This is the first review of the helminth fauna of the moor frog Rana arvalis Nilsson, 1842 from the Volga river basin (Russia). The article summarizes the authors’ and literature data on the helminthic fauna of this species. The method of complete helminthological dissection was used. Thirthy-eight helminth species were recorded from three classes: Cestoda (1), Trematoda (28), and Chromadorea (9). Nine helminth species are new to the moor frog in Russia: trematodes Gorgodera varsoviensis Sinitzin, 1905, Strigea falconis Szidat, 1928, larvae, Neodiplostomum spathoides Dubois, 1937, larvae, Tylodelphys excavata (Rudolphi, 1803), larvae, Pharyngostomum cordatum (Diesing, 1850), larvae, Astiotrema monticelli Stossich, 1904, larvae and Encyclometra colubrimurorum (Rudolphi, 1819), larvae, nematodes Strongyloides spiralis Grabda-Kazubska, 1978 and Icosiella neglecta (Diesing, 1851). The cestode Spirometra erinacei (Rudolphi, 1918), larvae were observed of this amphibian species in the Volga basin for the first time. The nematodes Rhabdias bufonis, Oswaldocruzia filiformis, Cosmocerca ornata and the trematode Haplometra cylindracea form the core of the helminth fauna of the moor frog. Information on species of helminths includes systematic position, localization, areas of detection, type and scheme of life cycle, geographical distribution, and degree of specificity to host amphibians.

This work presents an ecological analysis of the community and biodiversity of helminths of Pelophylax lessonae (Camerano, 1882) from floodplain and forest reservoirs of the European part of Russia. The material for the work was personal collections of helminths made from 2018–2021 in the National Park “Smolny” (Republic of Mordovia). Two hundred and thirty-five amphibian specimens were examined from nine reservoirs and three types of hydrobiocoenoses: (1) floodplains of a medium-sized river (in terms of catchment area); (2) floodplains of a small river; (3) a number of isolated forest reservoirs. Twenty-four species of helminths have been registered: Trematoda (20) and Chromadorea (4). Similar features (common species of trematodes and nematodes) were determined as well as differences in the composition and structure of the helminth fauna, the level of infestation by individual species and groups of helminths, diversity, and community structure. Amphibians of the river floodplain have a richer helminth fauna, they are more infected with a large number of helminths, and their community is more complex. Amphibians of isolated forest ponds, on the contrary, have fewer helminths, they are generally less infected, and their community is simplified (reduced). Having intermediate indicators of composition, structure, and degree of infestation, frogs from the forest floodplain of the small river—differ in the most diverse and maximally evenness community of helminths. The results of the study demonstrate the influence of biotopic factors on the formation of an amphibian helminth community.