Polydorid polychaetes on farmed molluscs: distribution, spread and factors contributing to their success

Species of the Polydora-complex (i.e. polydorids) are the most common shell-boring polychaetes found on cultured molluscs. However, which species become problematic depend on their ability to reach mollusc farms and flourish under culture conditions. We therefore hypothesise that the planktonic larval phases of pest polydorids on molluscs grown on-shore will be short (as is typical of adelphophagic larvae, which can maintain large local populations) while those of polydorids on molluscs grown off-shore will be long (as is typical of planktotrophic larvae, which can disperse long distances to farms). Principal component and discriminant analyses of information extracted from the literature partly supported this hypothesis by identifying larval developmental mode and pest species as contributing more to pest status than host species and culture mode, with differential influence on pest status in different situations and potential bias through incorrect identification of polydorid species. chi(2) analyses confirmed that pest status depended on host culture method and pest larval mode. Pest polydorids producing adelphophagic larvae in on-shore systems may reflect the development of large local populations on hosts with culture periods > 2 yr. The many records of pests in off-shore and near-shore systems with pest species producing planktotrophic larvae may reflect shorter host culture periods and the higher incidence of planktotrophy among polydorid species in general. Polydora websteri, P. uncinata, P. hoplura and P. haswelli are the most frequently recorded and widespread pest species globally, although the taxonomy of these and shell-boring P. ciliata and Boccardia polybranchia need to be clarified. The positive relationships between the numbers of alien shell-borers and pests, and the number of hosts cultured per country confirm that mollusc aquaculture is an important vector and reservoir of alien pest polychaetes.