| Summary: | The structural and functional organization of the pinniped eye, as represented by the harbour seal, Phoca vitulina, is much more complex than previously realized. The retina of the harbour seal is similar to that of the harp seal, in that it shows little zonation of the inner nuclear layer, possesses large horizontal cells, and has similar nuclear densities within the different retinal layers throughout the retina. The outer nuclear layer is the thickest of all the retinal layers. Photoreceptor cells of types B and C were observed with a ratio of roughly 1:23 respectively. This ratio and photoreceptor density along with a well-developed and extensive tapetum, indicates a highly light-sensitive retina. The existence of retinal folding in marine mammals is also questioned and discussed.
Refractive observations obtained through retinoscopy show that the harbour seal eye is very myopic and astigmatic in air, and hypermetropic with no discernable astigmatism in water. The axis of least corneal curvature is horizontal; current theories predict that the stenopeic pupil would thus correct for astigmatism in air. The functional significance of the astigmatism, which is in the order of 7 to 13 diopters, has not been adequately determined to date, however. These observations are in agreement with those recently reported for the harp seal.
Behavioural studies have also confirmed the high sensitivity of the harbour seal eye to light, as suggested by the histological observations. Two captive harbour seals were trained to select a two-bar pattern over a one-bar pattern in both air and water, discriminating for the presence of a gap. Their abilities to carry out this task were measured as the gap in the two-bar pattern was gradually reduced in size. The light intensity of the white parts of the stimulus cards was 284-369 candela/m² . Under these conditions, the harbor seal can detect a gap as small as 1 mm at a distance of 1.7 m in both air and water. These studies fail to provide values of the visual acuity of the harbour seal in the strict sense, however, owing to the physics of the pattern utilized. Rather, they indicate a measure of the absolute sensitivity of the eye to light. === Science, Faculty of === Zoology, Department of === Graduate
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