Variation in eye abundance among scallops reveals ontogenetic and evolutionary convergence associated with life habits

Eyes are remarkable systems to investigate the complex interaction between
ecological drivers and phenotypic outcomes. Some animals, such as
scallops, have many eyes for visual perception, but to date, the evolution
of multiple-eye systems remains obscure. For instance, it is
unclear whether eye number changes over a lifetime or varies among
species. Scallops are a suitable model group to investigate these
questions considering the interspecific variation of adult size and
ecological diversity. We tested whether eye abundance scales with
body size among individuals and species and whether it varies with life
habits. We performed comparative analyses, including a
phylogenetic ANCOVA and evolutionary model comparisons, based on eye count
and shell height (as a proxy of body size) across 31 scallop
species. Our analyses reveal that patterns of increasing relationship with
body size are not concordant among taxa and suggest ontogenetic
convergence caused by similar ecologies. Accordingly, selective optima in
eye numbers are associated with shifts in life habits. For
instance, species with increased mobility have significantly more eyes
than less mobile species. The convergent evolution of greater eye
abundance in more mobile scallops likely indicates a visual improvement
based on increased levels of oversampling of the surrounding environment.