cycles, the relationship between environmental 

 conditions and degree of infection. 

 Disease associated with Haplosporidiuni 



Excessive mortality of oysters in Delaware 

 Bay in a 6-week period of April and May 1957 

 wiped out from 35 to 85 percent of planted oysters 

 and almost completely ruined the oyster industry 

 of the State. A microorganism consistently 

 found in tissues of infected oysters was designated 

 by the code name MSX and later on was tenta- 

 tively identified by Mackin as one of the Haplo- 

 sporidia. The organism invades the connective 

 tissue surrounding the intestine and digestive 

 diverticula. Early plasmodial stages and en- 

 suing stages of development are shown in two 

 illustrations (figs. 375 and 376) made in the 

 laboratory from a preparation kindly supplied 

 by Haskin. 



Mortality of oysters on the eastern shore of 

 Virginia near Seaside was investigated from 

 1959 to 1961 by the Virginia Institute of Marine 



Science. The microorganism causing the disease 

 and fii'st designated as SSO was described by 

 Wood and Andrews (1962) as a sporozoan, 

 Haplosporidmm costale, n. sp., infecting connective 

 tissues of oysters and producing a truncate spore 

 encased in an operculum with a lid. An early 

 Plasmodium with 6 to 12 nuclei is from 6 to 8/i 

 in size (fig. 3~7). Haj^losporidium has been 

 found in live oysters as early as February, and in 

 mid-May to June the infection may cause high 

 mortality. How the parasite infects the oysters 

 is not known, and its life history is not fully 

 understood (Andrews, Wood, and Hoese, 1962). 



Shell disease 



This disease, which is probably associated with 

 an unidentified fungal infection of oyster shell, is 

 not particularly serious in C. virginica, but has 

 been reported to cause catastrophic mortalities in 

 the population of 0. eduHs in Oosterschelde, Hol- 

 land. The disease can be recognized by bottle- 

 green or orange-brown rubberlike warts and spots 



30 



Microns 



Figure 375. — Plasmodial stage of MSX in the connective tissue of heavily infected C. virginica from Delaware Bay. 



Bouin, hematoxylin -eosin. 



FACTORS AFFECTING OYSTER POPULATIONS 



417 



