Myxobolus medusae n. sp., a new species of Myxozoa with dendritic appendages.

The genus Myxobolus, belonging to the class Myxozoa, is a cnidarian genus characterized by ellipsoidal or rounded myxospores with two polar capsules located in the anterior sutural plane, commonly without caudal projections. However, diverse morphologies have been described in recent studies of myxospores, exhibiting various appendage forms. The present study describes the morphological and molecular characteristics of a new species of Myxobolus in the red piranha Pygocentrus nattereri. In 16 % (3/18) of the fish specimens captured in Lake Sacaizal, Amapá State, whitish cysts containing ovoid myxospores with dendritic caudal appendages were observed in the anterior chamber of the eye. The myxospores measured 17.7 μm long, 15.6 μm wide, and 14.0 μm thick and had two equally sized pyriform polar capsules 9.9 μm long and 4.2 μm wide. Phylogenetic analyses showed that the newly identified species, Myxobolus medusae n. sp., formed a clade with other species of Myxobolus without appendages and Henneguya, which infects Characiformes in Brazil, with a genetic distance above 12 %. These morphological and molecular data provide insights into a new species of the genus Myxobolus in P. nattereri, a fish species of socioeconomic importance in the Amazon region.