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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

Behavioral and auditory evoked potential (AEP) audiograms of a false killer whale were measured using the same subject and experimental conditions. The objective was to compare and assess the correspondence of auditory thresholds collected by behavioral and electrophysiological techniques. Behavioral audiograms used 3-s pure-tone stimuli from 4 to 45 kHz, and were conducted with a go/no-go modified staircase procedure. AEP audiograms used 20-ms sinusoidally amplitude-modulated tone bursts from 4 to 45 kHz, and the electrophysiological responses were received through gold disc electrodes in rubber suction cups. The behavioral data were reliable and repeatable, with the region of best sensitivity between 16 and 24 kHz and peak sensitivity at 20 kHz. The AEP audiograms produced thresholds that were also consistent over time, with range of best sensitivity from 16 to 22.5 kHz and peak sensitivity at 22.5 kHz. Behavioral thresholds were always lower than AEP thresholds. However, AEP audiograms were completed in a shorter amount of time with minimum participation from the animal. These data indicated that behavioral and AEP techniques can be used successfully and interchangeably to measure cetacean hearing sensitivity.

The time course of recovery from temporary threshold shift (TTS) was measured in a bottlenose dolphin, Tursiops truncatus, using an evoked‐potential procedure. The envelope‐following response (EFR), which is a rhythmic train of auditory brainstem responses (ABR) to sinusoidally amplitude‐modulated tones, was used as an indicator of the sound reception by the animal. Variation of the intensity of the stimulus allowed us to measure the animal's hearing via EFR thresholds. During each session, following an initial measure of threshold, the trained animal voluntary positioned itself within a hoop 1 m underwater while a 160 dB re 1 μPa noise of a 4–11 kHz bandwidth was presented for 30 min. After the noise exposure, thresholds were measured again at delays of 5, 10, 15, 25, 45, and 105 min. Measurements were made at test frequencies of 8, 11.2, 16, 22.5, and 32 kHz. The maximum TTS occurred 5 min after exposure and rapidly recovered with a rate of around 1.5 dB per doubling of time. TTS occurred at test frequencies from 8 to 16 kHz, with the maximum at 16 kHz. TTS was negligible at 22.5 kHz and absent at 32 kHz.

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.

Depth resolution of spectral ripples was measured in normal humans using a phase-reversal test. The principle of the test was to find the lowest ripple depth at which an interchange of peak and trough position (the phase reversal) in the rippled spectrum is detectable. Using this test, ripple-depth thresholds were measured as a function of ripple density of octave-band rippled noise at center frequencies from 0.5 to 8 kHz. The ripple-depth threshold in the power domain was around 0.2 at low ripple densities of 4-5 relative units (center-frequency-to-ripple-spacing ratio) or 3-3.5 ripples/oct. The threshold increased with the ripple density increase. It reached the highest possible level of 1.0 at ripple density from 7.5 relative units at 0.5 kHz center frequency to 14.3 relative units at 8 kHz (5.2 to 10.0 ripple/oct, respectively). The interrelation between the ripple depth threshold and ripple density can be satisfactorily described by transfer of the signal by frequency-tuned auditory filters.

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