Dispersal and genetic structure of Boccardia polybranchia and Polydora hoplura (Annelida: Spionidae) in South Africa and their implications for aquaculture

Abstract

Dispersal and observed genetic structure of sessile marine invertebrates such as the alien polydorid polychaete, Polydora hoplura, which is associated with cultured molluscs, may be influenced by biogeographic barriers and human-mediated dispersal. To test this hypothesis, we compared the population genetic structure of P. hoplura to that of Boccardia polybranchia since both species have similar larval developmental modes (and consequently natural dispersal potential) and distribution ranges in southern Africa. However, since B. polybranchia is not a pest, its population structure should reflect the effects of only biogeographic barriers on natural dispersal. Boccardia polybranchia was collected from five wild sites along the South African coast, and P. hoplura from seven wild sites and four oyster farms; two each were on-shore and off-shore. Analyses of mtDNA Cyt b sequence data indicate two distinct lineages for B. polybranchia, supporting Cape Point as a biogeographic barrier to the dispersal of larvae. In contrast, no distinct geographic lineages were identified for P. hoplura occurring in the wild, and haplotype sharing suggest possible cross infestation between farm and nearby wild sites. At farms the genetic diversity of P. hoplura is lower but also more structured, suggesting separate anthropogenic introductions at each site possibly from genetically distinct source populations. Furthermore, more limited genetic sharing was detected between worms from on-shore and nearby wild sites, whereas the opposite was observed for off-shore farms and associated wild sites. This suggests that populations in on-shore farms are more contained with most of the individuals carrying parasites corresponding to the source of introduction while those on off-shore farms display much more genetic exchange with wild populations. Thus oysters in off-shore facilities are probably more vulnerable to infestation by polydorid pests than are those in on-shore facilities. Statement of relevanceMovement of oysters spreads associated pests