We examined variation in the visual system both within and among seven species of darters, colorful freshwater fishes of the genus Etheostoma. Using microspectrophotometry, we found that darters possess rod photoreceptor cells, single cone photoreceptor cells containing middle wavelength sensitive (MWS) visual pigments, and twin photoreceptor cells containing (LWS) visual pigments. No variation in peak sensitivity was detected among species or individuals in the rod class. In the MWS class, significant variation was detected among species and a strong statistical trend suggests differences among individuals. By contrast, all differences in the LWS class could be attributed to variation among individuals. Patterns of variation detected among species, among individuals, and among cone classes suggest that complex patterns of selection may be shaping the visual system of these fishes. Further, differences among individuals may have important consequences for visually based behaviors.
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