Ultrastructural description of Agarella gracilis Dunkerly, 1915 (Myxozoa, Chloromyxidae) parasite of the dipnoan Lepidosiren paradoxa from the River Amazon

The myxosporean, Agarella gracilis, a parasite in Lepidosiren paradoxa collected in the estuarine region of the River Amazon, is described using light and transmission electron microscopy. The most evident life cycle stages were the spores, found in both the testis and ovary, which appeared scantily dispersed in the lumina. The prevalence of infection was 10/30 (33.3%). In mean dimensions the mature spore was 59.7 μm in total length, composed of an elongated body 17.7 μm long, 6.6 μm wide and 5.9 μm thick, with two long slightly divergent tail-like projections of the valves. There were four pyriform polar capsules, two longer (7.0×2.4 μm) and two shorter (3.8×2.4 μm), located anteriorly in the spore body; the polar filaments of all capsules had 6–7 turns. There was a posterior binucleate sporoplasm. The morphology agrees with the original description by light microscopy, and the taxonomy of the genus is discussed. The presence of the same genus of myxosporean parasites in both dipnoan and anuran species perhaps reflects their close phylogenetic relationship.