Topography of different photoreceptor cell types in the larval retina of Atlantic halibut (Hippoglossus hippoglossus)

The identities of single cone cells in the retina of Atlantic halibut (Hipp oglossus hippoglossus) larvae were studied by it situ hybridisation using R NA probes for the five different halibut opsins. Four different cone opsins (ultraviolet-, blue-, green- and red-sensitive) are expressed in Atlantic halibut at the end of the yolk-sac period, whereas rod opsin is expressed l ater in development. Photoreceptor cells expressing ultraviolet-sensitive o psin are found only in the ventral retina, presumably to optimise detection of the downwelling ultraviolet light. The majority of the photoreceptors ( approximately 90 %) in the retina express green-sensitive opsin and its dis tribution shows no regional differences. In contrast, blue- and red-sensiti ve opsins are expressed much less frequently (in approximately 10 % of phot oreceptors), although these two opsins are also found over the entire retin a. The expression patterns of the different visual pigments indicate some f orm of mosaic expression in the single-coned larval retina, and this is rem iniscent of the square mosaic expression found in post-metamorphic Atlantic halibut. These findings suggest plasticity in green-opsin-expressing cells during development, resulting in a square mosaic expression pattern.