Résumé
The distribution area of the nassariid gastropodCyclope neritea(L.) includes the Mediterranean and the Black Sea, as well as the Atlantic coasts of the southern Iberian Peninsula. The species has spread north to the eastern Bay of Biscay (Arcachon Bay in 1976, Marennes-Oléron area and Morbihan Gulf in 1983–84). This spread might be explained either by (1) a natural spread favoured by environmental changes (e.g. an increase of temperature) or (2) a sudden range expansion due to the introduction of individuals from distant native populations. Molecular genetic analyses based on mitochondrial markers suggest that the presentC. neriteapopulation in Arcachon Bay has been introduced, probably unintentionally with oyster transfers, from several source populations, genetically similar to the populations analysed in this study, i.e. those in the western Mediterranean and in south Portugal. Within its new distribution area,C. neriteacould potentially compete with the autochthonous nassariidNassarius reticulatus, both species being scavengers. AlthoughC. neriteatends to occur mainly in relatively clean sands in the intertidal andN. reticulatusin subtidal, organic rich sediments, the habitats of both species partially overlap. Laboratory experiments showed that in still water conditionsC. neriteawas more active and reached carrion faster thanN. reticulatus, thereby having a competitive advantage over the latter; flow conditions (~1 cm s–1) appeared to stimulate the activity ofN. reticulatus. Analysis of parasite load in both species in Arcachon Bay indicated thatN. reticulatuswas more heavily parasitized by digenean trematodes thanC. neritea. This suggests that the spread and population dynamics ofC. neriteaalong the French Atlantic coast has been favoured by the lack of parasites. Altogether, recurrent introduction, competitive ability and lack of heavy parasitic load might explain the successful settlement ofC. neriteaalong the French Atlantic coast.