Kin aggregations occur in eastern oyster Crassostrea virginica reefs despite limited regional genetic differentiation

Larval dispersal, particularly for sessile or sedentary marine organisms, significantly influences the scale of population structure in many species and fundamentally depends on the degree to which larvae from different populations are mixed in the plankton. In general, larval dispersal is thought to lead to well-mixed populations; however, recent evidence shows genetic structure at highly localized spatial scales in several benthopelagic species, raising important questions about realized patterns of larval dispersal and the scale of metapopulation connectivity. Here we use 22 microsatellite markers to characterize multi-scale patterns of genetic structure in the eastern oyster Crassostrea virginica, an ecologically and economically important foundation species located along the east coast of North America. At regional scales, we find limited evidence of spatial genetic structuring and weak population differentiation across 4 sites spanning 200 km of coastline. However, despite evidence of larval mixing and limited population structure, we find significant levels of kin structure at the scale of individual reefs, a pattern consistently found across samples. Such localized kin aggregations suggest that oyster larvae have significant larval retention within natal sites or exhibit non-diffusive larval movement, whereby siblings are more likely settle together. Importantly, these results show that larval mixing in the plankton is less extensive than previously believed, which has important implications for our understanding of population connectivity, gene flow, and the appropriate spatial management of marine resources.