The expression of deep-sea fish visual genes supports a conserved cone-to-

17 The ontogeny of the vertebrate retina is thought to follow a conserved pattern whereby cone 18 photoreceptors develop first and rod photoreceptors are added only at later developmental 19 stages. Whether this is also the case for species that use only rods as adults, such as many deep20 sea fishes, remained unclear. Since many deep-sea fishes start their lives in the shallow, well21 lit epipelagic zone cones might still be favoured during early developmental stages. Using a 22 comparative transcriptomic approach in 20 deep-sea species from eight teleost orders, we show 23 that while adults mostly rely on rod opsin (RH1) for vision in dim light, larvae almost 24 exclusively express mid-wavelength-sensitive cone opsins (RH2) in their retinas. The 25 phototransduction cascade genes follow a similar ontogenetic pattern of conefollowed by rod26 specific gene expression in most orders, except for the pearleye and sabretooth (Aulopiformes), 27 in which the cone cascade remains dominant throughout development. Hence, our findings 28 provide strong molecular support for a conserved cone-to-rod retinal development in 29