Changes in retinal structure during development and settlement of the goatfish Upeneus tragula.

The retinal structure of the goatfish Upeneus tragula was examined at various stages of the fish's development from pre-settlement pelagic larvae to benthic juveniles. Histological investigation revealed a double layer of cone inner segments in the dorsal retina and high cone and bipolar cell densities in both dorsal and ventral retina prior to settlement. During settlement reorganisation of the dorsal retina was found to occur, with the two cone layers slotting together to form a single layer. In both the dorsal and ventral retina cone and bipolar cell densities were rapidly reduced. Visual acuity, as calculated from histological data on cone density and lens diameter, was found to increase rapidly prior to settlement to an asymptote of approximately 27 minutes of arc, which corresponded to settlement. The changes in the visual system are thought to be associated with changes in both habitat and feeding behaviour at settlement when the fish leave the surface waters and begin a benthic mode of life using sensory barbels to search for sediment dwelling prey.