Modifications in shell structure, mantle, and adductor muscle are considered derived adaptations that allowed scallops to swim. This suggests that morphological properties of the adductor muscle and shell should relate to swimming performance in scallops. Various morphological characteristics of the shell (mass, aspect ratio, and volume between the valves) and the adductor muscle (size, position, and attachment to the shell) were measured in six scallop species (Amusium balloti, Placopecten magellanicus, Equichlamys bifrons, Pecten fumatus, Mimachlamys asperrima, and Crassadoma gigantea) with distinct swimming strategies, as documented by measurements of muscle use during induced escape responses. Morphological characteristics of the shell and adductor muscle differed markedly between the species, but did not always follow their swimming strategies. Principal components analysis revealed that shell width, shell and muscle masses, and associated morphological attributes were closely linked with swimming endurance. The intensity of the escape response was best predicted by the aspect ratio and the obliqueness of the adductor muscle.