n. sp. (Digenea: Notocotylidae) parasitizing the South American Black Oystercatcher and their atypical life cycle from the Patagonian coast.

By way of morphological and molecular analysis we describe a new species of notocotylid, n. sp., parasitizing from Argentina and we contribute to elucidate its life cycle. Within this genus, 4 groups can be morphologically distinguished according to body shape: ‘Oval’, ‘Pyriform’, ‘Elongate’, ‘Overlong’. The new species belongs to the ‘Elongate group’, which presents a wide variation in body length (597–4500 m). The new species, from Australia, from the Caribbean and from Russia share the smallest range of body size in this group (<1130 m). The new species more closely resembles but differs from it by cirrus-sac length, which is shorter in the new species (97–146 280–430 m in ), and egg size which is larger in the new species (25–33 18–20 m in ). n. sp. uses the limpet as both first and second intermediate hosts in which metacercariae encyst inside the redia. This is the first abbreviated cycle described for notocotylid species. Ribosomal RNA sequences provided for adults (ITS1, ITS2 and 28S) and metacercariae inside the rediae (ITS1) support the species identification and the correspondence among stages. Phylogenetic analysis based on 28S placed n. sp. close to other spp. and . Molecular results demonstrate that the hosts involved in the life cycles and the habitat more than morphological differences are determining the phylogenetic relationships in members of Notocotylidae.