VISUAL PIGMENT SENSITIVITY IN THREE DEEP DIVING MARINE MAMMALS

Morphological examination of both pinnipeds and cetaceans indicate that they are well suited to see in dim light conditions (Walls 1942, Jamieson and Fisher 1972). Some interspecies differences in scotopic (rod-based) visual sensitivity, however, have been observed that may reflect visual adaptations for functioning in variable underwater photic environments. These differences are determined primarily by the absorptive characteristics of the visual pigments (rhodopsins) located in the rod outer segments (ROS) of the retina, although other factors such as tapetal reflectance may also be important (Walls 1942, Collins and Morton 1949). While shallow-diving animals tend to encounter ambient light spread across much of the visual spectrum, deep-diving species experience a photic environment that is increasingly dominated by a narrow band of relatively short (blue) wavelengths of light centered around 475 nm (Kirk 1994). Avian and terrestrial mammalian rod spectral sensitivities typically fall within a narrow range of 492-506 nm (Dartnall 1962, Bowmaker et al. 1997), whereas those of fish and marine mammals are more broadly distributed (McFarland 197 1, Lavigne and Ronald 1975, Lythgoe 1979). The “sensitivity hypothesis” posits that the wide range of sensitivity observed in aquatic animals is a consequence of different species maximizing sensitivity to the broader range of light conditions available in various marine and freshwater environments (Clarke 1936). Accordingly, species active in deep marine environments are typically sensitive to the comparatively short wavelengths that predominate at depth (Lythgoe 1979). Short wavelength shifts (blue-shifts) in visual pigment sensitivity have been documented in a variety of mesoand bathypelagic fishes (Munt 1964, Knowles and Dartnall 1977a). For those marine mammals that have been studied, the limited number of deep-diving species examined tend to be more blue-shifted than shallower divers, suggesting a similar relationship between foraging depth and visual pigment sensitivity (Lythgoe and Dartnall 1970, McFarland 1971, Lavigne and Ronald 1975). California sea lions (Zalo p h s cal$omianar) and harbor seals (Phoca vitalina), both shallow divers (Feldkamp et al. 1989, Boness et al. 1994), have peak sensitivities of 502 nm and 497 nm, respectively (Cresticelli 1958, Lavigne and Ronald 1975). In contrast,