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The eye of aquatic mammals demonstrates several adaptations to both underwater and aerial vision. This study offers a review of eye anatomy in four groups of aquatic animals: cetaceans (toothed and baleen whales), pinnipeds (seals, sea lions, and walruses), sirenians (manatees and dugongs), and sea otters. Eye anatomy and optics, retinal laminar morphology, and topography of ganglion cell distribution are discussed with particular reference to aquatic specializations for underwater versus aerial vision. Aquatic mammals display emmetropia (i.e., refraction of light to focus on the retina) while submerged, and most have mechanisms to achieve emmetropia above water to counter the resulting aerial myopia. As underwater vision necessitates adjusting to wide variations in luminosity, iris muscle contractions create species-specific pupil shapes that regulate the amount of light entering the pupil and, in pinnipeds, work in conjunction with a reflective optic tapetum. The retina of aquatic mammals is similar to that of nocturnal terrestrial mammals in containing mainly rod photoreceptors and a minor number of cones (however, residual color vision may take place). A characteristic feature of the cetacean and pinniped retina is the large size of ganglion cells separated by wide intercellular spaces. Studies of topographic distribution of ganglion cells in the retina of cetaceans revealed two areas of ganglion cell concentration (the best-vision areas) located in the temporal and nasal quadrants; pinnipeds, sirenians, and sea otters have only one such area. In general, the visual system of marine mammals demonstrates a high degree of development and several specific features associated with adaptation for vision in both the aquatic and aerial environments. Anat Rec 290: 701-715, 2007. 2007 Key words: vision; ocular optics; retina; ganglion cells; retinal topography; aquatic mammalsComparative studies of the visual system in animals adapted to various living conditions have revealed new specific features of neuronal structures, have aided our understanding of mechanisms of visual perception, and have described the many ways in which sensory systems show adaptations to various environments. In recent years, there has been a great interest in the visual system of aquatic mammals: cetaceans (dolphins, porpoises, and whales), pinnipeds (seals, sea lions, and walruses), sirenians (manatees and dugongs), and sea otters. These species demonstrate various extents of adaptation to the aquatic environment. Many aquatic mammals (cetaceans, sirenians) spend their entire life in the water; however, air-breathing confines them to a near-surface layer of water. Other marine mammals (pinnipeds, sea otters) spend a significant part of their life on land. As a result, the visual systems of these groups feature remarkable morphological and functional specializations for both

Retinal topography, cell density and sizes of ganglion cells in the killer whale (Orcinus orca) were analyzed in retinal whole mounts stained with cresyl violet. A distinctive feature of the killer whale’s retina is the large size of ganglion cells and low cell density compared to terrestrial mammals. The ganglion cell diameter ranged from 8 to 100 µm, with the majority of cells within a range of 20–40 µm. The topographic distribution of ganglion cells displayed two spots of high cell density located in the temporal and nasal quadrants, 20 mm from the optic disk. The high-density areas were connected by a horizontal belt-like area passing below the optic disk of the retina. Peak cell densities in these areas were evaluated. Mean peak cell densities were 334 and 288 cells/mm² in the temporal and nasal high-density areas, respectively. With a posterior nodal distance of 19.5 mm, these high-density data predict a retinal resolution of 9.6′ (3.1 cycles/deg.) and 12.6′ (2.4 cycles/deg.) in the temporal and nasal areas, respectively, in water.

The distribution and size of ganglion cells in the retina of the bottlenosed dolphin are described. Ganglion cells concentrate at two spots of the highest density in the nasal and temporal quadrants, 15 to 16 mm (50 to 55°) from the optic disk. The mean peak cell density in both spots is about 670 cells/mm². With a posterior nodal distance of 14.5 mm (under water), this corresponds to 43 cells/deg², which provides a retinal resolution of about 9'' in water and 12'' in air. Mean cell size was from 26 to 31 µm in various parts of the retina.

The total number, size, topographic distribution and peak density of ganglion cells were studied in retinal wholemounts of the fur seal, Callorhinus ursinus. The cell distribution showed a distinct zone of high ganglion cell density. It was located in the temporal retinal quadrant, near the horizontal meridian, 10–12 mm (25–31°) from the optic disk. The peak cell density in this zone was 812–1332 cells/mm² (mean 1053 cells/mm²), i.e. 125–205 cells/deg² (mean 162 cells/deg²). These data predict a retinal resolution of 5.6.–7.1 cycle/deg. The ganglion cell soma size ranged from 10 to 50 µm. Cell size histograms were bimodal in shape with modes below and above 30 µn.