may be part of a mechanism for deterring predators, the research suggests.

Like many other deep-water organisms, the velvet belly lanternshark (Etmopterus spinax) produces a continuous glow on its underside to match the down-welling of sunlight, a process called counterillumination. The velvet belly’s underside is covered with thousands of tiny photophores that produce a long-lasting, blue-green luminescence. Julien Claes and colleagues report that this shark also produces a glow from the frontal edges of its dorsal fins, forming conspicuous arcs immediately behind its spines. The authors use visual modelling to show that the velvet belly’s potential predators can detect the dorsal luminescence from several meters away, whereas its main prey, Mueller's pearlside, can only do so from a short distance. This suggests that dorsal bioluminescence in the lanternshark may act as a warning beacon to deter approaching predators ? possibly by highlighting the spines themselves ? without jeopardizing prey capture.

The function of these glowing organs remains hypothetical, the authors caution, but the observations suggest that warning luminescence and counterillumination, two apparently contradictory strategies, could operate in a single organism at the same time.