The Morphological and Molecular Characterization of the Avian Trematodes Harrahium obscurum and Morishitium dollfusi (Digenea: Cyclocoelidae) from the Middle Volga Region (European Russia).

Simple Summary: Parasitic flatworms of the family Cyclocoelidae parasitize various inner organs of birds, mainly in their respiratory and digestive systems. The taxonomic position of many cyclocoelid species remains controversial. In our study, we studied two species of trematodes from the family Cyclocoelidae (Harrahium obscurum and Morishitium dollfusi) using classical morphological and molecular phylogenetic approaches. We provided original drawings, morphological descriptions, and the results of a molecular phylogenetic analysis of these species of helminths from birds in Russia for the first time. Our work shows the morphological variability in individuals of both flatworm species. Since the variability of individual cyclocoelids may be rather high, morphological features alone are insufficient for their identification, and confirmation with the use of molecular data is required. The taxonomic status of many species of the family Cyclocoelidae is still unclear. Two species of cyclocoelids, Harrahium obscurum and Morishitium dollfusi, were collected from the air sacs of birds (Turdus merula and Tringa ochropus) inhabiting the Middle Volga region (European Russia). Here, we provide the first detailed morphological description of these cyclocoelids and combine it with the first molecular phylogenetic analysis of Cyclocoelidae from birds in Russia based on partial sequences of their 28s rDNA and coI mtDNA genes. Specimens of both flatworm species from different host individuals differ slightly in body shape and size, which probably reflects host-induced intraspecific variability. For the first time, we have shown that a stable morphological character such as the length of the vitelline fields in the studied digeneans is variable at the species level and cannot be used in their morphological diagnosis. [ABSTRACT FROM AUTHOR]