All the information available to the brain for the interpretation of the visual scene comes from the number of photons absorbed by a very limited number of photoreceptor types which are characterized by their spectral sensitivity. In vertebrates there are considerable differences in the spectral absorption of the rods and cones making up the retinal mosaic of different animals and, in some cases, including fish and primates, there are considerable differences between the cone sets of

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... within a species. Broadly speaking, the spectral sensitivity of the photoreceptors is related to the spectral distribution of the ambient light and this is particularly true of the colour-biased light under water. When an animal migrates from one visual environment to another, its cone complement may change to that suited to the new conditions. However, significant differences between the cone sets of animals living within the same environment and colour vision polymorphism within a species suggest that visual tasks critical to survival or breeding success require particular visual pigment sets. A start has been made in trying to understand what tasks are best served by different pigment sets.