ULTRASTRUCTURAL-STUDY AND DESCRIPTION OF FLABELLIFORMA-MAGNIVORA SP-N(MICROSPORA, DUBOSCQIIDAE), A MICROSPORIDIAN PARASITE OF DAPHNIA-MAGNA (CRUSTACEA, CLADOCERA, DAPHNIIDAE)

The new microsporidium Flabelliforma magnivora sp. n. is described bas ed primarily on ultrastructural characteristics. All life cycle stages have isolated nuclei. Merogonial and sporogonial reproductions are by rosette-or finger-like budding. The sporogony yields 4-16, most frequ ently 12, sporoblasts. A fragile sporophorous vesicle is formed by the sporont. Spores are lightly pyriform, with both poles blunt, often wi th one surface slightly convex. Unfixed spores measure 2.34-3.03 x 4.0 7-4.93 mu m. The exospore, which is five-layered with an internal doub le-layer, measures 35-40 nm thick. The polar filament is isofilar with 14-17, 77-94 nm wide coils arranged in two irregular layers of coils in the posterior half of the spore. The polaroplast, which ends close to the anterior polar filament coils, is composed of two regions of re gularly arranged lamellae: narrow lamellae anteriorly, wider lamellae posteriorly. The host is the cladoceran Daphnia magna. The primary sit e of infection is the fat body, but infection was also observed in the hypodermic cells and the ovaries. In the laboratory the parasites are transmitted with nearly 100% fidelity from mother to offspring. Trans versely sectioned polyribosomes appeared as characteristic circular co nfigurations of 9 ribosomes. Teratological development, including inco mplete separation of sporoblasts, macrospores with supernumerary polar filament coils, and disturbed coiling, was observed. The identificati on of the species and the genus position are briefly discussed.