Supplemental diagnosis and phylogeny of Myxobolus absonus (Cnidaria, Myxozoa) from the eye of the freshwater fish Pimelodus maculatus (Siluriformes, Pimelodidae)

Abstract Myxobolus absonus Cellere et al., 2002 was originally described as having free cysts in the opercular cavity of the freshwater fish Pimelodus maculatus in Brazil. The present study provides a supplemental description of this parasite from the eye of its type host, with basis on morphological, ultrastructural, and molecular data of the SSU rRNA gene. The parasite formed spherical whitish cysts, which wall presented numerous microvilli that attached to the collagen layers of the corneal stroma. Myxospores were oval in valvular and sutural view, measuring 13.2 ± 0.4 μm in length, 8.5 ± 0.4 μm in width, and 6.6 ± 0.3 μm in thickness. Two asymmetric pyriform polar capsules were located at the anterior pole: the larger 6.2 ± 0.4 μm long and 3.6 ± 0.3 μm wide, containing a polar filament coiled in 6 turns; and the smaller 3.5 ± 0.3 μm long and 1.9 ± 0.1 μm wide, containing a polar filament coiled in 4 turns. At the posterior pole, the sporoplasm displayed two nuclei and numerous spherical sporoplasmosomes. Phylogenetic analysis using maximum likelihood, Bayesian inference and maximum parsimony revealed M. absonus clustering within a well-supported clade with poorly-resolved internal nodes, amongst the SSU rRNA sequences of other myxobolids that infect siluriform and characiform fish hosts, as well as the perciform-infecting species Myxobolus acanthogobii , Triangula percae and Cardimyxobolus japonensis . This clade appeared separated from the other clades comprising most of the species that infect siluriform and characiform hosts, showing that more than one myxobolid lineage evolved while parasitizing these taxonomic groups of fish.