DEVELOPMENTAL ANALYSIS OF ROD AND CONE PHOTORECEPTOR ARCHITECTURE IN THE RETINA OF THE GREEN SEA TURTLE (CHELONIA MYDAS)
All sea turtle species are threatened or endangered, some critically so. Vision is key to individual success in sea turtles; they use their vision for sea finding at hatching, foraging and predator avoidance throughout life, and mating and nesting as adults. This research project provides a comprehensive analysis throughout development (the first of kind) of the outer retina of the green turtle, Chelonia mydas. The research project included characterization of the retina biochemically via immunohistochemistry and histologically via light microscopy, both done over the course of development from hatchling through adult. At all stages, the retina was duplex, containing both rhodopsin and cone opsin types, with similar densities of both rods and cones throughout development of C. mydas. Rods were distributed throughout the retina while the cone photoreceptors were more regionalized with an increased density in the central region that persisted throughout development. There were no statistically significant differences in the densities of photoreceptors by retinal regions or developmental stages. However, the absolute density of cones was significantly higher than that of rods. The organization of the photoreceptor layer was architecturally distinct with the position of the ROS (rod outer segments) closer to the outer limiting membrane (OLM) compared to the COS (cone outer segments), which were closer to the retinal pigmented epithelium (RPE) in all three developmental stages studied. The results presented here show that sea turtles exhibit visual specializations consistent with their behaviors and environmental conditions. Their dynamic environment is reflected in the facts that their retinas contain both rods and cone photoreceptors with retinal specializations to maximize light capture. Evidence of a tiered architecture of the photoreceptor OS positioning in the sea turtle retina has not been documented before. This conclusion suggests that sea turtles may have a higher optical sensitivity than once thought compensated by this tiered architecture favoring the rods. These results indicate that sea turtles are well adapted to both bright light and dim light conditions.