PATTERNS OF ROD PROLIFERATION IN DEEP-SEA FISH RETINAE

In a sample of 37 species of deep-sea fish species from the sea floor of the Porcupine Seabight and the Gobal spur (North Atlantic) we inves tigated the overall structure of the retina with special respect for t he organization of rods, their length and their arrangement in multipl e banks. Using an immunocytochemical marker for cell proliferation (PC NA) we studied the mechanisms of rod proliferation, and, by means of s erial section reconstruction followed their integration into the exist ing population of rods. Furthermore, in three different species we hav e observed growth related changes in retinal thickness, rod density an d proliferation activity. We found evidence for two different principl es for the organization of rods in these deep-sea fish retinae. In the first group of species represented by Nematonurus armatus and Corypha enoides guentheri we found rods to be rather short (20-30 mu m) and ar ranged in three and more banks. In these species rod proliferation occ urred in a single band of cells immediately vitread of the external Li miting membrane, thus showing a high degree of spatial and temporal or der. In these species, young rods are inserted just sclerad of the ext ernal limiting membrane and the older outer segments pushed away from the incoming light towards the back of the eye. This may be linked to a progressive loss of function of the older rods and might represent a n alternative mechanism to the disk shedding in other vertebrates. In the second population (e.g. Conocara macroptera, Alepocephalus agassiz ii) we observed considerably longer rod outer segments (60-80 mu m) fo rming no more than two layers. These retinae had rod precursors arrang ed in disseminated clusters throughout the outer nuclear layer indicat ing the lack of clear spatio-temporal order in mitotic activity along with a more statistical pattern of integration of the newly formed out er segments. In our sample of species both populations were of about e qual size suggesting that the two mechanisms are equally effective and may have arisen independently.