Description of Metagonimus pusillus sp. nov. (Trematoda: Heterophyidae): phylogenetic relationships within the genus.

As a result of experimental studies conducted in the Russian southern Far East, adult worms from the genus Metagonimus were obtained. A comparative analysis of the morphometry of these worms with other Metagonimus representatives showed that they are most similar to M. katsuradai Izumi, 1935 and M. otsurui Shimazu & Urabe, 2002 found in Japan, due to the ratio of suckers and the positions of the testicle, uterus and vitellaria. However, Russian worms differ from species in Japan by other metric characters: they differ from M. otsurui by the maximum size of most organs and from M. katsuradai by body width, pharynx length, and maximum size of testes and ovary. At the same time, they are identical to a trematode from the Russian southern Far East, which was previously identified as M. katsuradai. The validity of this species was also confirmed by genetic data. According to the 28S gene and the internal transcribed spacer 2 (ITS2) region of rDNA, as well as the cytochrome c oxidase I (cox1) gene of mtDNA, the Metagonimus specimens found in Russia differ from published genetic data for other members of this genus. However, both morphological similarity and molecular data showed that M. pusillus sp. nov., M. katsuradai and M. otsurui are most likely cryptic species. Furthermore, additional data based on a mitochondrial marker were provided for M. suifunensis Shumenko, Tatonova & Besprozvannykh, 2017 from Russia.