Figure 10. — Oyster set at different ages and sizes. The smallest set seen on the shell at left is about 3 to 5 days old and 

 measures only about 1 mm. in length. The set on the shell at the right was photographed at the end of summer and 

 measures about 3/4-inch. The two shells In the middle show intermediate sizes. 



crustaceans, especially crabs, many varieties 

 of larval and adult fish, ascidians, and others. 

 It is probable that under natural conditions 

 oyster larvae are parasitized by fungi, suchas 

 S. zoophthorunn , and are infected by certain 

 bacteria, such as Vibrio and Pseudomonas . 

 Perhaps viruses also kill oyster larvae and 

 adult oysters. 



Oysters are parasitized with many micro- 

 organisms, some of which are responsible for 

 extensive mortalities. One of the most de- 

 structive cases of this nature is the so-called 

 Malpeque Bay disease, which struck oysters 

 of Prince Edward Island, Canada, around 1915. 

 Oysters affected by this disease become thin 

 and emaciated and sometimes have yellowish 

 pustules and abscesses. It took about 13 years 

 for the oyster beds of Malpeque Bay to re- 

 cover. Apparently, local oysters at that time 

 became largely immune to this disease, but 

 oysters originating in other locations, if trans- 

 planted to areas affected with Malpeque Bay 

 disease, became infected. It is still uncertain 

 what micro-organisms are responsible for the 

 disease, and some authorities think that the 



Malpeque Bay disease may be two or even 

 three diseases. 



Another widespread disease is caused by a 

 highly pathogenic fungus, Dermocystidium 

 marinum, which affects oysters from Dela- 

 ware Bay to Mexico. The range is not con- 

 tinuous because certain areas, such as the 

 Virginia seaside, appear free of this organism. 

 In the northern range of its distribution, D. 

 marinum disappears from oysters during cold 

 seasons but reappears in summer to cause 

 new losses. In the Gulf of Mexico, where 

 water temperatures even in winter are 

 relatively mild, considerable mortality may 

 occur even during the coldest part of the 

 year. Low salinities and strong tidal action 

 usually decrease the strength of the epi- 

 demics . 



In 1957 a heavy mortality of oysters oc- 

 curred in Delaware Bay. In April and May 

 from 35 to 85 percent of planted oysters died 

 on some beds. The losses continued during 

 1958 and later were reported from Chesa- 

 peake Bay and other areas. This disease is 

 now ascribed to a new organism called "MSX", 



18 



