large-scale deaths of oysters in Kanasawa Bay, 

 beginning in 1915 and continuing for a number 

 of years. Over 80 percent of the oysters in that 

 bay died annually. Ogasawara et al. (1962) re- 

 ported similar mass mortalities on the Miura 

 peninsula, beginning in 1927 and continuing 

 for 10 years. Oyster farms along the coast of 

 the peninsula lost 50 to 80 percent of their crop 

 annually. More recent mortalities of 2-year-old 

 oysters have occurred in Hiroshima Bay and 

 adjacent localities, beginning in 1945 (Fujita 

 et al, 1953). A 10-year study (Takeuchi, Mat- 

 subara, Hirokawa, and Tsukiyama, 1955, 1956; 

 Takeuchi, Matsubara, Hirokawa, and Matsuo, 

 1957; and Takeuchi et al., 1960) provided 

 somewhat inconclusive evidence that a bacterial 

 pathogen was responsible for the mortalities. 



A series of papers by Tohoku Regional I'ish- 

 eries Research Laboratory (Imai et al., 1965; 

 Kan-no et al., 1965; Mori, Imai, Toyoshima, 

 and Usuki, 1965; Mori, Tamate. Imai, and 

 Itikawa, 1965; Numachi et al., 1965; Tamate 

 et al., 1965) described mass mortalities of 

 oysters m Matsushima Bay, Miyagi Prefecture, 

 Japan, that have occurred annually in late 

 summer since 1961. Environmental, physio- 

 logical, and pathological factors were e.xam- 

 ined. Pathological changes were observed, and 

 mortalities were considered to be related to 

 metabolic changes during fattening and spawn- 

 ing. Mortalities exceeded 60 per cent per year 

 in certain areas of the Bay during 1961- 

 (M. A gram-positive bacterium was found in 

 multiple abscesses in as many as 20 percent of 

 oysters in certain samples (Numachi et al., 

 1965), but a causal relation with mortalities 

 was not establi.shed. Our later studies suggest 

 that the disease condition is the same as that 

 called "focal necrosis" in adult Pacific oysters 

 fiom Washington. The pathogen warrants fur- 

 ther observation, since the abscesses may repre- 

 st_'nt only the chronic stage of infection in re- 

 sistant hosts, whereas the acute disease may 

 have a significant eff'ect on mortality. An ame- 

 boid organism, often present in large numbers 

 and accompanied by pronounced host response, 

 has also been found in oysters from the Matsu- 

 shima Bay mortality area. 



Blue crab populations on the coasts of North 

 and South Carolina have been affected by exten- 

 sive mortalities beginning in 1965 (Lunz, 



1967). Significant impact on population size 

 was indicated by a marked drop in catch per 

 unit eff"ort in 1966, as compared with the previ- 

 ous 5 years. Newspapers stated that great num- 

 bers of crabs were washed up on beaches or lit- 

 tered the bottoms of creeks, and that catches 

 were drastically reduced. Disease has been sus- 

 pected (Lunz, 1967), but no clear evidence of a 

 pathogen has been obtained. Such mortalities 

 in wild crabs are apparently distinct from the 

 long-recognized high levels of deaths in crab 

 shedding floats (Beaven and Truitt, 1939), 

 which may in part be associated with parasites 

 or diseases (Couch, 1966; Sprague and Beckett, 

 1966). 



A few mass mortalities of marine inverte- 

 brates have been definitely ascribed to epizoo- 

 tics caused by specific pathogens (Sindermann, 

 1963). Recurring mortalities of American oys- 

 ters in the Gulf of Mexico were found to be 

 caused by the fungus Der»iocustidiMm mari- 

 iiKiii. Exerting its eff'ects in higher salinities 

 and temperatures among dense aggregations of 

 hosts, the pathogen can cause annual mortali- 

 ties in excess of 50 percent. Development and 

 use of a presumptive test, with thioglycollate 

 medium (Ray, 1952), has established the pres- 

 ence of the organism in oysters throughout the 

 Gulf of Mexico and northward along the Atlan- 

 tic coast as far as Connecticut. Although the 

 fungus may at times reach epizootic levels in 

 particular areas, its most significant eflFect is 

 probably that of continuing attrition, year after 

 year, during periods of high sea-water tempera- 

 ture. Eff'ects of the disease on commercial beds 

 are now controlled to some extent by planting 

 and harvesting at prescribed times of the year 

 and by spreading oysters thinly on the beds. 



Major mortalities, with consequent severe 

 depression of the oyster fishery, occurred in 

 Delaware and Chesapeake Bays on the U.S. east 

 coast, beginning in the late 1950's. A haplospo- 

 ridan parasite with distinctive characteristics. 

 M))ichiiiia uelsoiii, has been associated with the 

 mortalities. Epizootic areas have had oyster 

 losses in excess of 90 percent, and some indica- 

 tions are appearing of increased resistance 

 •imong survivors. The disease, like that caused 

 by Dermocystidium, exists and exerts severe 

 effects in salinities above 15 o oo. Seed beds 

 and oyster stocks in low-salinity areas have not 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



363 



