The deep-sea remains the least explored environment on earth although biological adaptations to this extreme habitat have led to exceptional discoveries. Due to limited access and resources, and to the difficulty in performing in-situ experiments, the behaviour of most deep-sea organisms remains a mystery. The study of sensory systems such as vision, can be used to shed light on the behaviour of these animals and allows us to understand what their day-to-day life might be like.

Using a multidisciplinary approach, we studied the visual system of one of the most abundant family of deep-sea fishes in the world’s ocean, the lanternfishes (Myctophidae). Lanternfishes inhabit the mesopelagic zone of the ocean (200-1000 m), a transition zone where both residual downwelling sunlight and bioluminescent signals are present. These fishes are part of the largest animal migration we know of, a diel vertical migration towards the surface at night in order to feed and a return to the depths to avoid predation from surface dwellers. Analysis of the eyes and bioluminescent organs of over 60 species suggests that lanternfishes are visually active at night with eyes specifically designed for the light conditions at their preferred depth. Different species possess distinct areas of high photoreceptors and ganglion cell density that reflect different interactions with congeners, prey and likely different predatory strategies.

An exciting new observation demonstrates how some species developed a novel intra-ocular filter to increase the detection of bioluminescent and/or fluorescent emissions in a specific part of their visual field. This filter is species-specific and sexually dimorphic suggesting the use of a private communication channel between potential mates.