Article: Adaptive significance of shell torsion in mytilid bivalves

A twisted commissure plane is a common feature in several species of the mytilid genus Modiolus. Observations on a semi-infaunal population of M. americanus in Bermuda suggests that the twisted shell morphology maximizes the length of posterior commissure raised above the sediment surface while keeping its profile low, with minimum risk of accidental damage. Thus, the shell morphology in the twisted Mytilidae represents adaptive convergence with the twisted Arcidae and Bakevelliidae. In these two families, however, the torsional direction is genetically fixed, while in the Mytilidae both the direction and the amount of torsion seem to develop as a phenotypic response to the shell orientation relative to the substrate.