Eye development in Atlantic halibut (Hippoglossus hippoglossus): Differentiation and development of the retina from early yolk sac stages through metamorphosis

The eye and retina in Atlantic halibut (Hippoglossus hippoglossus), from early yolk soc stages through metamorphosis, were examined after dissection by light microscope, and, for selected stages, by transmission electron microscope. At hatching the eyes were transparent and the retina undifferentiated. The retina differentiated slowly in sectors, and appeared functional around 150 degree-days (d degrees) posthatching (about 50% yolk absorption). This timing coincided with the development of functionality in other organs and observations of ability to feed. At the commercial time of first feeding (about 210 d degrees or 70% yolk absorption) the eyes were fully pigmented. Eye migration started at approximately 80 days posthatching, concurrent with the reorganization of the cones in the outer retina from single rows into a square-type cone mosaic. At 130 days, when metamorphosis was complete, the retina contained groups of rods and appeared to be mature, but until this stage, cones were the primary photoreceptor. The formation of a cone mosaic, recruitment of rods, and increasing size of the eye all indicated increased visual sensitivity at the time of settling. The results suggest that in culture, larvae can see to feed after 150 d degrees, and at settling, they can be successfully fed under dim light.