Résumé
Elucidating the mechanisms making a successful invader remains a central problem in invasion ecology. There is growing evidence supporting that the outcome of competition between species can be controlled by parasites. However our understanding of how parasites affect the interaction between native and invasive species is very limited. Here we explore the role of parasites as potential agents mediating the competitive exclusion of populations of Mediterranean brine shrimps Artemia by the exotic American A. franciscana, which is leading in most of the cases to the extinction of native Artemia populations. We compared the susceptibility of the invasive species and the native A. parthenogenetica to different cestode species using an exceptional case of sympatry in Aigues-Mortes saltern, South of France. The invader A. franciscana showed greatly reduced diversity, low prevalence and low burden of cestode larvae compared to its native congener. Infection in A. parthenogenetica was associated with high fitness costs. In particular, the most prevalent cestode, Flamingolepis liguloides caused castration in A. parthenogenetica. The results of this study suggest that the large impact of cestode on the native, but not the invading species, is likely to confer a decisive competitive advantage to the invader. It thus certainly contributes to explain the demographic success of A. franciscana in the Mediterranean region.