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Zoospores were obtained from a protist, parasitic in the clamMacoma balthica. It has been previously suggested that the parasite is either a closely related species or is the same species asLabyrinthomyxa marina (Dermocystidium marinum), which is found in the oysterCrassostrea virginica. The demonstration of zoosporulation and the description of zoospore micromorphology are here offered as evidence of similarity between the two parasites. The zoospores were biflagellated, pyriform cells with mastigonemes along one side of the anterior flagellum. Negative staining revealed substructure in the mastigonemes.

Dermocystidium marinum is a pathogen which kills oysters in Chesapeake Bay from July through October. The fungus proliferates readily only at temperatures above 25°C but overwinters in oysters as sub-clinical cases. Experiments were conducted in Nature to demonstrate the effects of proximity of infected oysters on epizootiology in isolated disease-free populations. Prevalence ofDermocystidium in the York River was moderate in 1957, heavy in 1959, and extremely low in 1963. In 1957, with 40 feet of isolation, groups mixed with infected oysters showed large increases in deaths and prevalences over controls. In 1959, the fungus was so widely prevalent in York River during a long hot summer that relatively minor reductions in death rates were obtained by 50 feet of isolation. In 1963, with the fungus nearly absent, a failure in laboratory infection techniques caused late timing of the experiment, and results were inconclusive. A new disease caused by “MSX”, a protistan parasite, developed high prevalences in the 1963 lots. This organism, by destroying oyster, populations in high-salinity waters (above 15‰ in late summer) has effectively reducedDermocystidium to a minor role as an oyster pathogen in lower Chesapeake Bay. Losses fromDermocystidium in Chesapeake Bay are dependent upon duration of warm seasons and survival of cases through winter. MSX, is slightly more sensitive to low salinities but much more tolerant of low temperatures thanDermocystidium. The decline ofDermocystidium with the disappearance of oyster populations suggests that the pathogen may be dependent solely upon transmission from oyster, to oyster.