Tidepools are well-known structures at tidal coastal ecosystems. They mainly occur on rocky shores, while they are less abundant on sedimentary shores such as the Wadden Sea in the south-eastern North Sea due to the lack of structures, that hold back the water during low tide. However, with the introduction of the ecosystem engineering Pacific oyster Magallana gigas (former Crassostrea gigas) and its establishment on native blue mussel beds (Mytilus edulis), new biogenic structures developed, which also caused an increase of tidepools within the reefs. These tidepools offer constant submersion in the intertidal zone, which may cause changes in the occurrence of mussel and oyster reef associated species. In this study, I determined the densities of barnacles, amphipods and the common periwinkle Littorina littorea within and outside of tidepools. Furthermore, I examined the prevalence of the copepod parasites Mytilicola intestinalis and Mytilicola orientalis and the metacercarial stage of the trematode parasite Renicola roscovita in native blue mussel M. edulis occurring inside and outside of tidepools. My results revealed a lower barnacle coverage on mussels and oysters in- than outside of tidepools. An experimental removal of grazers and predators in- and outside of tidepools resulted in a strong increase of barnacle recruitment in both habitats. Densities of periwinkles were similar in- and outside of tidepools. However, the grazing and bulldozing activity of the common periwinkle L. littorea is increased in permanently flooded areas, leading to higher displacement of juvenile barnacles from hard substrate resulting in a lower barnacle coverage within tidepools. The trematode R. roscovita showed higher abundances in blue mussels collected outside of tidepools, whereas the parasite copepod Mytilicola spp. is more abundant in blue mussels within tidepools. The density of amphipods was also found to be higher inside of tidepools. The two parasites have a difference in life cycle with R. roscovita having an increased number of free- living phases compared to Mytilicola spp. To explain their difference in infection rates in mussels inside and outside of tidepools, we suggest a stronger predation pressure by amphipods on free-living stages of R. roscovita than on the free-living stages of Mytilicola spp. My results indicate that tidepools may serve as catalysator for the spread of Mytilicola spp. in mixed mussel and oyster reefs, but do not represent a favourable habitat for barnacles and trematodes.