A new Platynosomoides (Trematoda: Dicrocoeliidae) parasitic in the sigmodontine rodent Akodon cursor from the Atlantic Forest of Brazil, with the first molecular genetic characterization from a species within the genus.

We describe a new species of the genus Platynosomoides Yamaguti, 1971 (Trematoda: Dicrocoeliidae) from the bile ducts of the cricetid Akodon cursor (Winge, 1887) (Rodentia: Sigmodontinae) from Itatiaia National Park (Parque Nacional do Itatiaia, PNI), in the state of Rio de Janeiro, Brazil. The species is characterized by the following diagnostic features: body size; dimensions and relative proportions of the testes in relation to the body length; extracecal and asymmetrical vitelline glands commencing at the level of the acetabulum, the left vitellarium terminating at the mid-region of the second third of the body, and the right one extending to the anterior margin of the posterior third; and the relative size of the suckers, with the ventral sucker being approximately 60% larger than the oral sucker. The morphological characteristics were described with the assistance of optical microscopy images to observe the internal structures, and their external details were provided by scanning electron microscopy (SEM). Additionally, molecular phylogenetic analyses based on the 18S rRNA gene were conducted to assess the phylogenetic placement of the genus within Dicrocoeliidae. These analyses accompany the description of Platynosomoides macrotestis sp. nov. and contribute to a better understanding of trematode diversity. Furthermore, this study highlights the importance of parasitological research in conserving helminth biodiversity. Its occurrence in conservation areas, such as the PNI, supports the maintenance of unique ecological interactions and species conservation.