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

Williams, Lee-Gavin (2015-03)

Thesis (MSc) -- Stellenbosch University, 2015.


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.

AFRIKAANSE OPSOMMING: Polydorid polikete infesteer kommersiële belangrike skulpvisse soos die oester, Crassostrea gigas, en dit kan tot finansiële skade in die industrie lei. Vroëe verskepingsritte vanaf Europa na Suid-Afrika en die invoer van oesters vanaf die VSA, Frankryk, Engeland, Chillie en Namibië het tot die invoer van indringer uitheemse skulp-borende polydorids in Suid Afrika gelei. Aanvullend tot dit word oesters tussen plase verskuif en dit versprei die peste verder. Die mees algemeenste polydorids wat geboerde C. gigas besmet in suidelike Afrika is die inheemse Boccardia pseudonatrix, die indringer spesies Polydora hoplura en 'n spesie wat voorlopig as Polydora ciliata/calcarea geidentifiseer is. Die doelwitte van die studie is om 1) die identiteit van P. ciliata/calcarea te bevestig en 2) om die genetiese struktuur van die drie peste te bepaal en te vergelyk met 'n kontrole vir natuurlike verspreiding (Boccardia polybranchia) om vas te stel a) of peste met oesters versprei word, b) of daar beweging tussen plase en wilde lokaliteite is of c) natuurlike beweging tussen lokaliteite gefasiliteer word deur seestrome langs die suidelike Afrika kuslyn. Tradisionele taksonomiese karakters was gebruik om spesies te identifiseer en het bewys dat P. ciliata/calcarea morphologies baie na aan Polydora websteri van Japan en Australiё is. Om hierdie identifikasie te bevestig is 'n 18S rRNA filogenie van 1759 bp gekonstrueer vir P. ciliata/calcarea, P. websteri van Japan, Australiё en die VSA en ander morfologies soortgelyke spesies. Die filogenie het die morfologiese data ondersteun, suidelike Afrikaanse eksemplare verskil slegs 2 bp (0.1%) van die Japanese en Australiese eksemplare. Hierdie groep het egter grootliks verskil (29 bp/1.6 %) van P. websteri wat versamel is naby die tipe lokaliteit in die VSA. Die gevolgtrekking was dat Amerikaanse eksemplare die “ware” P. websteri verteenwoordig en dat suidelike Afrika, Japenese en Australiese eksemplare 'n morfologiese soortgelyke, maar genetiese spesifieke spesies verteenwoordig, hier verwys na as Polydora cf. websteri. Analise van die mtDNA Cytochrome b en nuDNA ATPsα datastelle het bewys dat die Cyt b meer sensitief is om genetiese differensiasies tussen bevolkings op te spoor waar die ATPsfix merker 'n tekort van filogeografiese struktuur gewys het. Die Cyt b haplotiepe network gekonstruktueer vir B. polybranchia toon 'n hoё vlak van genetiese struktuur tussen oos en weskus bevolkings wat in ooreenstemming is met die gedokumenteerde hindernisse tot geenvloei by Kaappunt. Genetiese struktuur tussen die ooskusbevolkings was nie geaffekteer deur hindernisse vir geenvloei in daardie area nie. Die genetiese verspreiding van B. polybranchia suggereer dat verspreiding word primêr deur plaaslike see strome beïnvloed. Haplotipe netwerke vir B. pseudonatrix toon geringe genetiese struktuur tussen plase wat op onafhanklike bronne van besmetting dui, en plaaslike beweging tussen wilde en geboerde lokasies met moontlike antropogeniese beweging tussen Kleinzee en Hamburg plase. Populasies van P. hoplura toon genetiese struktuur tussen naburige lokaliteite, waarskynlik as gevolg van gelokaliseerde verspreiding van larwas, maar daar was genoegsame bewys vir antropogeniese verspreiding van die spesies. Polydora cf. websteri het 'n enkel Cyt b haplotipe vir alle populasies bewys wat dui op „n enkele vestiging. As gevolg van die feit dat alle diere dieselfde haplotipe deel kon geen uitspraak gemaak word oor antropogeniese verspreiding nie. Oor die geheel, toon beide spesies geen duidelike biogeografiese genetiese struktuur nie wat moontlik kan wys op antropogeniese verspreiding. Hierdie resultate suggereer dat die beweging van skulpvisse versigtig gedoen moet word aangesien skulp-borende spesies ook saam beweeg kan word.

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