Comparative visual function in elasmobranchs: Spatial arrangement and ecological correlates of photoreceptor and ganglion cell distributions

The topographic analysis of retinal ganglion and photoreceptor cell distributions yields valuable information for assessing the visual capabilities and behavioral ecology of vertebrates. This study examines whole-mounted retinas of four elasmobranch species, the ornate wobbegong, Orectolobus ornatus; the whitetip reef shark, Triaenodon obesus; the epaulette shark. Hemiscyllium ocellatum; and the east Australia shovelnose ray, Aptychotrema rostrata, for regional specializations mediating zones of improved visual ability. These species represent a range of lifestyles: benthic, mid-water. diurnal, and nocturnal. Both photoreceptors (visualized using differential interference contrast optics) and ganglion cells (stained with cresyl violet) in the retina are extensively sampled, and their spatial distribution is found to be nonuniform. exhibiting areae or visual streaks. In general, the topographic distribution,; of both cell populations are in register and match well with respect to the location of regions of high density. However, the location of peaks in rod and cone densities can vary within a retina, indicating that preferential sampling of different regions of the visual field may Occur in photopic and scotopic vision. Anatomical measures of the optical limits of resolving power. indicated by intercone spacing, range from 3.8 to 13.1 cycles/deg. Spatial limits of resolving power, calculated from ganglion cell spacing, range from 2.6 to 4.3 cycles/deg. Summation ratios, assessed by direct comparison of cell densities of photoreceptors (input cells) and ganglion cells (output cells), at more than 150 different loci across the retina, show topographic differences in signal convergence (ranging from 25:1 to over 70:1 Species-specific retinal specializations strongly correlate to the habitat and feeding behavior of each species.