The introduction of non-indigenous species may cause strong effects on biodiversity, species interactions and functioning of native coastal ecosystems. Especially introduced bioengineering organisms may fundamentally change native soft-bottom ecosystems by modifying existing benthic habitat structures and thereby habitat-specific species interactions. The introduction of Pacific oysters Crassostrea gigas into the sedimentary coastal area of the south-eastern North Sea and its preferred settlement on native blue mussel shells caused a large-scale shift from pure epibenthic Mytilus edulis beds to current mixed reefs of mussels and oysters. To investigate whether the newly developed biotic habitat affects the occurrence of associated key organisms and their ecological functions, we studied the long-term occurrence of periwinkles Littorina littorea and their grazing activity on barnacles attached to Pacific oyster shells in the northern Wadden Sea. We found no negative effects of oyster density increase on periwinkle population dynamics in the last two decades, which spans a time-period from the beginning of Pacific oyster establishment on mussel beds to oyster dominance today. A manipulative field experiment revealed that density of L. littorea significantly affects the recruitment success of barnacles Semibalanus balanoides on oyster shells. The highest number of barnacle recruited at periwinkle exclusion. Thus, snail density and resulting grazing activity may control barnacle overgrowth on oyster shells, which is known to cause detrimental effects on overgrown bivalves. The company of introduced oysters and native periwinkles in the Wadden Sea shows that alien species may find native friends in newly conquered ecosystems with mutual benefits.