A taxonomical re-evaluation of Schistocephalus sp. (Cestoda: Diphyllobothriidea) from Siberian cottid fishes (Russia).

Schistocephalus spp. are widely distributed Holarctic cestodes that comprise key model species for host-parasite studies. S. solidus and S. pungitii, the most extensively studied species, exhibit narrow host specificity. Another species, S. cotti, is known from the European bullhead (Cottus gobio); however, information on its host range, geographical distribution, and molecular data remain poorly characterized. In Siberia, plerocercoids of Schistocephalus found in sculpins were previously recorded from endemic cottid fishes of Lake Baikal and were identified as S. solidus, which is inconsistent with the known host specificity of this species. The present study aims to clarify the species identity of Schistocephalus from Siberian cottid fishes using morphological and molecular data, and to test the hypothesis of conspecificity of Schistocephalus plerocercoids from sculpin hosts across Eurasia and North America. Morphologically, Siberian plerocercoids correspond to the description of S. cotti, supporting their assignment to this species. The phylogenetic analysis confirmed the identity of the plerocercoids as S. cotti and demonstrated the existence of two distinct Schistocephalus lineages in North America and Siberia. These findings indicate that at least two different Schistocephalus species infect sculpins in the Old and New Worlds.