Direct and indirect effects of seastars Asterias rubens on mussel beds (Mytilus edulis) in the Wadden Sea

Near the island of Sylt in the Wadden Sea (German Bight, North Sea), seastars Asterias rubens (L.) co-occur with their preferred prey, mussels Mytilus edulis (L.), which form extensive beds from the intertidal down to the subtidal zone. Mussel density within these beds is significantly lower in the subtidal than the intertidal zone. Laboratory and field experiments were conducted to check if this was due to seastar predation. Feeding experiments did not indicate size refuge of M. edulis from predation by A. rubens, but showed that seastars preferred clean subtidal mussels above barnacle-overgrown intertidal ones. This preference coincided with higher abundances of both large (arm length>5.5 cm) and smaller seastars in the shallow subtidal, but their abundance was too low to account for the decreased subtidal mussel density in the area studied. However, seastars may indirectly reduce mussel recruitment in the subtidal zone. This is caused by juvenile seastar predation upon the barnacles that grow on mussels, because such epigrowth strongly enhances recruitment in mussels. Such an indirect effect on mussel recruitment may affect mussel density more than adult seastar predation. An exception may be mass invasions of A. rubens on subtidal mussel beds. One such event happened during this study, clearing a large patch of mussels.