Exploitation of Anodonta piscinalis (Bivalvia) by trematodes: parasite tactics and host longevity

In a long-lived molluscan host, trematode adaptations that minimize the risk of parasite-induced host mortality may evolve because the residual reproductive value of a parasite would not be limited by a short life-span of the host. Two features of the host exploitation (site selection and timing of the reproductive period) were examined for the bucephalid trematodes Rhipidocotyle fennica and R. campanula in their first intermediate host Anodonta piscinalis which is a long-lived freshwater clam. The gonad, assumed to be less critical for survival of the host, was the organ most frequently parasitized by both of the parasite species. Except for the kidney in the case of R. campanula, the prevalence of infection was low in organs that were expected to be important for host survival (kidney, gills, digestive gland). We also made a literature survey on the site selection of larval trematodes of aquatic molluscs in general. We found that the longer the life-span of the host, the higher the proportion of trematode-mollusc associations where the gonad is the main site of infection. Infected A. piscinalis do not reproduce due to parasitic castration but the uninfected individuals develop their offspring, presumably, during the period of most abundant resources in the seasonally fluctuating environment. Therefore, if the parasites adjust their cercarial production to the same period as the uninfected clams they should minimize mortality caused by an energetic stress for the host. We found the reproductive period of R. fennica to co-occur with that of the host, while the cercarial production of R. campanula started 2 weeks earlier than the host reproduction. In summary, R. fennica followed a “damage avoidance tactic” but R. campanula to a lesser extent.