THE EFFECT OF LIGHT-REARING AND DARK-REARING ON THE DEVELOPMENT OF THE EYES OF ATLANTIC HALIBUT (HIPPOGLOSSUS-HIPPOGLOSSUS) YOLK-SAG LARVAE

The development of the visual system in Atlantic halibut was studied f rom hatching until the time of first feeding by following the organoge nesis of the retina and by analysing the resolving power at various st ages. Larvae reared under continuous darkness were compared with larva e reared in 0.3 mu Em(-2)s(-1) white light. The retinal morphology in the two groups followed the same pattern of development. This was also true for the thickness of the various cell layers, lens diameter and photoreceptor density. The only significant difference we observed was that the inner plexiform layer was 1.5% thicker in the light-reared g roup. Visual acuity calculated from the optomotoric response was about 50 for halibut larvae at the stage of first feeding. Visual acuity in this period, calculated from histological sections, was approximately 0.6 degrees. Acuity was not different between the two groups. The res ults indicate that light is not a critical factor for normal morphogen esis of the retina of halibut to the time of first feeding.