Towards the molecular resolution of the bird digenean Hyptiasmus oculeus complex (Cyclocoelidae).

Hyptiasmus Kossack, 1911 is a genus of cyclocoelid digeneans infecting the body cavity, air sacs, and lungs of mainly anseriform and gruiform birds. Some species of this genus have extensive ranges, spanning more than one continent. Therefore, this taxon is potentially of interest for studying patterns of genetic diversity and the associated problems of taxonomic delimitation of species. We focused on the examination of Hyptiasmus specimens initially identified as Hyptiasmus oculeus Kossack, 1911. These digeneans were collected from the nasal sinuses of specimens of Eurasian Coot Fulica atra Linnaeus, 1758 taken in both European and Asian parts of Russia. Phylogenetic analyses based on 28S rRNA, nd1, and cytb markers, in combination with morphological data, indicate that the newly collected digeneans are in fact members of H. oculeus complex represented by true H. oculeus and a putative new species Hyptiasmus sp. The morphological differences between these species appear to be the length of the postovarian space and the position of the genital pore. A detailed morphological study revealed the presence of rudimentary ventral and oral suckers in gravid specimens of H. oculeus complex. We did not find any geographic structuredness of the haplotype diversity in true H. oculeus. Our findings do not support the recent proposal to resurrect H. singilum Witenberg, 1923 and H. brumpti Dollfus, 1948. In addition, we hypothesise that Paracyclocoelum Vainutis, Andreev & Voronova, 2024 is a synonym of Cyclocoelum Brandes, 1892.