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Dense beds of mussels of the family Mytilidae occur worldwide on soft-bottoms in cold and warm temperate coastal waters and have usually been considered hot spots of biodiversity. We examined intertidal mussel beds at four distant locations around the globe with the same sampling method, to Wnd out whether this "hot spot" designation holds universally. We studied species assemblages within the matrices of byssally interconnected mussels engineered by Mytilus edulis in the North Sea, by mixed Perumytilus purpuratus and Mytilus chilensis at the southern Chilean coast, by Musculista senhousia in the Yellow Sea and by Xenostrobus inconstans at the coast of southern Australia. In all cases, species assemblages inside mussel beds were signiWcantly diVerent from those outside with many species being restricted to one habitat type. However, species richness and diversity were not generally higher in mussel beds than in ambient sediments without mussels. In the North Sea (M. edulis) and at the Chilean coast (P. purpuratus, M. chilensis), mussel beds have markedly higher species numbers and diversities than surrounding sediments, but this was not the case for mussel beds in Australia (X. inconstans) and the Yellow Sea (M. senhousia) where numbers of associated species were only slightly higher and somewhat lower than in adjacent sediments, respectively. In conclusion, although soft bottom mytilid mussels generally enhance habitat heterogeneity and species diversity at the ecosystem level, mussel beds themselves are not universal centres of biodiversity, but the eVects on associated species are site speciWc.

The faunal assemblages of a mussel bed (Mytilus edufis L.) and ambient sand_flat were compared to study how a bioherm of suspension feeding organisms affects benthic communities in a tidal flat. During a survey of mussel beds in the Wadden Sea at the island of Sylt {North Sea}, a total of 52 macrofaunal species and 44 meiobenthic plathelminth species were detected. They occupied different microhabitats in the mussel bed. 56 % of the macrofauna species were dwelling in the sediment beneath the mussels and 42 % were epibenthic or epiphytic. The latter were restricted in their occurrence to the mussel bed. Along a transect from the sandflat to the mussel bed the mean species densities of macrofauna did not differ significantly, while abundances were significantly lower in the mussel bed than in the sandflat. The composition of the assemblages shifted from a dominance of Polychaeta in the sandflat to Oligochaeta in the mussel bed. Surface filter-feeding polychaetes of the sandflat (Tharyx mariom) were displaced by deposit feeding polychaetes under the mussel cover (Capitella capitata, Heteromastus filiformis}. The total meiobenthic density was lower and single taxa (Ostracoda, Plathelminthes, Nematoda} were ~ignificantly less abundant in the mud of the mussel bed. The plathelminth assemblage was dominated by grazing species (Archaphanostoma agile}, and differed in community structure from a sandflat assemblage. An amensalistic relationship was found between the suspension-feeding mussels and suspensionfeeding infauna, while deposit-feeders were enhanced. The presence of epibenthic microhabitats results in a variety of trophic groups co-occurring in a mussel bed. This is hypothesized as trophic group amelioration and described as an attribute of heterotrophic reefs.

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Since the late 1990s, the Pacific oyster (Crassostrea gigas) has spread into the East Frisian Wadden Sea (Germany). This invasion provided an opportunity to study the population dynamics and the patterns of spread during the initial bioinvasion process. With its source area in The Netherlands, the bioinvasion continues in an eastward direction, as documented by a gradient of high abundances in the west and low abundances in the east during the first study year. One year later, abundances of the Pacific oyster were more heterogenic and differed between adjacent tidal basins. The increase in population sizes at all study sites was very high, reaching levels similar to native occurrence populations. The growth constant (K) varied between 0.300 and 0.990 year -1 . The mussel bed with the highest densities had a mean abundance of [300 ind. m -2 , and a maximum of 1,460 ind. m -2 . Furthermore, the bioinvasion was facilitated by a low mortality (Z) found for populations between 0.5 and 1.5 years old (Z = 0.03-0.13 year -1 ). At present, Pacific oysters are well established at several locations in the East Frisian Wadden Sea and may become with these reproductive potential self-sustaining populations.

Macrozoobenthos communities in the North Sea showed pronounced changes over the past decade in relation to an increasing number of invasive species and climate change. We analysed data sets spanning 22 years on abundance, biomass and species composition of intertidal soft bottom mussel beds near the island of Sylt (German Bight) in the Northern Wadden Sea, based on surveys from 1983/ 1984, 1990, 1993 and from 1999 to 2005. Mussel bed area and blue mussel biomass decreased, and a change in the dominance structure in the associated community comparing 1984 to mid-1990s with the period from 1999 to 2005 was observed. Coverage of the mussel beds with the algae Fucus vesiculosus decreased since the end of the 1990s. Within the study period biomass and densities of the associated community increased significantly. Dominance structure changed mainly because of increasing abundances of associated epibenthic taxa. Apart from the Pacific oyster Crassostrea gigas all other alien species were already present in the area during the study period. Community changes already started before Pacific oysters became abundant. An attempt is made to evaluate effects on the observed changes of decreasing mussel biomass, ageing of mussel beds, decreasing fucoid coverage and increasing abundances of invader. All four factors are assumed to contribute to changing community structure of intertidal mussel beds.