Oswaldocruzia ukrainae (Nematoda: Molineidae)—A Parasite of European Green Toad Bufotes viridis: Morphological and Molecular Data

SIMPLE SUMMARY: Roundworms of the genus Oswaldocruzia are common parasites of the small intestine of amphibians and reptiles. Our recent study revealed that only one species, Oswaldocruzia filiformis, parasitizes anurans, lizards and snakes in European Russia. In this work we studied nematodes of the genus Oswaldocruzia in the European green toads from the Middle Volga region using traditional morphological and molecular genetic methods. Descriptions, original drawings, and data on molecular phylogenetic analysis for studied nematodes were produced. The results of our study showed that green toads are parasitized by two Oswaldocruzia species, Oswaldocruzia ukrainae and O. filiformis. We found broad variability in morphology of the studied nematodes, which can lead to inaccurate identification of closely related species of Oswaldocruzia. The data obtained in our study should contribute to the knowledge of Oswaldocruzia nematodes and host-parasite associations in general. ABSTRACT: Nematodes of the genus Oswaldocruzia are common parasites of the small intestine of amphibians and reptiles. Our recent molecular analysis of Oswaldocruzia nematodes revealed that only Oswaldocruzia filiformis, which possesses high morphological variability, parasitizes amphibians and reptiles in European Russia. Here we present the study of Oswaldocruzia nematodes from the European green toad Bufotes viridis (Anura, Bufonidae) collected at different localities of the Middle Volga region in 2018–2022. We analyzed the morphological characteristics of the Oswaldocruzia spp. taxonomy together with novel molecular phylogenetic data. The data on phylogenetic analysis (based on partial CoxI mtDNA gene sequences) showed that Bufotes viridis is parasitized by two Oswaldocruzia species, the host-specific parasite Oswaldocruzia ukrainae and species generalist Oswaldocruzia filiformis. Broad morphological variability was revealed in O. ukrainae nematodes both from the same host specimen and from various toad individuals from different localities. Our results highlight the need for further biodiversity research of morphologically similar Oswaldocruzia species from amphibians and reptiles in the Western Palearctic using molecular genetic methods.