Shift from native mussels to alien oysters: Differential effects of ecosystem engineers

Blue mussel (Mytilus edulis) beds in the intertidal Wadden Sea (coastal North Sea) have been replaced by introduced Pacific oysters (Crassostrea gigas). To test the effects of these habitat-generating suspension feeders on associated macrozoobenthos, a field experiment was designed. Circular plots (‘rings’) were composed either of oysters, mussels, both together or none at all. These four treatments were arranged in June 2006 in 5 blocks along low tide line. Rings enclose 3 m2 of bare muddy sand surrounded by a 1-m-wide belt (10 m2) of the densely packed epibenthic suspension feeders. Sediment, infauna, mobile epifauna and settling sessile epifauna were sampled. Epibenthic suspension feeders caused an accumulation of fine particles and organic content in the sediment. This was particularly true for mixed treatments indicating interactive effects of both bivalves on sediment characteristics. Mussels caused finer sediment grain-size composition compared to bare sediment. This did not occur among oysters but both bivalves increased organic content. The presence of mussels and oysters increased the abundance of infaunal and epibenthic mobile species differently. The polychaete Lanice conchilega was more abundant on oyster rings and the oligochaete Tubificoides benedeni on mussel rings. Densities of juvenile shore crabs Carcinus maenas and young periwinkles Littorina littorea (< 10 mm shell height) were higher on mussel rings; while abundance of L. littorea ≥ 10 mm shell height was higher on oyster rings. Juveniles of the barnacle Elminius modestus and of mussels showed no preference while oyster spat preferentially settled on conspecifics. We conclude that a shift in dominance from mussels to oysters alters habitat structures which entail differential abundances of associated organisms. This exchange of suspension feeder species is not neutral to community structure because resident mussels and alien oysters function differently as ecosystem engineers.