Effects of the alien Pacific oyster ( Crassostrea gigas ) on subtidal macrozoobenthos communities

Since 2006, the Pacific oyster Crassostrea gigas has been a permanent resident on the west cost of Sweden. Because C . gigas is nonindigenous in Scandinavia, it may modify ecosystems and affect the resident biota. Individuals of C . gigas often settle in large aggregations, and the physical structure of the resulting reef provides attachment points and refuges for many secondary species. However, C . gigas also has the potential to change the macrofaunal community structure of the associated sediment, for example by stabilization or enrichment of the sediment. Here, we assess the macrozoobenthos community of sediments within C . gigas reefs and contrast the results with the comparable community within beds of the native blue mussel ( Mytilus edulis ) and with that of uniform bare sediment. We show that the communities within oyster reefs and mussel beds contained more species and had a higher abundance of organisms compared with the bare sediment. In addition, we show significant differences between the communities within oyster reefs and mussel beds and consistently a larger total abundance in the former.