TRANSMISSION FAILURE OF PARASITES (DIGENEA) IN SITES COLONIZED BY THERECENTLY INTRODUCED INVASIVE ALGA CAULERPA-TAXIFOLIA

The recently introduced invasive tropical seaweed Caulerpa taxifolia h as by now invaded large areas of the western Mediterranean coast betwe en Nice (France) and Imperia (Italy). The labrid fish Symphodus ocella tus, which usually inhabits Posidonia oceanica meadows or lives among photophilic algae growing on rocky substrates, is also present in area s which are thickly covered with C. taxifolia. This fish is territoria l and sedentary, and its life span is never more than 3 yr. Since C, t axifolia has been present since 1987 in the areas studied, the S, ocel latus individuals living there can be assumed to have probably spent t heir whole post-larval Lives in the vicinity of the seaweed. At the co lonized sites, the invertebrate benthic prey of S. ocellatus have unde rgone both quantitative and qualitative changes. The effects of these changes on the transmission of parasites were studied using the digene ans of the digestive tract of S. ocellatus as a model. At the control sites, 6 digenean species were identified: Helicometra fasciata, Macvi caria 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%; cumulativ e abundance = 0.02). Among the possible reasons explaining the nearly complete absence of digeneans parasitizing S. ocellatus, the rarefacti on of intermediate hosts in the invaded areas can probably be ruled ou t, 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, mig ht be responsible for the near-complete disappearance of the digeneans of S. ocellatus.