Genetic homogeneity among geographically distant populations of the blister worm Polydora websteri

The shell-boring polychaete worm Polydora websteri, also known as blister worm, is a pest species that infests the shells of several commercially important shellfish species, including those of the oysters Crassostrea virginica and C. gigas. Historical records indicate that infestations of blister worm have impacted shellfish culture worldwide for over 100 yr. Although a lot of attention has been given to the burrowing habits of and damage created by blister worms, few studies have examined the levels of population genetic structure in this cosmopolitan species. We examined the patterns of sequence divergence at the mitochondrial cytochrome c oxidase subunit I (mtCOI) gene for individual P. websteri specimens sampled from oysters and scallops collected from shellfish farms along the US Atlantic and Gulf coasts, and Hawaii. The mtCOI sequences we obtained were aligned and compared with published sequences for P. websteri adults and larvae sampled at an oyster farm in Guangdong Province, China. We observed little genetic variation, overall, and no differentiation between populations of P. websteri, a pattern that suggests high levels of connectivity among locations. It is unlikely that natural dispersal alone can account for this lack of differentiation; rather, our results are consistent with the hypothesis that human-mediated introductions have led to genetic homogeneity across large geographic distances for this pest polychaete species.