mass mortalities of species such as oysters and 

 mussels is voluminous. Disease has sometimes 

 been demonstrated to be the cause of deaths ; 

 in other situations, disease has been strongly 

 suspected, or the cause has not been deter- 

 mined. Within the past decade knowledge about 

 molluscan shellfish diseases has increased at a 

 greatly accelerated pace, largely because of con- 

 cern about mortalities which have occurred in 

 widely scattered populations. Literature on 

 oyster diseases is most abundant ; that on mus- 

 sel and clam diseases is less voluminous. 



OYSTERS 



The 20th century has been a difficult and trou- 

 blesome period for oysters (family Ostreidae) 

 in many parts of the world (Orton, 1924a; 

 Roughley, 1926; Gross and Smyth, 1946; Logie, 

 1956; Mackin, 1961; Sindermann, 1966c). De- 

 cline in abundance of oysters actually started 

 late in the 19th century, probably cau-sed in 

 large part by indiscriminate harvesting and 

 destruction of beds. Extensive mortalities from 

 unknown causes also contributed to decreased 

 oyster production. The rate of decline on the 

 North American east coa.st and in other geo- 

 graphic areas has recently increased because 

 of large-scale mortalities, several of which have 

 been caused by disease. Largely because of 

 their worldwide economic importance, oysters 

 are among the m.ost thoroughly .studied of ma- 

 rine animals — especially their diseases and 

 parasites. Interest in oyster diseases has logi- 

 cally arisen from catastrophic mortalities in 

 many parts of the world. Many unsolved prob- 

 lems remain but the body of literature is large, 

 and is growing rapidly. 



Among the important diseases of oysters are 

 microbial diseases and those cau.sed by hel- 

 minths and parasitic crustaceans. 



Microbial Diseases 



Bacteria, fungi, and protozoans are the prin- 

 cipal causes of microbial diseases in oysters. 



Bacteria. — Reports of mass mortalities of 

 Pacific oysters, Crassostrea gigas (Thunberg), 

 have been published recently in Japan (Fujita, 

 Matsubara, Hirokawa, and Araki, 1953, 1955; 

 Takeuchi, Takemoto, and Matsubara, 1960; 

 Ogasawara, Kobayashi, Okamoto, Furukawa, 

 Hisaoka, and Nogami, 1962; Imai, Numachi, 



Oizumi, and Sato, 1965; Kan-no, Sasaki, 

 Sakurai, Watanabe, and Suzuki, 1965; Mori, 

 Imai, Toyoshima, and Usuki, 1965; Mori, 

 Tamate, Imai, and Itikawa, 1965; Numachi, 

 Oizumi, Sato, and Imai, 1965; Tamate, Numa- 

 chi, Mori, Itikawa, and Imai, 1965). Takeuchi 

 et al. (1960) implicated a gram-negative, 

 motile, 1- to 3-/i bacillus, probably an Achromo- 

 bacter, in large-scale mortalities in Pacific 

 oyster culture areas of Hiroshima Bay since 

 1946. Experimental infections were achieved 

 with cultured bacteria, but the organisms could 

 be isolated from healthy as well as sick oysters, 

 and from sea water. Moribund oysters had 

 diffuse cell infiltration, massive increase in bac- 

 terial numbers, and tissue necrosis. 



Numachi et al. (1965) found up to 20 per- 

 cent infection with gram-positive bacteria (not 

 further identified) in oysters during mass mor- 

 talities in Matsushima Bay, Japan, in the early 

 1960's. The disease was called "multiple ab- 

 scesses," but the authors did not think that a 

 causal relation existed between bacteria and 

 mortalities. A similar disease was found in 

 1965 by stafi" members of the BCF Biological 

 Laboratory, Oxford, Md., in seed oysters (less 

 than 1 year old) imported to the U.S. we.st 

 coast from Matsushima Bay, and in adult oys- 

 ters from Willapa Bay, Wash. The disease has 

 been labeled "focal necrosis" (fig. 1). Studies 

 of the etiologic agent and its pathogenicity 



Figure 1. — "Focal necrosis" in connective tissue of 

 Pacific oyster from Willapa Bay, Wash., sur- 

 rounded by extensive leucocytic infiltration ( x 700 ) . 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



337 



