Morphological convergence of shell shape in distantly related scallop species (Mollusca: Pectinidae)

Morphological convergence is a central concept in evolutionary biology, but convergent patterns remain understudied in nonvertebrate organisms. Some scallop species exhibit long-distance swimming, a behaviour whose biomechanical requirements probably generate similar selective regimes. We tested the hypothesis that shell shape similarity in long-distance swimming species is a result of convergent evolution. Using landmark-based geometric morphometrics, we quantified shell shape in seven species representing major behavioural habits. All species displayed distinct shell shapes, with the exception of the two long-distance swimmers, whose shells were indistinguishable. These species also displayed reduced morphological variance relative to other taxa. Finally, a phylogenetic simulation revealed that these species were more similar in their shell shape than was expected under Brownian motion, the model of character evolution that best described changes in shell shape. Together, these findings reveal that convergent evolution of shell shape occurs in scallops, and suggest that selection for shell shape and behaviour may be important in the diversification of the group.