Cross-transplantations of Omphiscola glabra between two French departments (Haute-Vienne and Indre) were carried out to study snail settlement in these new sites during six years and to determine if this snail species was well adapted to its local parasites (Fasciola hepatica and Paramphistomum daubneyi). Local snails placed in new sites for each department were used as controls. In Indre furrows, the number of adults per population and the area of each habitat peaked on the third year post-introduction. Similar findings were also noted in the Haute-Vienne furrows occupied by local snails but peaks were only observed on the fourth year. In contrast, in Haute-Vienne sites populated by introduced snails, there was a progressive decrease in snail abundance and habitat area from the second year. The local O. glabra transplanted in Haute-Vienne kept their same degree de susceptibility to F. hepatica and P. daubneyi, thus demonstrating a good adaptation of these snails to their parasites. In other groups of transplanted snails, the first natural infections were only noted from the second or third year post-introduction and their prevalence progressively increased over time. Local adaptation of these latter snails to parasites would be more progressive in time and would stretch over several snail generations.

The effect of experimental non-host snail (NHS) communities on the infection of Lymnaea truncatula with Paramphistomum daubneyi and Fasciola hepatica miracidia was studied. The results obtained indicated that NHS had variable effects on basic population parameters of host infection. Specifically, NHS were able to produce aggregation of sporocyst number/snail in the experimental host populations. Proposed hypotheses for decoy effect mechanisms failed to explain completely the experimental results. A new mechanism is suggested, based on a distance response of miracidia to snail chemo-attractants, to explain effects of NHS on miracidial host finding behaviour. It is assumed that miracidia have evolved the ability to discriminate among individual snail chemical attractants in a community already at a distance. This attribute of miracidial behaviour allows NHS to induce variations in the effectiveness of individual host localization by miracidia. The resulting aggregation of successful infections/host is assumed to be the specific component of snail decoy effect at the community level. It was also established that NHS affected trematode reproduction in the host inducing a compensatory increase in the intramolluscan population. Intraspecific competition among developmental stages is suggested to be the mechanism of this compensatory reaction.