Abstract
In the Etang de Thau (Herault - France), Helicometra gobii, H. fasciata and H. pulchella parasitize the Calliostoma striatum, Gibbula adansoni and Tricolia tenuis gastropods respectively. A comparative analysis of these host populations and their trematodes reveals a relation between the spatial and temporal structures of these populations, the parasite specificity and the specific richness of parasite settlements of these molluscs. Thus, although it has by far the rarest gastropod and has the strictest specificity relative to the definitive host fish, H. pulchella nevertheless has a capacity for colonizing its first intermediate host comparable to it congeners. This is also the only species with no known potential competitor (digene) in its mollusc. The evolutionary implications of the demographic strategies of the three phylogenetically closely related trematodes are finally discussed.