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Marine intertidal hard substrates provide an ecologically important habitat for many sessile or semi-sessile organisms. As shelter, food source, nursery or living space, they are of central importance in marine ecosy- stems worldwide (Bellan-Santini et al., 1994). Particularly in the Wadden Sea, dominated by soft substrates and extensive tidal flats, the few hard substrates are essential to many sessile organisms. Therefore, such habitats require special protection. During the last forty years, the pacific oyster became more dominant in the North Sea. While it appears to be in compe- tition with the blue mussel, it could provide potential new niches for sessile organisms, on the other side (Dietrich, 2005). This study aims to make a comparative investigation of the biodiversity on the different types of hard substrates in the intertidal zone of the North Sea on Sylt Island. In total, 53 species belonging to three different ecological guilds (algae, sessile and mobile animals) could be documented on four dif- ferent substrates under two conditions. Besides the attached species’ list, different ecological parameters were calculated, including species abundance, Evenness, and the Shannon-Wiener Index (H). The latter was highest for rocks from the rock-blue mussel reef and lowest in detached rocks taken from the mudflat. In bivalves, the observed biodiversity in sessile organisms was higher in pacific oysters than it was on blue mussels. The opposite is true for mobile organisms, which showed a higher biodiversity on blue mussels found in reef than on isolated oysters. This study has shown that the isolated oysters provide a comparable biodiverse habitat for species living on hard substrate in the Wadden Sea, showing a comparable biodiversity. The native blue mussel as well as the rock reefs shows a great abundance of species, being a shelter for specialist taxa like the hydrozoans. In contrast to the expectations, the oysters offer living space for many native taxa. Therefore, as an extension to the original habitats, they might represent an enrichment of the hard substrate habitat in the Wadden sea on Sylt.

Additional file 1. Table S1: Primer names, sequences and sources of protocols used for their amplification; Table S2: GenBank accession numbers of previously published sequences of cytb and cox1 genes of Polychromophilus melanipherus; Table S3: Overview of Polychromophilus melanipherus cytb and cox1 haplotypes obtained; Figure S1: Haplotype network of concatenated sequences of genes cytb and cox1 of Polychromophilus melanipherus 1524 bp long.