Pacific oysters (Crassostrea gigas Thunberg 1793) have been introduced into the Wadden Sea (North Sea) by aquaculture in the 1980s. Subsequently, natural spatfalls occurred and wild oyster populations became established. For settlement, oyster larvae need hard substrates to which they attach themselves permanently. By settling on top of each other, they may create massive biogenic reefs. On the sedimentary tidal flats of the Wadden Sea, epibenthic mussel beds (Mytilus edulis L.) represent the main insular hard substrates, wherefore the oysters attached themselves mainly to the shells of living and dead blue mussels. Resident mussel beds became more and more overgrown by C. gigas and the question arose, whether they all might soon be replaced by oyster reefs. In this context, the bjective was to assess the impact of C. gigas on the native ecosystem by investigating the population development in the northern Wadden Sea, and by evaluating the scope for coexistence with resident mussels. In general, this may be a test case whether an introduced species is capable of displacing a native analogue in a sedimentary shore environment.The invasion of C. gigas in the northern Wadden Sea started in 1991 when the first wild oysters had settled on an intertidal mussel bed in the vicinity of an oyster farm that has started its business in 1986 in the List tidal basin (island of Sylt, Germany). At first, abundances on intertidal mussel beds remained low and patchy (1995: 3.56 ± 3.21 individuals m-2; 1999: 3.71 ± 3.79 individuals m-2). The population slowly expanded its range from intertidal to subtidal locations as well as from Sylt north- and southwards along the coastline. However, a succession of three summers (2001 2003) with anomalous high water temperatures led to a massive increase in oyster abundances (2003: 125.80 ± 119.47 individuals m-2, 2004: 244.44 ± 172.84 individuals m-2). It is assumed that the further invasion of C. gigas in the northern Wadden Sea will benefit from high late-summer water temperatures when these oysters reproduce. However, length frequency distributions revealed that successful cohorts survived for at least 5 years, allowing for population persistence even when warm summers are rare.Studies on recruitment showed differential settlement of oysters and mussels that may lead to niche separation and coexistence of both species. As oysters settle preferentially on conspecifics, a positive feedback of adults on recruitment may facilitate rapid reef formation. Mussels may find a refuge underneath a cover of the brown macroalga Fucus vesiculosus. Potentially, mussels may overgrow oyster reefs in high recruitment years especially if a facilitating barnacle cover is high. However, biotic interactions with C. gigas that reaches about three to four times the size of mussels may prevent M. edulis to become abundant on oyster reefs.Growth experiments revealed a faster growth of C. gigas compared to M. edulis in intertidal and subtidal habitats. Whereas oyster growth is not hampered by the presence of oysters, mussels, and barnacle epigrowth, the growth of mussels is reduced in the presence of these species, thus suggesting competitive inferiority.In field experiments, a high survival rate of juvenile oysters was found and presumably caused by very low predation pressure. About 70% of juvenile C. gigas survived the first three months on an intertidal mussel bed and about 40% reached their first reproductiveperiod one year after settlement. Only early recruitment in the subtidal zone was reduced due to predation. Laboratory feeding preference experiments confirmed that the main benthic predators, shore crabs (Carcinus maenas L.) and starfish (Asterias rubens L.), strongly prefer mussels to oysters. Size selective feeding by the main mussel predators together with an early size refuge from predation due to faster growth and larger size may facilitate a competitive advantage of C. gigas over M. edulis.As C. gigas is well adapted to the Wadden Sea ecosystem and competitive superior to their native congeners, a further increase of the oyster population in the Wadden Sea is expected. The development of massive intertidal and possibly also subtidal oyster reefs that may contain a variable amount of mussel epigrowth depending on recruitment success in different years is considered as a likely future scenario. As oyster recruitment depends on high summer water temperatures whereas high mussel recruitment usually follows severe winters, a possible climate change leading to warmer summers and milder winters will further support the displacement of M. edulis by C. gigas. This regime shift is expected to have profound impacts on the Wadden Sea ecosystem, mainly because oysters are less integrated in the basic food web. A massive increase of the oyster population may lead to food limitation of other suspension feeders, especially in the wake of decreasing eutrophication, and to a decline of benthic predators. However, in which way the resident community will adapt to this new invader will be a future task to tackle. I conclude that the invasion of C. gigas in the Wadden Sea is facilitated by a high efficiency of using space and food resources and by low predation pressure by resident predators.