Opsin gene expression in larval and adult deep-sea fishes supports a conserved cone-to-rod pathway in teleost visual development

Deep-sea fishes show extraordinary visual adaptations to an environment where every photon of light that is captured might make the difference between life and death. While considerable effort has been made in understanding how adult deep-sea fishes see their world, relatively little is known about vision in earlier life stages. Similar to most marine species, larval deep-sea fishes start their life in the well-lit epipelagic zone, where food is abundant and predation relatively low. In this study, we show major changes in visual gene expression between larval and adult deep-sea fishes from eight different orders (Argentiniformes, Aulopiformes, Beryciformes, Myctophiformes, Pempheriformes, Scombriformes, Stomiiformes and Trachichthyiformes). Comparison between 18 species revealed that while adults mostly rely on rod opsin(s) (RH1) for vision in dim-light, larvae mostly express green-sensitive cone opsin(s) (RH2) in their retinas. Adults of the scombriform and three aulopiform species also expressed low levels of RH2, with the latter using different copies of the gene between ontogenetic stages. Cone opsins in adult fishes are rather surprising as most deep-sea fishes have lost their cone photoreceptors in favour of a highly sensitive pure rod retina. The expression of RH2 in larvae, on the other hand, shows that even in species that might not have any cones as adults, the larval retina is likely to be cone dominated first, before rod photoreceptors are added through ontogeny. Our study therefore supports a conserved pathway for the cone-to-rod developmental sequence of the teleost or even vertebrate retina.