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L Peichl, K Kovacs, C Lydersen; Absence of S-cones in the Retinae of Further Marine Mammals (Whales and Seals) . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3762.
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Purpose: Most terrestrial mammals are cone dichromats, having short-wave sensitive (S-)cones and middle-to-long-wave sensitive (M/L-)cones. In stark contrast, the marine whales and seals studied to date completely lack S-cones (Peichl et al., 2001, Europ. J. Neurosci 13: 1520-1528). In order to check the hypothesis that whales and seals generally lack S-cones, we have studied the spectral cone types in further whale and seal species. Methods: Eyes were obtained from stranded autopsied whales, and from seals sacrificed for an unrelated project. In formalin-fixed retinal flatmounts, spectral cone types and their distributions were assessed using the antisera JH 492 against M/L-cone opsin and JH 455 against S-cone opsin (kindly provided by J. Nathans). Results: Cone types and cone densities were analyzed in the bearded seal Erignathus barbatus (Carnivora, family Phocidae) and in three species of toothed whales (Odontoceti): the striped dolphin Stenella coeruleoalba (family Delphinidae), Dall's porpoise Phocoenoides dalli (Phocoenidae) and the northern bottlenose whale Hyperoodon ampullatus (Ziphiidae). All four species completely lack immunoreactive S-cones, but possess regular populations of M/L-cones. M/L-cone densities in the bearded seal range from 9000/mm² in central retina to 6000/mm² in the periphery, and in Dall's porpoise from 12,000/mm² to 5000/mm². Peripheral to midperipheral M/L-cone densities are 3000-4000/mm² in the striped dolphin, and 2000-2500/mm² in the northern bottlenose whale. These densities are similar to those reported for other whales and seals (Peichl et al., 2001). Conclusion: The absence of S-cones in further whale and seal species supports the hypothesis that both groups of marine mammals have generally lost their S-cones. None of the 10 toothed whale and 8 seal species studied to date possesses S-cones. As the underwater lighting conditions in open ocean waters become successively blue-shifted with depth, the adaptive value of the S-cone loss remains enigmatic.
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