An investigation of mortalities in a group of cardinal tetras Paracheirodon axelrodi Meyers, 1936, a popular ornamental fish, revealed myxozoan parasites in ventricles of the brains in 3/10 fish and the ocular retina of a fourth. Parasite impacts were unclear, as additional histopathological findings were present, including bacterial dermatitis and meningitis. Ethanol-preserved specimens pooled from multiple fish were used for morphological characterization of myxospores. Elongate, teardrop myxospores were 20.5 ± 0.7-μm (mean ± SD; range = 19.0-21.8 μm) long, 6.6 ± 0.5-μm (5.7-7.9 μm) wide, and 5.1 ± 0.4-μm (4.8-5.9 μm) thick (valvular width). Two, unequally sized, apical, pyriform polar capsules were in the same plane as the sutural ridge. The larger measured 9.9 ± 0.8-μm (8.0-11.2 μm) long and 3.8 ± 0.3-μm (3.2-4.8 μm) wide. The smaller was 4.1 ± 0.3-μm (3.5-4.5 μm) long and 2.0 ± 0.1-μm (1.8-2.3 μm) wide. Identical 1912 bp 18S rRNA sequences were obtained from two pooled spore samples from tetra brains, which did not match any sequences in the NCBI nr/nt database. Phylogenetically, these parasites grouped loosely within a clade containing Myxobolus spp. from other South American characins and Unicauda spp. from siluriform catfish. Myxospores shared some morphological similarities with Myxobolus inaequus from the unrelated glass knifefish (Order: Gymnotiformes), but were genetically divergent (<85 % similarity) from other myxozoan parasites of South American characins and shared few morphological features or tissue predilection sites. Based on host and tissue tropism, spore morphology, and 18S rRNA sequencing, we report this isolate as a previously unknown species, Myxobolus axelrodi n. sp.
Keywords: Brain; Cardinal tetra; Myxobolus; Myxozoa; Paracheirodon axelrodi.