Morphological and ultrastructural description of Pleistophora dammami sp. n. infecting the intestinal wall of Saurida undosquamis from the Arabian Gulf, Saudi Arabia.

Pleistophora dammami sp. n. is described from Saurida undosquamis from the Arabian Gulf in Saudi Arabia. Infection appeared as whitish cysts in the intestinal wall. Cysts ranged in size from 1 to 4 mm. The prevalence of the infection across both fish sexes was 17.5% (24/420). Two kinds of spores were recognized, microspores and macrospores, and each were ovoid in shape. The microspores measured ~2.5 × 2.0 μm in size, while the macrospores measured ~6.0 × 3.0 μm. Ultrastructurally, the parasite did not form xenoma but it formed cysts surrounded by thick cyst wall. All stages of development as meronts, sporonts, sporoblast and spores occurred in the cytoplasm of the host cells within sporophorous vesicles. The stages of development occurred asynchronously and thus all stages were randomly distributed within the cysts. Meronts were elliptical and multinucleated, with unpaired nuclei which constantly divided giving rise to new sporonts. During the transition to sporonts, the border of the meronts increased in thickness to form dense discontinuous cell coat. Later, the sporont divided into sporoblast cells which gradually differentiated the typical organelles of the spores. In mature spores, the polar filament was arranged in 20-24 coils in two rows either side of the posterior vacuole. All ultrastructural and morphological criteria indicate that the described species belongs to the genus Pleistophora.