Ultrastructure of Gametocyst of Parasitic Protozoan, Nematopsis sp. in Black Tiger Shrimp Penaeus monodon from the Gulf of Thailand

Ultrastructure of gametocyst of Nematopsis sp., a protozoa parasite of black tiger shrimp Penaeus monodon from the Gulf of Thailand is described. Ball-shaped gametocysts of about 110–160 µm diameter were found in close contact with the intestinal wall of shrimps. Surface of the gametocyst cyst wall or capsule is wrinkled with a circular bare area at one pole that contains a central pore 4–5 µm in diameter. The interior of the gametocyst is composed of numerous gymnospores and membranous sacs. Gymnospores varied in size with an average diameter of 6–8 µm. Ball-shaped gymnospores were composed of numerous, radially arranged, cone shaped sporozoites. Average width and length of sporozoites were 0.8–1.2 µm and 3–5 µm, respectively, with their rostral part pointing outward and caudal part, inward connecting to the residual cytoplasm in the centre of a gymospore. The rostral part of the sporozoite contains an oval nucleus, rough endoplasmic reticulum, mitochondria and a group of secretory granules. Membranous sacs were composed of two types of globular granules; large electron lucid granules and small dense granules