Description of Nematopsis mytella n. sp (Apicomplexa), parasite of the mussel Mytella guyanensis (Mytelidae) from the Amazon estuary and description of its oocysts

Numerous uninucleated phagocytes with several parasitophorous vacuoles (PV) each containing a single oocyst of an apicomplexan gregarine parasite of t he mussel Mytella guyanensis (Mytelidae) from the Amazon estuary were descr ibed. The phagocytes were in close contact with the gill tissue. Each phago cyte was surrounded by a thin layer composed of electron-lucent material. T he plasmalemma of the phagocyte presented numerous microvilli irregularly p rojected to the surrounding thin layer. The phagocyte cytoplasm was occupie d by large light areas, numerous small vesicles and among them a variable n umber of PV each containing a single oocyst. The oocysts were 11.5 mu m lon g and 8.2 mu m wide and the wall was 0.7 mu m thick. The apical zone of the wall contained a circular micropyle covered by an operculum attached to th e PV membrane by a dense cord. Externally, the oocyst wall was surrounded b y numerous, long and anastomosed microfibrils attached to the wall and conn ected to the PV membrane. Several dense bodies within the PV were observed. Each oocyst contained a single vermiform uninucleated sporozoite. This gre garine was named Nematopsis mytella n. sp. and the morphological characteri stics of its oocysts were described. This parasite became the first record of Nematopsis sp. and the first gregarine from the Amazonian mussel.