A new species of polyclad flatworm, Idiostylochus tortuosus gen. nov., sp. nov. (Polycladida, Idioplanidae), from Arcachon Bay (France) is described. This description is based on a morphological analysis and a molecular analysis using partial sequences of the 28S and cytochrome Oxidase I (COI) genes. After the molecular analysis Idiostylochus gen. nov. appears to be the second genus of the Family Idioplanidae and closely related to the family Latocestidae as well as the genera Leptostylochus and Mirostylochus. The molecular data revealed that the new species may belong to an Indonesian or Indo-Pacific family, closely related to genera with origins in South Pacific Ocean waters. This species was found feeding on the oysters and mussels of the Arcachon farms.
Keywords: Acotylea; Idioplanidae; Magallana gigas; Non-native species; Oyster culture.