Eye numbers were quantified on the mantles of sea scallops (Placopecten magellanicus) to determine whether they varied with scallop size (correlated with age), and to compare the numbers of eyes present on the upper (left) and lower (right) mantles. We used Poisson regressions to model eye addition with growth and found that larger scallops had significantly more eyes than their smaller conspecifics, although eye addition occurred only on the upper mantle. Consistent with predictions of an adaptive evolutionary response in eye numbers driven by the need for visualizing objects in the water column, we found scallops had significantly more eyes on the upper mantle than on the lower mantle. This may favor detection of predators, food, or habitat. We also documented significantly greater densities of eyes on the mantles of the smallest scallops, which may indicate vision is especially important in juveniles, which are most vulnerable to predation.