In marine sedimentary coastal environments such as the Wadden Sea (coastal south-eastern North Sea) shells of epibenthic molluscs are the only major available settlement substrate for an increasing number of alien sessile organisms. We investigated the effects of Pacific oyster Crassostrea gigas epibionts on body weight, mobility and fecundity of the native common periwinkle Littorina littorea. Body dry weight of snails without oyster overgrowth was twice as high compared to periwinkles covered with oysters. Also crawling speed of snails with oyster epigrowth was significantly slowed down and about ten times lower than in unfouled periwinkles. Additionally, oyster epibionts caused a strong decrease in reproductive output. In laboratory experiments, egg production of fouled L. littorea was about 100-fold lower than in periwinkles free of oysters. Field surveys in different years and habitats demonstrated that up to 10% of periwinkles occurring on epibenthic bivalve beds and up to 25% of snails living on sand flats may be fouled by C. gigas. However, oysters are not the only epibionts on L. littorea in the area. In previous studies it was shown that periwinkles can also be strongly overgrown by other alien epibionts such as Australian barnacles Austrominius modestus and American slipper limpets Crepidula fornicata. These epibiotic structures may cause similar effects as revealed for C. gigas. Thus, we conclude that an alien epibiont accumulation on L. littorea may have a significant impact on periwinkle population dynamics and its ecological functions in the Wadden Sea. © 2011 The Author(s).