Myxobolus axelrodi n. sp. (Myxosporea: Myxobolidae) a parasite infecting the brain and retinas of the cardinal tetra Paracheirodon axelrodi (Teleostei: Characidae)

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 (