For whales and seals the ocean is not blue: a visual pigment loss in marine mammals

Most terrestrial mammals have colour vision based on two spectrally differe nt visual pigments located in two types of retinal cone photoreceptors, i.e . they are cone dichromats with long-to-middle-wave-sensitive (commonly gre en) L-cones and short-wave-sensitive (commonly blue) S-cones. With visual p igment-specific antibodies, we here demonstrate an absence of S-cones in th e retinae of all whales and seals studied. The sample includes seven specie s of toothed whales (Odontoceti) and five species of marine carnivores (ear ed and earless seals). These marine mammals have only L-cones (cone monochr omacy) and hence are essentially colour-blind. For comparison, the study al so includes the wolf, ferret and European river otter (Carnivora) as well a s the mouflon and pygmy hippopotamus (Artiodactyla), close terrestrial rela tives of the seals and whales, respectively. These have a normal complement of S-cones and L-cones. The S-cone loss in marine species from two distant mammalian orders strongly argues for convergent evolution and an adaptive advantage of that trait in the marine visual environment. To us this sugges ts that the S-cones may have been lost in all whales and seals. However, as the spectral composition of light in clear ocean waters is increasingly bl ue-shifted with depth, an S-cone loss would seem particularly disadvantageo us. We discuss some hypotheses to explain this paradox.