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Figure 3. — Sporocysts of Minchinia nelso7ii (formerly known as "MSX"), the etiolic agent of "Delaware Bay 

 disease" in an American oyster^fresh preparation at left, and stained section at right (X 1,000). 



Farley (1965), after completing a 5-year 

 histopathological study of Chesapeake Bay 

 oysters, categorized natural infections accord- 

 ing to extent of invasion and nature of host 

 response. 



As an example of the effects of M. nelsoni 

 on the U.S. oyster fishery, landings in New 

 Jersey waters of Delaware Bay in the late 

 1940's and early 1950's had fluctuated around 

 6 million pounds of shucked meats until the 

 mid-1950's, when disease decimated the stocks. 

 Landings fell precipitously to a low of 167,000 

 pounds in 1960, and no significant recovery has 

 occurred (fig. 4). Comparable effects have 

 been felt in the high-salinity waters of lower 

 Chesapeake Bay, another major oyster produc- 

 ing area. 



Thus far, one alleviating influence seems to 

 be salinity. M. nelsoni occurs in waters whose 

 salinity consistently exceeds 15 o/oo; during 

 3 years (1963-65) of drought along the Atlan- 

 tic coast, the pathogen invaded areas of middle 

 Chesapeake Bay formerly free of the disease 

 (Rosenfield and Sindermann, 1966). Tempera- 

 ture may also be important, since the pathogen 

 appears to be quiescent during the winter. 



Although several life history stages have 

 been recognized for both species of Minchinia, 



and concurrent infections have been found 

 (Couch, 1967b), routes of infection and meth- 

 ods of transmission are still unknown. Because 

 several research groups are actively concerned 

 with oyster diseases on the U.S. east coast, par- 

 ticularly with M. nelsoni, increased understand- 

 ing of this and other pathogens should develop 

 rapidly. Recently, we have seen in Pacific oys- 

 ters from Taiwan plasmodial parasites mor- 

 phologically very similar to those found in 

 American oysters. 



Leger and Hollande (1917) described an- 

 other haplosporidan, Chytridiopsis ovicolu, in- 

 fecting the eggs of European oysters taken at 

 Marennes, France. The parasite was relatively 

 rare and occurred only in certain ovarian fol- 

 licles of parasitized oysters. 



Nematopsis ostrearum Prytherch, a grega- 

 rine parasite of the American oy.ster, was held 

 (Prytherch, 1938, 1940) to be the cause of 

 extensive mortalities in Virginia and Louisiana. 

 Later studies (Sprague, 1949; Sprague and 

 Orr, 1955) indicated, however, that Nematop- 

 sis did not cause deaths of oysters and sug- 

 gested that Dennocystidium infections may 

 have complicated earlier results. Owen, Wal- 

 ters, and Bregan (1952) found no correlation 

 between Nematopsis infections and oyster mor- 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



341 



