Ultrastructural characteristics of the in vitro cell cycle of the protozoan pathogen of oysters, Perkinsus marinus

Ultrastructural characteristics of vegetative and zoosporangial stages of c ultured Perkinsus marinus, a pathogen of the eastern oyster, Crassostrea vi rginica. were examined by transmission electron microscopy. An axenic cell culture was propagated from infected Chesapeake Bay oyster hemolymph. Diffe rent stages of the in vitro cell cycle, including schizonts and different s ize trophonts, were examined. Trophonts had spherical nuclei with wide peri nuclear spaces, mitochondria with tubular cristae, and vacuoles with vacuop lasts. There were micropores on the inside of cell walls. A tubular network in the cytoplasm connected lomasomes to vacuoles, and contained vacuoplast precursor material. Vacuoplasts and precursor material diminished when cel l cultures were not fed. suggesting a function in metabolite storage. Cells divided by schizogony or binary fission. Daughter cells in a schizont were not alike, and may specialize for different functions. Some of the daughte r cells in a schizont died. Some hypnospores, directly isolated from infect ed oyster hemolymph enlarged in Ray's fluid thioglycollate medium, and were induced to zoosporulate. Zoosporangia contained varicose, hypha-like struc tures, whose apical tips gave rise to prezoospores. Ultrastructural charact eristics of the vegetative and zoosporangial stages did not resemble any ap icomplexan parasites other than members of the genus Perkinsus.