Counterillumination in a large demersal bioluminescent shark

Counterillumination is a widespread camouflage strategy in mesopelagic organisms, whereby ventral light emission reduces visibility from below. While its predator-avoidance camouflage function is well established, its potential role in predation remains poorly supported. Here, we investigate bioluminescence in the kitefin shark (Dalatias licha), a large demersal predator for which no natural predators have been recorded. Using in vivo luminescence measurements, we show that this species produces a spatially uniform ventral emission-with no significant difference in radiance across body regions-whose intensity and spectral peak are consistent with a counterillumination function. However, its ecological context, combined with conspicuous dorsal luminescence and a broad angular emission pattern, instead points to a primary role in prey detection and capture rather than predator-avoidance camouflage. These findings expand the functional framework of counterillumination and highlight its ecological relevance in deep benthopelagic environments.