Parasitism and introduced species: epidemiology of trematodes in the intertidal mussels Perna perna and Mytilus galloprovincialis

The Mediterranean mussel Mytilus galloprovincialis was accidentally introduced onto the South African west coast in the late 1970s and has since become invasive. It is spreading onto the south and east coasts where it has the potential to outcompete the indigenous mussel Perna perna. One possible explanation for this phenomenon is that it is not affected by the natural parasites of the area where it was introduced. A survey of trematodes was done at different spatial scales for populations of the indigenous mussel Perna perna and the invasive Mytilus galloprovincialis in South Africa, as well as for indigenous populations of M. galloprovincialis in Spain. Both South African and Spanish populations of M. galloprovincialis were found to be free of trematodes, whilst P. perna was commonly infected by trematodes. The most important trematodes found in this mussel were metacercariae of Proctoeces and bucephalid sporocysts, with infection rates up to 62 and 49%, respectively. Both Proctoeces and the bucephalid sporocysts were randomly distributed within a mussel bed. Bucephalid sporocysts were found in very low prevalences at most sites along the South African coast. Prevalences were highly dependent on the size of the mussel, and there were no seasonal variations in infection rates with this parasite. Proctoeces was only found between Cape Agulhas on the south coast and Sodwana Bay on the east coast, and infected significantly more females than males. Prevalence of infection was highly dependent on the size of the mussel in the case of females but not in the case of males. There were no seasonal variations in prevalence of infection with Proctoeces in either females or males. Mytilus galloprovincialis has proved to be a competitively superior species which has replaced the indigenous Aulacomya ater as the dominant mussel on the west coast. The fact that it is free of trematodes, whilst Perna perna on the south and east coasts is commonly infected by trematodes, may be one of the reasons for its success in South Africa as a whole and its incipient invasive behaviour on the south coast. (C) 1998 Elsevier Science B.V.