Une nouvelle espèce de ver polyclade marin non indigène dans les pertuis charentais, Postenterogonia Orbicularis (Schmarda, 1859) (= Idiostylochus Tortuosus) (Platyhelminthes : Polycladida)

Postenterogonia orbicularis is a marine flatworm of the order Polyclada, native to New Zealand and Australasia. This species was unknown in France before 2020 and was first reported in the Arcachon Basin and then in the Pertuis Charentais straits as a non-indigenous species (NIS). The studied species was initially mistakenly described as new (Idiostylochus tortuosus) before being correctly identified through genetic analysis as P. orbicularis. Polyclads are flatworms that are difficult to identify morphologically, often requiring molecular analysis. This invasive alien species (IAS) is described using two approaches: morphological and molecular. Its taxonomic classification is presented, along with its biology, habitat, and biogeography. Its presence in the Pertuis straits is discussed, although it is not yet locally invasive; its expansion could pose a risk to shellfish farming.