Ark clams and relatives (Bivalvia: Arcida) show convergent morphological evolution associated with lifestyle transitions in the marine benthos

One of the most intriguing puzzles in macroevolutionary studies is to understand how distantly related taxa can evolve towards similar phenotypes in response to similar ecological conditions. Ark clams and their relatives (Arcida) display two main ecologies represented by epifaunal and infaunal lifestyles. Their mantle margin includes features, such as photosensory and muscular organs, that may coincide with each habit, making these bivalves a suitable model to explore evolutionary convergence in the marine benthos. To test for the evolutionary association between lifestyles and morphology, we gathered data on the mantle margin for 64 species across all six extant arcidan families. A molecular phylogeny of Arcida was inferred based on four gene sequences from 54 species and used to study trait evolution. Our results support the hypothesis that photoreceptor organs had a single origin and that infaunal lineages lost these structures in independent events, suggesting a correlated pattern of evolution. In addition, the enlargement of the posterior inner fold, which acts as a functional siphon, favoured the occurrence of convergent transitions to infaunal habits during the Mesozoic. We provide evidence of ecomorphological associations and putative adaptations in a bivalve clade that sheds light on the underlying factors driving evolution of the marine benthos.