Ultrastructural description of the life cycle of Nosema monorchis n. sp. (Microspora, Nosematidae), hyperparasite of Monorchis parvus (Digenea, Monorchiidae), intestinal parasite of Diplodus annularis (Pisces, Teleostei)

Life cycle stages of a new species of the genus Nosema Naegeli, 1857 (Microspora, Nosematidae), were examined by light and electron microscopy. It parasitizes the gut and the uterus of the digenean Monorchis parvus (Monorchiidae), in Diplodus annularis (Pisces, Teleostei). All stages were in close contact with the cytoplasm of the host cell and were probably all diplokaryotic. The divisions of meronts and sporonts were recognizable by the formation of spindle plaques at the surface of the nucleus. Spores were oval, measured 3.2±0.3×2.5±0.2 μm on ultrathin sections, and had a polar filament with 16-17 coils. The polaroplast presented two parts: an anterior region with closely packed lamellae and a posterior part with wider lamellae. This Nosema species is compared with the other microsporidian parasites of digeneans. This new species is named Nosema monorchis n. sp., after the generic name of its host.