M. margaritifera is shown here to be a largely sedentary animal, which can move short distances, however, if its habitat changes. It requires a stable substrate of coarse sand, where it burrows to varying depths, and it does not occur in streams where the substrate is substantially resorted by torrents. In these it fails to establish and is washed away. Recolonisation of denuded areas can only be achieved very slowly by adult mussels and the main source of colonists is certainly young mussels being released from host fish. When large mussels suddenly reappear in denuded areas it is probable that these are specimens which were previously completely buried and have since burrowed up into view.