Retinal ganglion cell topography in juvenile harbor seals (Phoca vitulina)

Retinal topography in juvenile harbor seals (Phoca vitulina) was analyzed in retinal wholemounts stained with cresyl violet. A region of highest ganglion cell density, i.e., an area centralis, was identified in the temporal retina at a mean distance of 7.7 mm from the papilla (mean peak ganglion cell density 3,952 cells/mm(2)). With an estimated posterior nodal distance of 21 mm, this value corresponds to 531 cells/deg(2). Retinal resolution was calculated as 11.5 cycles/deg under water and 8.7 cycles/deg in air, which is better than the corresponding behaviorally assessed visual acuities. In the region of high ganglion cell density, the ganglion cell layer comprised 1-2 cell layers. From there a moderate visual streak with a ganglion cell density of approximately 1,000 cells/mm(2) extended into the nasal retina. The latter two features together with the absolute ganglion cell density render the harbor seal retina more similar to that of terrestrial carnivores than to that of other pinnipeds.