Parasitism and invasive species: effects of digenetic trematodes on mussels

The brown mussel Perna perna in South Africa is threatened by the introduction of the invasive Mediterranean mussel Mytilus galloprovincialis. Whilst the indigenous P. perna has been found to be commonly infected by digenetic trematodes, the invasive M. galloprovincialis is free of trematodes, which may give it a competitive advantage in direct or indirect interactions between the 2 mussels. The most common parasites infecting P, perna are 2 species of digenetic trematodes: metacercariae of the genus Proctoeces and bucephalid sporocysts. The influence of these 2 parasites on the ecological fitness of their host was tested by examining their effects on survival and competitive ability of P, perna. The results showed significant negative effects. Both parasites significantly depressed condition, but only after spawning, when the mussels were already stressed. Neither parasite affected mortality rate or gaping behaviour of I? perna exposed to air. Proctoeces did not affect the force required to open mussels or the amount of water lost by mussels in air. In contrast mussels infected with bucephalid sporocysts were easier to open and. lost significantly more water than non-infected individuals, possibly because their valves did not seal properly. There were no significant differences in either number or size of oocytes in females infected. with Proctoeces compared with non-infected females. However, bucephalid sporocysts had a dramatic effect on reproduction by castrating the host. Proctoeces reduced growth both in summer and in winter, whilst bucephalid sporocysts had no significant effect on growth. Neither parasite had a significant effect on filtration rates or oxygen consumption of the host. All these results indicate that both Proctoeces and the bucephalid sporocysts have detrimental effects on P. perna. Proctoeces affects primarily growth, while bucephalid sporocysts affect reproduction, adductor muscle strength and water loss. The effects of both parasites are concentrated on those size classes of mussel which channel most energy into the portion of the energy budget affected by the parasite. Proctoeces affects growth only in the smaller individuals, which in normal conditions would put most energy into growth; bucephalid sporocysts castrate the bigger mussels, which would expend most energy on reproduction. In energetic terms, the absence of effects on filtration and respiration indicates that there was neither re-allocation nor compensation for the energy lost from production, but that it was simply re-routed to the parasite. These negative effects, together with the high prevalence of both parasites in P. perna along the South African coast and their absence in M. galloprovincialis, suggest that lack of these parasites may contribute to the success of M, galloprovincialis.