Transmission failure of parasites (Digenea) in sites colonized by the recently introduced invasive alga Caulerpa taxifolia

The recently introduced invasive tropical seaweed Caulerpa taxifolia has by now invaded large areas of the western Mediterranean coast between Nice (France) and Imperia (Italy). The labrid fish Symphodus ocellatus, which usually inhabits Posidonia oceanica meadows or lives among photophilic algae growing on rocky substrates, is also present in areas which are thickly covered with C. taxifolia. This fish is territorial and sedentary, and its life span is never more than 3 yr. Since C, taxifolia has been present since 1987 in the areas studied, the S, ocellatus individuals living there can be assumed to have probably spent their whole post-larval Lives in the vicinity of the seaweed. At the colonized sites, the invertebrate benthic prey of S. ocellatus have undergone both quantitative and qualitative changes. The effects of these changes on the transmission of parasites were studied using the digeneans of the digestive tract of S. ocellatus as a model. At the control sites, 6 digenean species were identified: Helicometra fasciata, Macvicaria alacris, Proctoeces maculatus, Holorchis pycnoporus, Lecithaster stellatus and Genitocotyle mediterranea (cumulative prevalence of all species = 46.3%; cumulative abundance of all species = 0.95). At the sites colonized by C. taxifolia, only 2 digenean species were present: H, fasciata and L. stellatus (cumulative prevalence = 1.5%; cumulative abundance = 0.02). Among the possible reasons explaining the nearly complete absence of digeneans parasitizing S. ocellatus, the rarefaction of intermediate hosts in the invaded areas can probably be ruled out, at least in the case of 2 digenean species. Secondary metabolites (caulerpenyne and other terpenes) synthesized by C, taxifolia, and then released into the environment or transmmitted along the food web, might be responsible for the near-complete disappearance of the digeneans of S. ocellatus.