<p>Largehead hairtail performs both seasonal and diel vertical migrations, occurring at depths ranging from the shallow surface water to the deeper mesopelagic zone. This species feeds actively at dawn and at dusk, with its dusk migration synchronized to prey occurrence. To understand how the largehead hairtail has adapted to dim light conditions, we investigated visual pigment genes and the distribution of photoreceptor cells in the retinas extracted from individual fish that were experimentally light and dark-adapted. We isolated one rhodopsin (RH1) and one green opsin (RH2) gene, for which we estimate visual pigment peak absorption wavelengths of λ_max = 490 and 507&#xa0;nm, respectively from amino acid key sites. Histological sections revealed that even in the dark-adapted retinas, in which pigment granules have become fully aggregated on the scleral side, most cone cells aligned on the outer limiting membrane, with few dispersed ones in the photoreceptor layer. For the retinas of dark-adapted largehead hairtail, we postulate that under dim light conditions their cones interact with the incident light received by the retina first, before it reaches the rods, and that highly sensitive rods then detect the residual light that has passed through the cone layer. In this way, largehead hairtail may exhibit color vision with RH1 and RH2 pigments even under dim light conditions.</p>