Part III: The evolution of photopic visual pigments in fishes

Abstract

The feeding strategies of diurnal, tropical marine fishes correlate with the visual pigments extracted from their retinae and with the photic environments in which they hunt prey. Two basic feeding modes prevail: (1) silhouetting prey from below against the surface background light; and (2) contrasting prey against other fields of view, which are more monochromatic. Fishes that silhouette prey from below possess a single visual pigment matched to the spectral distribution of downwelling light. This maximizes the contrast between the brighter background and darker target. To predators that view prey in the horizontal fleld, their targets may appear either darker or brighter than the background. The photocontrast of a nonreflective target is maximized by a class of cones with a matching visual pigment. But the contrast of reflective (“bright”) targets is enhanced by visual pigments offset from the spectral distribution of the monochromatic blue background. Thus, the evolutionary selection of multiple photopic systems, and of color vision itself, is probably related to the maximization of visual contrast against monochromatic backgrounds.