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Why is the blind side of an adult flatfish usually white or unpigmented?

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Body plan and sexual dimorphism in the common sole (Solea solea) and dusky flounder (Syacium papillosum). — pleuronectiform Body plan and sexual dimorphism in the common sole (*Solea solea*) and dusky flounder (*Syacium papillosum*).

Adult flatfish (order Pleuronectiformes) are bottom-dwelling creatures that habitually rest on one side of their bodies. Because this lower side remains against the substrate and is not exposed to light or potential predators, it typically lacks the pigmentation found on the upper, eye-bearing side.1

The upper side is pigmented to provide camouflage, allowing the fish to blend into its surroundings, while the unpigmented, white appearance of the blind side is simply the natural state for the surface that is not visible to observers in the water column.21