are in progress. The disease affects seed 

 oysters as young as 6 months, as well as 

 adults. The necrotic foci or multiple abscesses 

 may represent the resistant or arrested disease 

 state, and the active, fulminating phase may 

 have already killed susceptible members of the 

 population. 



Several bacterial pathogens of bivalve larvae 

 have been isolated (Guillard, 1959; Tubiash, 

 Chanley, and Leifson, 1965). Identified only as 

 Aerotnonas sp. or Vibrio sp., the organisms 

 killed larvae and juveniles of five bivalve spe- 

 cies tested, including American oysters, Craft- 

 sostrea virginica (Gmelin), and European 

 oysters, Ofitrea eduli'^ L., but did not affect 

 adults. 



Funqi. — Oysters have several fungus dis- 

 eases, some of serious consecjuence. Identifica- 

 tion of fungus pathogens, especially in the 

 early literature, has often been tentative, and 

 i»nly a few adequate characterizations of etio- 

 logic agents have been made. 



A relatively well-known fungus infecting 

 oysters from the Atlantic and Gulf coasts of 

 the United States is Dermncystidium marininu. 

 First de.scribed by Mackin, Owen, and Collier 

 (1950), the pathogen has been the subject of 

 much research. A useful diagnostic technique, 

 b.ised on antibiotic fortified fluid thioglycollate 

 medium, was devised by Ray (1952, 1966b) 

 (luring attempts to culture the organism. A 

 number of authors have shown that D. niari- 

 iinm causes oyster mortalities (Mackin et al., 

 1950; Mackin, 1953; Andrews and Hewatt, 

 1954; Hewatt and Andrews, 1954; Ray, 1954a, 

 19o4b, 1954c). Pathological changes in infected 

 oysters were described by Mackin (1951). In- 

 vasion takes place through the gut epithelium 

 and possibly through the mantle. The epi- 

 thelium is destroyed; the parasite lyses the 

 basement membrane and is distributed by the 

 blood to all parts of the body (fig. 2). All 

 tissues are invaded and damaged, and multiple 

 abscesses are formed. Normal gonad develop- 

 ment is inhibited, infected oysters become 

 .severely emaciated (Ray, Mackin, and Boswell, 

 1953; Ray, 1954b), and growth is retarded 

 (Menzel and Hopkins, 1955b). 



Temperature is important in the epizooti- 

 ology of dermocystidium disease (Hewatt and 



Figure 2. — Dermocystidium disease of American oys- 

 ters. Histologic section of infected oyster showinj;: 

 diagnostic "hypnospore" (a) and "rosette" (b) 

 (X 1,000). 



Andrews, 1956). Infections and associated mor- 

 talities rise during the warmer months and 

 decline during colder periods. Mortalities de- 

 cline in winter, prob.ibly because of reduced 

 parasite metabolism, rather than elimination of 

 the organism. Failure to find D. m(ni)nim con- 

 sistently north of Chesapeake Ray suggests 

 that prolonged low temperature may be a sig- 

 nificant limiting factor. Andrews (1965) found 

 that D. Diariinini proliferates readily only at 

 temperatures above 25' C., and overwinters as 

 subpatent infections. His further observation, 

 that the partial destruction of oyster popula- 

 tions by a iirotozoan disease resulted in de- 

 creased prevalence of the fungus organism, 

 suggests that Domoci/stidiimi is dependent 

 upon direct transmission from one oyster to 

 .mother. Other evidence for direct transmission 

 was provided by Ray and Mackin (1955). 



Infections and resulting mortalities are re- 

 duced in salinities below 15 o oo. Ray (1954b) 

 found evidence that low salinity retarded de- 

 velopment of terminal infections in laboratory 

 populations. Ray and Chandler (1955) sug- 

 gested that e.xcessively high salinities may also 

 be unfavorable for Dermocystidium. Mackin 

 (1956) found a positive correlation between 

 high salinity and high incidence of the fungus, 

 but he observed that the salinity tolerance 

 range was wide in experimental studies. Dilu- 

 tion of infective elements by inflow of fresh 

 water was suggested as an important limiting 

 factor and a possible control measure. 



338 



U.S. FISH AND WH DLIFE SERVICE 



