DEVELOPMENT OF MULTIBANK ROD RETINAE IN DEEP-SEA FISHES

We studied the development of multibank rod retinae by monitoring the size-related addition of new layers of rod inner and outer segments in four species of deep-sea fishes and found two different growth paradi gms. In the mesopelagic Chauliodus sloani, new banks of rod inner and outer segments are added as long as the fish increases in size, as obs erved earlier by Locket (1980). By contrast, in three bathybenthic spe cies (Antimora rostrata, Coryphaenoides (Coryphaenoides) guentheri, an d Coryphaenoides (Nematonurus) armatus), the final complement of banks is reached when the specimens have grown to between 20 and 47% of the ir maximal size, suggesting that the visual system is mature only afte r this stage. Increase in retinal area, density of rod nuclei, and den sities of rod inner and outer segments were also studied in these and additional species. Taken together with previous data on rod prolifera tion patterns and outer segment membrane synthesis, our findings indic ate that at least in species with no continual addition of new banks, there is no major functional difference between the innermost and oute rmost banks of rod inner and outer segments. While Chauliodus spends a ll its Life in the mesopelagic environment, the three bathybenthic spe cies live in this environment during early development and descend tow ards greater depths only upon maturation. We speculate that this coinc ides with the stage when the full complement of rod banks is formed in the retina, as a possible prerequisite for a life outside the reach o f sunlight.