For whales and seals the ocean is not blue

April 19, 2001

Discovery of a paradoxical colour-blindness in marine mammals

Most mammals have reasonable colour vision on the basis of two spectral types of cone photoreceptors, the blue cones and green cones. Researchers at the Max Planck Institute for Brain Research in Frankfurt/M., the Alfred Wegener Institute in Bremen, and the University of Lund (Sweden) have now discovered that whales and seals do not possess blue cones (European Journal of Neuroscience, Vol. 13, pp. 1520-1528, April 2001). These marine mammals only have green cones and hence are colour-blind, because colour discrimination is impossible with only one type of cone. In contrast, the terrestrial relatives of whales and seals possess both cone types. The loss of the blue cones in marine mammals appears particularly enigmatic as in clear ocean waters the penetrating light becomes increasingly blue-shifted with depth.

Man and many other primates have rather acute colour vision. Their so-called 'trichromatic' colour vision is based on the presence of three types of cone photoreceptors with different spectral sensitivities in the retina: blue, green and red cones. Most other mammals have reasonable but less refined colour vision, as they possess only two spectral types of cone, blue cones and green cones (blue cones and red cones in some species). Such dichromatic colour vision is the basic mammalian pattern. However, a German-Swedish group of researchers now discovered that two major groups of marine mammals, the whales and seals, do not possess any blue cones; they only have green cones and hence are colour-blind. With just one spectral type of cone (cone monochromacy) colour discrimination is not possible. Furthermore, without blue cones the detection of contrast and brightness (i. e. non-chromatic cues) is very poor in the blue part of the spectrum. Given the fact that in clear oceanic waters the penetrating light becomes increasingly blue-shifted with depth, the loss of blue cones in marine mammals appears paradoxical.

Fig. 1: Black-and-white photograph of a bottlenose dolphin, alluding to the colour-blindness of whales and seals. These marine mammals lack blue cones, which seems an odd adaptation to a marine environment where the underwater light field is increasingly blue-shifted with depth.

Photograph: R. Kröger

When studying the eyes and retinae of various marine mammals, Leo Peichl from the MPI for Brain Research in Frankfurt/M., Günther Behrmann from the Alfred-Wegener-Institute for Polar and Marine Research in Bremen, and Ronald Kröger from the Zoological Institute of Lund University (Sweden) observed a surprising deficit: All 14 investigated species of toothed whales (dolphins), eared seals (sealions) and earless seals consistently lacked the blue cones. The retinae of these species only contained green cones and rod photoreceptors, the latter being important for achromatic vision at low light levels (scotopic 'night' vision). The deficit was demonstrated immunocytochemically with antibodies against the cone visual pigments. This method allows to analyze the cone types in preserved eyes of animals that had stranded or died in zoos.

On the basis of their taxonomically broad sample of species, Peichl, Behrmann and Kröger believe that all whales and seals consistently lack blue cones. Whales and seals belong to different mammalian orders. The ancestors of whales were terrestrial artiodactyls (even-toed ungulates), the closest terrestrial relative of the whales is the hippopotamus. Seals have evolved from terrestrial carnivores, among their close relatives are the wolf, ferret and river otter. In all of these terrestrial relatives, the researchers found blue cones. The loss of the blue cones in the marine representatives of such distant mammalian groups strongly argues for convergent evolution and hence for an adaptive advantage of that loss in the marine habitat. The big puzzle is what this advantage could be. When light travels through clear water, e. g. in the open ocean, the longer waves are preferentially scattered and the remaining light becomes more and more blue-shifted with depth - an effect well known to divers. In these conditions the loss of blue cones appears as a particularly bad adaptation. Even if colour vision (on the basis of two spectral cone types) is of little importance in a monochrome blue underwater world, there should be some advantage in keeping the cone type best suited to the spectral environment for contrast and brightness detection. In fact, many fish living in similar lighting conditions possess blue cones.

Fig. 2: Cone photoreceptors in the retina of the ringed seal, immunocytochemically labeled by an antiserum against the visual pigment of green-sensitive cones. Seals and whales only possess this green-sensitive cone type. The spaces between the cones are occupied by the more numerous rod photoreceptors which operate at low light levels.

Photograph: Max Planck Institut for Brain Research / L. Peichl

The authors hypothesize that the blue cone loss occured at an early stage of evolution, when the ancestors of modern whales and seals at their return to the sea initially inhabited coastal waters. Coastal waters contain a larger amount of inorganic and organic debris (due to land-drainage and a richer flora), which preferentially scatters the shorter wavelengths. Hence the underwater light field is shifted to longer wavelengths and contains little blue. In these conditions, the loss of 'idle' blue cones may have been advantageous or at least a neutral event. A loss of colour vision could have simplified visual information processing and hence freed cortical capacity for other sensory abilities. Indeed, many whales have evolved echolocation, and seals are able to track prey by detecting turbulences with their whiskers. At least for the species that have remained in coastal waters, the blue cone loss continues to be advantageous. Those species that have ventured out into the open ocean during further evolution might now profit again from blue cones. But the genetic defect probably is too profound to be undone. Max Planck researcher Leo Peichl summarizes: "Perhaps the colour-blindness of whales and seals is the price these mammals had to pay for access to the wealth of food in the seas."
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