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), seasta rs Asterias rubens (L.) co-occur with their preferred prey, mussels Mytilus edulis (L.), which form extensive beds from the intertidal down to the sub tidal zone. Mussel density within these beds is significantly lower in the subtidal than the intertidal zone. Laboratory and field experiments were co nducted 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-overgr own intertidal ones. This preference coincided with higher abundances of bo th 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 muss el density in the area studied. However, seastars may indirectly reduce mus sel recruitment in the subtidal zone. This is caused by juvenile seastar pr edation upon the barnacles that grow on mussels, because such epigrowth str ongly enhances recruitment in mussels. Such an indirect effect on mussel re cruitment may affect mussel density more than adult seastar predation. An e xception may be mass invasions of A. rubens on subtidal mussel beds. One su ch event happened during this study, clearing a large patch of mussels. (C) 2001 Elsevier Science B.V. All rights reserved.