TAYLOR, S/M; GRACE, M/S*; Florida Institute of Technology: Evolution and Ecology of Vision: Development of Photoreceptor Arrays in Elopomorph Fish

The Atlantic tarpon (Megalops atlanticus) and the speckled worm eel (Myrophis punctatus) belong to the teleost subdivision Elopomorpha, an assemblage of four orders grouped primarily on the basis of remarkably similar larval form. As adults, however, tarpon are large visually-guided surface predators while speckled worm eels are small nocturnal burrowers that rely primarily on chemoreception for feeding. Because these species exhibit dramatic morphological and behavioral divergence during development, we hypothesized concomitant changes in retinal structure and function in support of species-specific life history changes. We studied ultrastructure, opsin content, and rod and cone distributions in these two species at four developmental stages: premetamorphic pelagic larval, settlement/metamorphosis, postmetamorphic juvenile, and adult. Prior to metamorphosis, both species had rod-dominated (but duplex) retinas, in stark contrast to pure-cone or cone-dominated retinas of most teleosts. In larval tarpon, cone opsin was present only in the mid-temporal region; in larval eels, only in the ventral region. During and post-metamorphosis, tarpon rods and cones became more differentiated, and exhibited distinct distributions. In the worm eel, rod and cone opsins were present at metamorphosis, but the two cell types were morphologically indistinct. During subsequent development, worm eel retinas contained only rhodopsin, and all photoreceptors were morphologically rod-like. Our results show that the Elopomorpha may be united on the basis of shared larval retinal organization in addition to overall form, and that development of the Elopomorph retina is dramatically different from that in most teleosts. Furthermore, our results show that retinal organization changes dramatically in concert with changing habits and habitats.