Ocular morphology in antarctic notothenioid fishes.

Beneath the sea ice at McMurdo Sound, Antarctica, notothenioid fishes are subject to extreme seasonal variation in the annual light cycle including 4 months of continual darkness. Gross and microscopic anatomy of the eyes of 18 species revealed ocular morphology that was generally similar to that of coastal fishes elsewhere in the world, and unlike that of deep sea fishes living in perpetual darkness. The spectacle was well developed as were hyaloid arteries at the vitreoretinal interface. Fourteen species had a choroid body, and its presence was considered a primitive character state for notothenioids. The choroid body was absent in phyletically derived groups. The choroid body was especially large in Dissostichus mawsoni, the only species with a rod dominated retina. Retinae were 154-279 μm thick with layering and sublayering typical for teleosts. Although all species had both rods and cones, there was marked interspecific variation in the ratio of cones:rods and in the total number of visual cells. Non-Antarctic notothenioids from New Zealand had more visual cells than most species from McMurdo Sound. Retinae appeared balanced for vision under dim but seasonally variable light conditions and not specially adapted to the 4-month period of winter darkness. Retinal histology reflected the ecology and depth range of most species. Based on ecology and retinal histology, four groups of species were recognized: (1) Non-Antarctic, (2) cryopelagic (including two visually oriented benthic species), (3) pelagic and benthopelagic, and (4) benthic.