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

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

“…In contrast, worms at 'onshore' farms (< 0.5 km from shore) showed lower diversity and the presence of unique haplotypes, which Williams et al (2016) attributed to the aquaculture-mediated introductions along with the subsequent development of large local populations supported by a preponderance of local recruitment by adelphophagic larvae. In some cases, Williams et al (2016) noted that haplotype-sharing among 'onshore' farms was likely the result of the transfer of host shellfish between farms. Overall, Williams et al (2016) concluded that long-term importation of shellfish, proximity of farms to ports where pests may have been introduced via ballast water discharge, along with local, regional or interregional movement of shellfish hosts has led to the patterns of genetic diversity without any appreciable population structure they observed among P. hoplura populations in South Africa.…”

“…Even so, Selkoe & Toonen (2011) have shown that there is a consistent pattern of increasing genetic distance with increasing geographic distance, with the shape of this relationship dependent on planktonic larval duration, which serves as a proxy for dispersal in many coastal marine invertebrates. In a recent study, Williams et al (2016) compared the degree of population structure for 2 spionid polychaetes, Boccardia polybranchia, which is not associated with shellfish aquaculture, and P. hoplura, which is. For B. polybranchia, they found evidence for isolation by distance among mtDNA sequence clades observed on either side of Cape Point, South Africa.…”

“…The comparative lack of population genetic structure for pest polychaete species on shellfish farms indicates that processes other than natural dispersal are influencing the patterns of genetic diversity. For example, Williams et al (2016) found that P. hoplura associated with oysters in 'offshore' farms (>1 km from shore) near ports had the highest levels of genetic diversity, ostensibly due to a combination of previous aquaculture-based introductions along gene flow from wild populations that were established via ballast water introductions. In contrast, worms at 'onshore' farms (< 0.5 km from shore) showed lower diversity and the presence of unique haplotypes, which Williams et al (2016) attributed to the aquaculture-mediated introductions along with the subsequent development of large local populations supported by a preponderance of local recruitment by adelphophagic larvae.…”

“…For example, Williams et al (2016) found that P. hoplura associated with oysters in 'offshore' farms (>1 km from shore) near ports had the highest levels of genetic diversity, ostensibly due to a combination of previous aquaculture-based introductions along gene flow from wild populations that were established via ballast water introductions. In contrast, worms at 'onshore' farms (< 0.5 km from shore) showed lower diversity and the presence of unique haplotypes, which Williams et al (2016) attributed to the aquaculture-mediated introductions along with the subsequent development of large local populations supported by a preponderance of local recruitment by adelphophagic larvae. In some cases, Williams et al (2016) noted that haplotype-sharing among 'onshore' farms was likely the result of the transfer of host shellfish between farms.…”

“…In some cases, Williams et al (2016) noted that haplotype-sharing among 'onshore' farms was likely the result of the transfer of host shellfish between farms. Overall, Williams et al (2016) concluded that long-term importation of shellfish, proximity of farms to ports where pests may have been introduced via ballast water discharge, along with local, regional or interregional movement of shellfish hosts has led to the patterns of genetic diversity without any appreciable population structure they observed among P. hoplura populations in South Africa.…”