Genetic structure of pest polydorids (Annelida: Spionidae) infesting Crassostrea gigas in southern Africa : are pests being moved with oysters?

[摘要] ENGLISH ABSTRACT: Polydorid polychaetes infest commercially important shellfish such as the oyster, Crassostrea gigas, and can cause financial losses to the industry. Early shipping voyages from Europe to South Africa, and the importation of oyster spat from USA, France, UK, Chile and Namibia, has most likely led to the introduction of non-native shell-boring polydorids in South Africa. Additionally, oysters are often moved between farms which may spread these pests further. The most prevalent southern African polydorids infesting farmed C. gigas are the indigenous Boccardia pseudonatrix, the introduced Polydora hoplura and a species tentatively identified as Polydora ciliata/calcarea. The aims of this study were therefore to 1) confirm the identity of P. ciliata/calcarea and to 2) determine the genetic structure of the three pests and compare these structures to a control for natural dispersal (Boccardia polybranchia) to determine if pests worms are a) being moved with oysters, b) moving between farm and wild sites or c) moving naturally between sites, facilitated by ocean currents along the southern African coast. Traditional taxonomic characters were used to identify species, and revealed that P. ciliata/calcarea morphologically closely resembles Polydora websteri from Japan and Australia. To confirm this identity, an 18S rRNA phylogeny of 1759 bp was constructed for P. ciliata/calcarea, P. websteri from Japan, Australia and USA and other morphologically similar species. The phylogeny supported the morphological data; southern African specimens differed by only 2 bp (0.1%) from Japanese and Australian P. websteri specimens. However, they all differed markedly (29 bp/1.6%) from P. websteri from near the type locality in the USA. It was therefore concluded that American specimens represent the 'true P. websteri, and that southern African, Japanese and Australian specimens represent a morphologically similar, but genetically distinct species, here referred to as Polydora cf. websteri. Analysis of the mtDNA Cytochrome b and nuDNA ATPsα datasets revealed that Cyt b was more sensitive in detecting genetic differentiation among populations, whereas the ATPsα marker showed a lack of phylogeographic structure. The Cyt b haplotype network constructed for B. polybranchia showed a high level of genetic structure between east and west coast populations, which is concordant with a documented barrier to gene-flow at Cape Point. However, genetic structure among east coast populations was discordant with all other documented barriers to gene-flow in that region. The genetic distribution of B. polybranchia suggests that dispersal is primarily influenced by local ocean currents. Haplotype networks for B. pseudonatrix show some genetic structure among farms, suggesting independent sources of infestation and localised movement between wild and farmed sites, with some inconclusive evidence for anthropogenic movement between Kleinzee and Hamburg farms. Populations of P. hoplura show some genetic structure among neighbouring sites, probably due to localised dispersal of larvae, however, there is substantial evidence for the anthropogenic dispersal of this species. Polydora cf. websteri revealed a single Cyt b haplotype for all populations, providing some evidence for a single introduction from a single source population. Due to the absence of variation in this marker it is not possible to make any inferences on anthropogenic dispersal Overall, both introduced species show no evidence of genetic structure which could be attributed to anthropogenic dispersal. These results suggest that caution should be exercised with the movement of molluscs since shell-boring polydorids are likely to be moved with them.

[发布机构] Stellenbosch University