Hemiurid digeneans conspecific with Stomachicola muraenesocis Yamaguti, 1934 (the type species of the genus Stomachicola Yamaguti, 1934) were collected from the stomach of the daggertooth pike conger Muraenesox cinereus (Forsskål) off the Persian Gulf of Iran. This study aimed to provide a detailed characterization of Stom. muraenesocis, including measurements, illustrations and scanning electron microscopy (s.e.m.) representations. Comparisons with the original and previous descriptions revealed morphological and metrical variations in several features (i.e. body size and shape, arrangement of reproductive organs, soma to ecsoma length ratio, position of genital opening, number of vitelline tubules and extension of uterine coils) between Stom. muraenesocis from different hosts and localities. This study presents the first molecular sequence data associated with the small (18S) and large (28S) subunit nuclear ribosomal RNA genes (rDNA) for Stom. muraenesocis. Phylogenetic analyses of the 18S dataset placed Stom. muraenesocis as sister lineage to a clade formed of a group of species of Lecithaster Lühe, 1901 (Lecithasteridae Odhner, 1905). In contrast, phylogenetic analyses based on the 28S consistently recovered a sister relationship between Stom. muraenesocis and representatives of the Hemiuridae Looss, 1899. Further comprehensive phylogenetically based classification in light of morphology and taxonomic history of the Hemiuridae and Lecithasteridae is required to infer phylogenetic affinities and historical biogeography of Stomachicola. A comprehensive list of previously reported species of Stomachicola together with their associated hosts, localities and morphometric data is provided.

Morphological discrimination of species is problematic in many digenean taxa. Parasites of marine fish from the genus Lecithaster Lühe, 1901 are a good example of this. Our goal was to understand which species of Lecithaster infect fish in the White Sea, and reveal their life cycles. We collected specimens of maritae from nine fish species, analysed their morphology and sequenced 28S ribosomal DNA and internal transcribed spacer 2 (ITS2). Contrary to previous accounts, all of them belong to a single species, Lecithaster salmonis Yamaguti, 1934, which was previously only recorded from the Pacific. Morphologically, our maritae specimens were highly variable, sharing characters of L. salmonis, Lecithaster confusus Odhner, 1905 and Lecithaster gibbosus (Rudolphi, 1802) Lühe, 1901. This variability did not correlate with the moderate differences in ITS2 among the specimens, and neither did the fish host species. Members of the subfamily Salmoninae appear to be the best suited definitive hosts, judging from the intensity rates. The intermediate hosts were also discovered: the first is Cryptonatica affinis (Gmelin, 1791) and the second are planktonic copepods. These lifecycle data from the White Sea are consistent with L. salmonis species identification and with the distribution of this species in the North Pacific. The geographical range of L. salmonis seems to be interrupted, and we discuss possible ways of L. salmonis expansion.