Ultrastructure of Myxobolus brycon n. sp (Phylum Myxozoa), Parasite of the Piraputanga Fish Brycon hilarii (Teleostei) from Pantanal (Brazil)

Light and electron microscopy studies of a myxosporean, parasitizing the gill filaments of the freshwater fish Brycon hilarii (Valenciennes, 1850) (Characidae) collected in the Paraguay River (18 degrees 49'S, 57 degrees 39'W) (Pantanal), Brazil, is described. This parasite produces spherical to ellipsoidal polysporic histozoic plasmodia (Pmd) (up to similar to 180 mu m in diameter) delimited by a double membrane and with several pinocytic channels. The plasmodial cyst contained the youngest developmental stages at the cortical periphery and immature and mature spores more internally. The Pmd developed near the cartilaginous structure of the gill filament, forming a prominent deformation where the gill lamellae disappear. Pyriform spores measured 6.9 +/- 0.6 (range 6.5-7.2) mu m long, 4.2 +/- 0.5 (range 3.9-4.8) mu m wide, and 2.5 +/- 0.7 (range 1.9-2.8) mu m thick. The spores composed of two equal shell valves (similar to 70 nm thick), adhering together along the straight suture line, surrounded two equal symmetric and elongated to pyriform polar capsules (PC) 4.2 +/- 0.6 (range 3.8-4.7) x 1.9 +/- 0.6 (1.7-2.5) mu m; each PC contained a coiled polar filament with eight or nine (rarely 10) turns and a binucleated sporoplasm cell. Dense irregular masses were observed among the polar filaments coils. An intercapsular appendix was not observed. The sporoplasm contained several globular sporoplasmosomes randomly distributed among an extensive rough endoplasmic reticulum system with numerous vesicles and cisternae. Based on the morphological and ultrastructural differences and specificity of the host, we establish the new species, Myxobolus brycon n. sp.