OLIN, Jill A; MORRISSEY, John F; Hofstra University; Hofstra University: Histological Comparison of the Retinal Structure of Deep-Water and Epipelagic Sharks

Morphological characteristics of the retina of a deep-water shark (Order: Squaliformes) were compared to three epipelagic shark species of two taxonomic orders (Carcharhiniformes and Lamniformes) to investigate elasmobranch visual adaptations at differing oceanic depths. The common thresher Alopias vulpinus, the short fin mako Isurus oxyrinchus, and the blue shark Prionace glauca, whose depth distribution overlap in the epipelagic zone between the surface and 500m, have well-developed eyes with retinas composed of both rod and cone photoreceptors in ratios of 17:1, 16:1 and 20:1 respectively. On the contrary, Centrophorus cf. uyato, a mesobenthic species found at depths greater than 400m, has a retina composed solely of rod photoreceptors. These data suggest that the rod-rich, relatively cone-poor retina of these elasmobranchs comprise an eye that has low visual acuity, rather high sensitivity. Further investigations into eight different regions of the retina (four in the central and four in the peripheral) exhibited no indication of interspecific variations within species of the following retinal attributes: photoreceptor abundance, distribution, photoreceptor outer segment length, photoreceptor inner segment length, or photoreceptor width, suggesting that there is no specific area of increased visual acuity or sensitivity in these retinas. As expected, significant differences in the aforementioned characteristics were found between the deep-water and pelagic species while no significant differences existed between the taxonomically related A. vulpinus and I. oxyrinchus. Retinal structure appears to be concomitant with the habitats and taxonomic positions of these shark species.