In Mar Chiquita coastal lagoon (Argentina), the reefs of the invasive polychaete Ficopomatus enigmaticus (Serpulidae) serve as concentration areas for invertebrates and vertebrates and as potential foci of parasite transmission (mainly digeneans). To analyse the role of F. enigmaticus as facilitator of parasite transmission, and to evaluate the influence of the habitats selected by 2 species of cochliopid snails (Heleobia conexa and Heleobia australis), on the richness and on the prevalence of the digenean assemblages that parasitize them, 1945 snails were collected from 2 sampling sites. The comparisons between larval digenean communities parasitizing both snail hosts revealed significant differences related to the overall prevalence and the prevalence contributed by birds in the snails collected from reefs. These results support the idea that the reefs may increase the number of links between intermediate and definitive hosts of digeneans, facilitating parasite transmission only when the aggregates of F. enigmaticus become the unique habitat of the snail host. This first report of an exotic species acting as facilitator of parasite transmission in an estuarine area highlights the importance of analysing the effects of the assimilation process of exotic species by recently colonized environments (including the possible effects on parasite transmission).

Factors which may determine trematode–snail interactions were assessed in the present study. Compatibility was examined using a bacterial clearance assay to detect the modulatory effects of both compatible and incompatible trematode infections on the activity of haemocytes from Lymnaea stagnalis, during the early stages of infection. Exposure to and injection with Trichobilharzia ocellata, a compatible trematode, or the incompatible Schistosoma mansoni, resulted in modulation of haemocyte activity. However, T. ocellata activated haemocytes 1·5 h post-infection (p.i.) and then suppressed activity 24–72 h p.i. whereas with S. mansoni no suppression, only activation of haemocytes was observed throughout the test period (1·5–72 h p.i.). In previous studies, modulation of the haemocyte clearance activity by T. ocellata was found to be mediated by 2 E–S fractions, an activating fraction and a suppressing one. Investigations to assess whether the lack of suppression of haemocyte activity, observed in the S. mansoni–L. stagnalis incompatible trematode–snail interaction studied, was due to either the absence or ineffectiveness of the suppressing E–S fraction, were performed on a second incompatible combination, T. ocellata–Planorbis corneus. Using this combination it was revealed that only the activating E–S fraction had modulatory effects on P. corneus haemocytes, indicating that the suppressing E–S fraction, which actively interferes with the clearance activity of haemocytes from L. stagnalis, appears to act in a host-specific manner. In conclusion, the suppressing E–S fraction determines, at least in part, compatibility in the trematode–snail association studied. This is also probably likely in other trematode–snail combinations.