Light and scanning electron microscopy of Henneguya arapaima n. sp. (Myxozoa: Myxobolidae) and histology of infected sites in pirarucu (Arapaima gigas: Pisces: Arapaimidae) from the Araguaia River, Brazil

In this report, we describe Henneguya arapaima n. sp., a parasite of the gill arch and gall bladder of Arapaima gigas (pirarucu) collected in the Araguaia River, in the municipality of Nova Crixás, Goiás State, central Brazil. The plasmodia were white, round or ellipsoidal and measured 200–600 μm. Parasite development was asynchronous and the mature spores were fusiform and had smooth wall. The spores measurements were (range, with means ± S.D. in parentheses): total length—48.4–53.1 μm (51.6 ± 3.4 μm), body length—13.5–15.2 μm (14.2 ± 0.8 μm), body width—5.1–6.1 μm (5.7 ± 0.5 μm), body thickness—4.7–5.3 μm (4.9 ± 0.2 μm) and caudal process length—38.0–41.2 μm (38.3 ± 2.9 μm). The polar capsules were elongated and of unequal size, with lengths of 6.3–6.8 μm (6.5 ± 0.2) and 6.2–6.6 μm (6.3 ± 0.1) for the longest and shortest axes, respectively. Capsule width was 1.4–1.6 μm (1.5 ± 0.1). Histological analysis showed that the plasmodia occurred in the tunica adventitia of the gall bladder and were delimited by a thin capsule of connective tissue. In the gill arch, the plasmodia were also surrounded by connective tissue similar to the endomesium of striated skeletal muscle cells. Sixty-five juvenile specimens of A. gigas weighing 1.0–25.0 kg were examined, 17 (26.1%) of which were infected. Of these, 14 (82.3%) had cysts in the gall bladder, two (11.7%) had cysts in the gill arch and only one (5.9%) had cysts in both organs. When the fish were grouped by weight, the prevalence of infection in fish weighing up to 10.0 kg (20.7%) was significantly lower than in fish weighing 10.1–25.0 kg (50%) (G = 3.93; d.f. = 1; p < 0.05).