Description of a new species of Myxobolus infecting Rhamdia quelen (Actinopterygii: Siluriformes), on the island of Marajó, Brazil.

Description of a new species of Myxobolus parasitizing the freshwater fish Rhamdia quelen, in the municipality of Cachoeira do Arari, Marajó Island, Pará, Brazil. Thirty specimens were analyzed, with 60% exhibiting rounded or ovoid, whitish plasmodia containing mature myxospores, scattered across the palate and the lower lip of the hosts. These myxospores measured 13.9 ± 0.3 μm in length and 9.9 ± 0.3 μm in width, in valvar view. Two pear-shaped polar capsules were observed in the anterior region, each containing 6 to 7 turns of the polar tube. Histological analysis revealed the development of plasmodia within the submucosal layer, causing compression of adjacent tissues and exhibiting features suggestive of bone loss. Myxospores were identified diffusely distributed in the vicinity of the plasmodia, possibly because of the rupture of a plasmodium, and were associated with an intense local inflammatory response. In the molecular phylogeny, the new species can be positioned in a clade with high nodal support, predominantly formed by species that infect fish of the order Siluriformes, with M. marajoensis, an intestinal parasite, identified as its sister species. The combined morphological and phylogenetic evidence confirm the description of the new species, Myxobolus cachoeirensis n. sp. and contribute to the knowledge of myxozoan fish parasite biodiversity and phylogenetic relationships in the Amazon biome.