Native and nonnative bivalve settlement: Potential competition for spatial resources in a northeast Florida estuary

Introduced species often have negative effects on native species that occupy the same habitats in the area of introduction. Habitat preferences of organisms can therefore play a large role in determining the severity of interactions such as competition or predation. Previous laboratory studies have shown that the introduced mussel species Perna viridis can decrease the settlement and early survival of the native oyster species Crassostrea virginica. Both species can often be found in the same general areas, but it is unknown whether this habitat overlap is complete, or whether there are certain locations that may act as refuges for native bivalve species. The present study investigated the settlement patterns of the introduced P. viridis and two groups of native bivalves, oysters (C. virginica and Ostrea equestris) and mussels (Ischadium recurvum, Brachidontes exustus and Geukensia demissa), to determine whether preferential settlement depths and habitats differed among the groups. Most of these groups showed significantly greater settlement on deeper substrates that were located in the main channel of the Atlantic Intracoastal Waterway of northeast Florida. While native oysters also settled in adjacent feeder creeks in fairly large numbers, settlement of P. viridis in these areas was relatively rare. In main channel locations, the overlap in settlement depth is likely to result in competition between introduced and native bivalve species, but feeder creeks may provide areas of refuge for native oysters. The spatial and temporal overlap of all three groups suggests that competition among nonnative and native bivalves may occur in nature