its etiologic agent as a bacterium, MijotoniKs 

 ostrearmn, but further definitive studies of the 

 causative organism are needed. The disease is 

 localized in the shell under the attachment of 

 the adductor muscle, where it causes roughen- 

 ing and blistering of the shell and degeneration 

 of adjacent muscle tissue. The muscle may be- 

 come detached as irregular cysts are formed. 

 Major mortalities occurred on oyster beds at 

 Arcachon, France, in 1877 (Hornell, 1910; 

 Orton, 1937). The cause was not determined, 

 but some evidence of "foot di.sease" was found. 

 Galtsoff (1964) reported the rare occurrence 

 of the disease in the American oyster from the 

 southern United States, but did not consider it 

 a serious threat to oyster populations. Durve 

 and Bal (1960) reported the rare occurrence 

 of a shell di-sease which they considered to be 

 "maladie du pied" in the backwater oyster, 

 C. gnjplioides (Schlotheim) from India. 



Davis, Loosanoff, Weston, and Martin (1954) 

 isolated a fungus, later described as Sirolpidiion 

 zoophthorum (Vishniac, 1955), from hatchery- 

 produced oyster and clam larvae. The infec- 

 tions were rare, but they produced occasional 

 epizootics that killed mo.st of the cultured larval 

 population in 2 to 4 days. Juvenile as well as 

 larval bivalves were infected ; growth ceased 

 and death followed soon after infection. In- 

 fected cultures of bivalve larvae contained 

 large numbers of motile biflagellate zoospores 

 of the fungus. The authors speculated that an 

 epizootic of the fungus could occur among 

 lamellibranch larvae in nature. 



There are several inconclusive reports of 

 organisms resembling actinomycetes in oysters. 

 Eyre (1924) reported Cladothrix dichotoma 

 from oysters examined during the great mor- 

 talities of 1919-23 in western Europe. The 

 isolate was not pathogenic in experimental 

 studies. Dollfus (1921a) .stated that Eyre's iso- 

 late was a species of Nocardia. Pettit (1921) 

 also identified a Nocardia from O. edidis, but 

 Dollfus (1921b) considered this to be merely 

 normal cell reticulum of the oyster. Mackin 

 (1962) described a "mycelial disease" of 

 Ci-assostrea virginica, which he thought might 

 be caused by an actinomycete. We have recently 

 seen an organism similar to that described by 

 Mackin in C. angulata from France. 



Protozoa. — A variety of Protozoa parasitize 



oysters (Rosenfield, 1964; Sindermann, 19G6b), 

 and certain Sporozoa ' are serious pathogens. 

 Two haplosporidans, MbicJniiia costalis (Wood 

 and Andrews) and M. nclsviii Haskin, Stauber, 

 and Mackin, have caused oyster mortalities on 

 the North American east coast within the past 

 decade (fig. 3). 



M. costalis is found in seaside bays of Mary- 

 land and Virginia, along the lower eastern 

 shore of Virginia, and in Delaware Bay (An- 

 drews, Wood, and Hoesc, 1962; Wood and 

 Andrew.s, 1962; Sprague. 1963; Haskin, Stau- 

 ber, and Mackin, 1966; Couch, 1967b). First 

 recognized in moribund and dead oysters from 

 Hog Island Bay, Va., in 1959 by Wood and 

 Andrews, M. co.stalis was held responsible, on 

 the basis of epizootiological evidence, for sharp 

 peaks of mortality in early summer. The patho- 

 gen and mortalities caused by it continue to 

 characterize Maryland and Virginia .seaside 

 oyster populations. 



The second haplospoiidan species, M. )i(lsoiii, 

 has a wider distribution — from Connecticut to 

 North Carolina. It has caused extensive mor- 

 talities and drastic decline of the oy.ster fishery 

 in Delaware Bay beginning about 1957 and in 

 lower Chesapeake Bay beginning in 1959 

 (Mackin, 1960; Engle and Ro.senfield, 1963; 

 Andrews, 1964). In each affected area, mortali- 

 ties have often exceeded 95 percent for several 

 years. Because of the severe impact of the M. 

 nelsoni epizootic on oyster stocks of the Middle 

 Atlantic States, a number of research group.s — 

 university. State, and Federal — have partici- 

 pated in scientific studies since the late 1950's, 

 and significant papers have been published 

 recently. Haskin et al. (1966) named the 

 plasmodial stage of the parasite as Mivchinia 

 nelsoni: Couch, Farley, and Rosenfield (1966) 

 associated the Plasmodium with spore and pre- 

 spore .stages; and Barrow and Taylor (1966) 

 confirmed, with immunological techniques, the 

 a.s.sociation of Plasmodium and spore. Andrews 

 (1964, 1966) described aspects of the epizoo- 

 tiology of the disea.se in Virginia waters, and 

 Haskin, Canzonier, and Myhre (1965) briefly 

 summarized the epizootiology in Delaware Bay. 



' ThrmiKhimt this paper an attompt has been made to conform 

 to the reviscci rlassincation of the Protozoa as propose.) by Honig- 

 bcrg. Balamuth. Bovce, CorH.ss, Gojdics, Hall. Kudo, I.evine, Loeb- 

 lich. Weiser, and Wenrich (1964), and modified by Sprague (1966a). 



340 



U.S. FISH AND WILDLIFE SERVICE 



