Retinal properties of the mesopelagic fish <i>Diaphus</i> sp.: vision under dim light conditions

Abstract

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Deep-sea fishes inhabit an environment where light plays a crucial ecological role with consequences on their behaviour, but little is known on how visual adaptations evolved to these specific light conditions. The mesopelagic zone is the transition from the upward epipelagic photic zone to the downward abyssopelagic aphotic zone. In this dim environment only monochromatic blue (470-490 nm) light is available, from the faint down-welling sunlight and the abundant bioluminescent species. Behaviour and survival depend on well-developed visual adaptations, once light is responsible for inter- and intraspecific interactions (e.g. communication, foraging, defense, and reproduction). Generally, mesopelagic fishes are diel vertical migrators, swimming to shallower depths to feed at night and stay in dormancy at mesopelagic depths during daytime. This migratory behaviour is influenced by vertical distribution of potential prey and visual predators. Deep-sea fishes possess high-density pure rod retina with high rhodopsin concentrations, conferring them a sensitive scotopic vision well adapted to the mesopelagic light conditions. In this study, retinal wholemounting technique was applied to study characteristics and sensitivity of the myctophid Diaphus sp. retinae. This technique produces topographic maps of photoreceptor and retinal ganglionar cells, showing how these fishes are adapted to the specific light regime of mesopelagic zone.

Keywords

• Retina
• Vision
• fish
• deep-sea
• mesopelagic
• Lanternfish
• myctophid
• diaphus