Effect of dose and frequency of exposure to infectious stages on trematode infection intensity and success in mussels

Marine parasites such as trematodes often compromise the fitness of their hosts. Sucheffects are generally considered to be density-dependent, i.e. the greater the infection intensity inthe host, the greater the detrimental impact on host fitness. However, the mechanisms determininginfection in marine hosts are still poorly understood. Here, we investigated the effect of cercarialdose and exposure frequency (single vs. trickle infections) of a marine trematode parasite,Himasthla elongata (Trematoda: Echinostomatidae), on infection intensity and success in its secondintermediate host, the blue mussel Mytilus edulis, an abundant and widely distributed bivalvein European coastal waters. In our laboratory experiment, we tested 4 levels of parasite doses andshowed that mussels faced higher parasite infection intensity at higher doses of cercarial exposureand that they acquired more infections when repeatedly exposed to smaller doses compared to asingle high dose. However, the infection success of cercariae did not differ among 4 dose levelsbut was only significantly different between trickle and single exposures. This indicates that cercariaewere not subjected to a dose-dependent regulation of their infectivity, suggesting thatinfection intensity in mussels is largely driven by factors mediating the abundance of infectivestages. With the combined investigation of the effect of cercarial dose and exposure frequency atrealistic dose levels, our study contributes to our currently very limited understanding of the de -terminants of infection intensity in marine hosts and highlights the usefulness of experimentalstudies in advancing our knowledge in this field