A century of taxonomic uncertainty: re-description of two species of tapeworms (Diphyllobothriidea) from Arctic seals

Diphyllobothriid tapeworms are well-known parasites of mammals including humans. Most species are known for centuries, but the validity of many species and their classification are still poorly understood. Based on new collections and re-observation of museum material, we focus on widely distributed taxa infecting several species of seals (Phocinae) in Arctic and sub-Arctic waters. The existence of two morphologically similar diphyllobothriid species, Diphyllobothrium lanceolatum and Diphyllobothrium schistochilos, is revealed through detailed analyses of morphological and morphometric data as well as supported by genetic data published recently. Both species show a striking similarity in their body shape, which historically resulted in numerous misidentifications and erroneous records in the literature. Despite previous attempts to unequivocally recognize the species identity of these congeners, D. lanceolatum can be differentiated from D. schistochilos by the presence of a triangular scolex with a posterior velum, a rapid maturation of the strobila and the pointed or slightly rounded appearance of the last proglottid. In addition to the re-descriptions of both species, we provide novel information on their host spectrum and biogeographic distribution.