A New Species of Myxozoa, Henneguya rondoni n. sp. (Myxozoa), from the Peripheral Nervous System of the Amazonian Fish, Gymnorhamphichthys rondoni (Teleostei)

.Henneguya rondoni n. sp. found in the peripheral lateral nerves located below the two lateral lines of the fish Gymnorhamphichthys rondoni (Teleostei, Rhamphichthyidae) from the Amazon river is described using light and electron microscopy. Spherical to ellipsoid cysts measuring up to 110 μm in length contained only immature and mature spores located in close contact with the myelin sheaths of the nervous fibres. Ellipsoidal spores measured 17.7 (16.9–18.1)‐μm long, 3.6 (3.0–3.9)‐μm wide, and 2.5 (2.2–2.8)‐μm ( n =25) thick. The spore body measuring 7.0 (6.8–7.3)‐μm long was formed by two equal symmetric valves, each with an equal tapering tail 10.7 (10.3–11.0) μm in length. The tails were composed of an internal dense material surrounded by an external homogeneous sheath of hyaline substance. The valves surrounded two equal pyriform polar capsules measuring 2.5 (2.2–2.8)‐μm long and 0.85 (0.79–0.88)‐μm ( n =25) wide and a binucleated sporoplasm cell containing globular sporoplasmosomes 0.38 (0.33–0.42) μm ( n =25) in diam. with an internal eccentric dense structure with half‐crescent section. Each polar capsule contains an anisofilar polar filament with 6–7 turns obliquely to the long axis. The matrix of the polar capsule was dense and the wall filled with a hyaline substance. The spores differed from those of previously described species. Based on the ultrastructural morphology of the spore and specificity to the host species, we propose a new species name H. rondoni n. sp.