Visual system development of the spotted unicornfish, Naso brevirostris (Acanthuridae).

Ontogenetic changes of the visual system are often correlated with shifts in habitat and feeding behaviour of animals. Coral reef fishes begin their lives in the pelagic zone and then migrate to the reef. This habitat transition frequently involves a change in diet and light environment as well as major morphological modifications. The spotted unicornfish, Naso brevirostris, is known to shift diet from zooplankton to algae and back to mainly zooplankton when transitioning from larval to juvenile and then to adult stages. Concurrently, N. brevirostris also moves from an open pelagic to a coral-associated habitat before migrating up in the water column when reaching adulthood. Using retinal mapping techniques, we discovered that the distribution and density of ganglion and photoreceptor cells in N. brevirostris changes primarily during the transition from the larval to the juvenile stage, with only minor modifications thereafter. Similarly, visual gene (opsin) expression based on RNA sequencing, although qualitatively similar between stages (all fish mainly expressed the same three cone opsins; SWS2B, RH2B, RH2A), also showed the biggest quantitative difference when transitioning from larvae to juveniles. The juvenile stage in particular seems mismatched with its reef-associated ecology, which may be due to this stage only lasting a fraction of the lifespan of these fish. Hence, the visual ontogeny found in N. brevirostris is very different from the progressive changes found in other reef fishes, calling for a thorough analysis of visual system development of the reef fish community.