Ecology of reef fish parasites at Moorea island, French Polynesia

In coral reef ecosystems, fish populations are among the most diversified and rich communities, and like other organisms they do not escape parasitism. For a long time parasites have been ignored in ecosystem studies and only recently ecologists have come to that parasites play a role in the functioning of ecosystems and their stability. The parasite-host systems studied consist of three reef fish species and their parasites selected for their suitability for experimental manipulations (Stegastes nigricans and Dascyllus aruanus) or because of their economical importance (Cephalopholis argus). The present thesis was divided into several distinct but complementary phases which have brought the following results. - The description of the parasite species and the study of their host specificity have revealed that most of the species reported at Moorea island were unknown species, but this seems to be mainly due to the lack of taxonomical studies in the coral reef habitat rather than a real endemicity. Parasite exchanges between some neighbouring hosts are likely to have led to an increase of parasite diversity in the field. - Investigations on intra- and interspecies interactions within the gill habitat have suggested a lack of interspecies competitions for space and emphasize the importance of cross fertilization in monogeneans. - The study of the host-parasite relationships has shown close relationships between host traits and parasitism (size, age, behaviour, diet). Parasite distribution within host population is aggregated and the community structure seems to be located between a isolationist and interactive type of community structure. - The study of temporal variations of parasite parameters has revealed the influence of factors such as host reproduction on parasitism. Mortality due to parasite on natural host populations seems to be not important. - The study of the spatial variability of parasitism in the coral reef ecosystem has shown the existence of spatial heterogeneity of parasite burden within the coral reef habitat. - Experiments devoted to the infection dynamics of S, nigricans and D. aruanus monogeneans in situ and in aquarium have shown fast recruitment processes in the lagoon and rapid saturation of the gill habitat evoking infrapopulation regulating mechanisms. - The study of host density influence on parasite recruitment within the host population has revealed a lack of influence of host population density increase or decrease on the parasite infrapopulations, suggesting the existence of a continuous flow of infective larval stages in the lagoon and supporting the hypothesis of infrapopulations regulating mechanisms.