The Cyathocotylidae Mühling, 1898 is a family of primitive diplostomoid trematodes important for understanding the evolution of the superfamily Diplostomoidea. However, cyathocotylids remain poorly studied with the use of molecular techniques. In this study we sequenced the 5.8S + ITS2 region, 28S rRNA, and cox1 genes of two cyathocotylid species and obtained new morphological data on them. We propose Georduboisia nom. nov. instead of the preoccupied name Duboisia Szidat, 1936 (junior homonym of Duboisia Stremme, 1911). Adults of Georduboisia cf. teganuma (Ishii, 1935) and Paracoenogonimus ovatus Katsurada, 1914 were collected from fish-eating birds in the south of the European part of Russia. Georduboisia cf. teganuma was very similar to G.teganuma but differed from it in the shape of the testes. The 28S rRNA gene dataset provided the best-resolved phylogeny of the Cyathocotylidae to date. In the phylogram based on partial sequences of this gene, P. ovatus was close to members of Holostephanoides Dubois, 1983, Neogogatea Chandler & Rausch, 1947 and Gogatea Szidat, 1936. Georduboisia cf. teganuma clustered with members of Cyathocotyle Mühling, 1896 and Holostephanus Szidat, 1936. Phylogenetic analysis based on the 5.8S + ITS2 dataset showed that adults of P. ovatus examined in our study were conspecific with the metacercariae from the musculature of fish collected in Hungary and Italy. It also revealed probable misidentifications of larvae and adults of cyathocotylids whose sequences are deposited in GenBank NCBI.

Notocotylidae is a family of digeneans with some aberrant morphological features (absence of ventral sucker), which as sexual adults parasitise herbivorous and molluscivorous birds and mammals. The phylogenetics and taxonomy of this family have many unresolved problems concerning the relatedness of genera and species and the reliability of identification of specimens for which molecular data are available. The aim of this study is morphological and molecular characterisation of the type species of Notocotylus Diesing, 1839 ( Notocotylus triserialis Diesing, 1839) and its two congeners, Notocotylus pacifera (Noble, 1933) and Notocotylus sp., and verification of the monophyly of the genus Notocotylus within the taxonomic boundaries established in the recent revision. Notocotylus triserialis was collected from a Greater White-fronted Goose Anser albifrons (Scopoli, 1769) taken down in the Russian Karelia, and the other two digenean species were derived from European Coots Fulica atra Linnaeus, 1758, from different locations in Eurasia (Eastern Europe and the Far East). We provide detailed morphological descriptions of the examined digeneans, supplemented with original line drawings. Notocotylus sp. is morphologically similar to N. pacifera but differs in cirrus sac length. Phylogenetic inference based on the 28S rRNA gene dataset places N. triserialis within a well-supported clade comprising several congeners, whereas N. pacifera is recovered outside this assemblage, forming a closer phylogenetic association with Pseudocatatropis dvoryadkini Izrailskaia, Besprozvannykh, Tatonova, Nguyen and Ngo, 2019. Thus, our data do not support the monophyly of the genus Notocotylus.

Phylogenetic studies of aberrant species are of considerable scientific interest because their taxonomic rank in traditional systems based on morphological characters is not infrequently overestimated. Apopharynx bolodes (Braun, 1902) is one of the few psilostomid digeneans devoid of the pharynx. This is considered a sufficient basis for assigning it and similar species to the subfamily Apopharynginae. We found A. bolodes in Fulica atra Linnaeus, 1758 from Belarus, described it morphologically, and genotyped it by the 28S rRNA gene and the ITS2 region. It is the first molecular data on A. bolodes and the first record of this digenean species in Belarus. The phylogenetic analysis based on partial sequences of the 28S rRNA gene showed that A. bolodes is closely related to the Sphaeridiotrema spp. (Sphaeridiotrematinae). However, this phylogenetic inference has not received yet support with data on the ITS2 region.

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.