Renicola spp. (Digenea, Renicolidae) of the ‘Duck Clade’ with Description of the Renicola mollissima Kulachkova, 1957 Life Cycle

Renicolid digeneans parasitise aquatic birds. In molecular trees, they are divided into three clades, one of which, the ‘duck clade’, parasitises anatids. Renicola mollissima, a member of this clade, parasitises sea ducks, mainly eiders. Its life cycle remains unknown. We verified the diagnosis of R. mollissima using integrated morphological and molecular data and provided the first information on its life cycle in northern Palaearctic. We proved that intramolluscan stages of R. mollissima, previously known as Cercaria pacifica 2, develop in intertidal snails Littorina squalida and L. saxatilis. We provided a detailed morphological description of cercariae and adults of R. mollissima and a discriminative analysis with closely related species. Molecular data demonstrated an amphiboreal distribution of R. mollissima and the existence of a single population in Europe and the North Pacific. Using molecular methods, we also found metacercariae of an unknown renicolid species from the ‘duck clade’, designated as Cercaria cf. nordica I, in subtidal mussels of the Barents Sea. All individuals of C. cf. nordica I examined in our study were represented by the same haplotype. We discuss possible ways of formation of this phylogeographic structure, the composition of the ‘duck clade’ and the evolutionary pathways of the family Renicolidae.