Ultrastructural observations on different developmental stages of Goussia sinensis (Chen, 1955), a parasite of the silver carp (Hypophthalmichthys molitrix Valenciennes, 1844)

The development of Goussia sinensis, a coccidium parasitizing the intestine of the silver carp (Hypophthalmichthys molitrix) was studied by electron microscopy. All stages developed in the epithelial cells, less frequently in the goblet cells, and were located within a parasitophorous vacuole. In some cases one cell was invaded by several merozoites. Eight to sixteen merozoites were formed within the meront by ectomerogony. The ultrastructural processes characteristic of gamogony were the same as those found for Goussia spp. parasitizing other species of fish. A hitherto unknown mechanism of oocyst wall formation was observed. The oocyst membrane developing within the zygote surrounded only part of the zygote material. Thus, a small part of the zygote material left the oocyst proper. It is suggested that this zygote residue and the necrotic host cell constitute the so-called "yellow bodies" which include the excreted oocyst. The oocyst wall was 40 to 60 nm thick. Oocyst sporulation took place within the fish. The sporocysts consisted of two hemispheres connected by sutures and had a 100 to 120 nm thick double wall. They were surrounded by sporocyst veils fixed to the oocyst wall by membranes.