"■.:^5 



"^^^^.■■; 



20 



Microns 



Figure 376. — Later stage of development of MSX in 

 the connective tissue of heavily infected C. virginica 

 from Delaware Bay. Formalin 10 percent, iron 

 hematoxylin. 



Figure 377. — Haplosporidium costale. A— mature spore ; 

 B — early Plasmodium. From Wood and Andrews, 

 fig. 1, Science, vol. 136, 1962, p. 711. 



on the inner surfaces of the shell and, in more 

 advanced cases, by deformation of the shell edges 

 and hinge. Examination of thin slides of shell 

 show abundantly branching fungus. The shell 

 disease in Oosterschelde was studied by Korringa 

 (1951a), who discovered that it spreads at water 

 temperatures above 19° C. and that the higher the 

 temperature the more vigorous the attack. The 



fungus was not isolated from Dutch oysters and 

 remains unidentified. Korringa believes that it 

 survives in the old green cockle shells scattered as 

 cultch over the bottom and that its spores are 

 probably carried by the water currents. Whole- 

 sale cleaning, removal of old shells, and disinfecting 

 of young infected oysters with a solution of "an 

 organic salt of mercury" (not fully specified by 

 Korringa) are recommended as control measures. 

 Shell disease in Dutch oysters has been known 

 since 1902, but at that time occurred only in a 

 limited percentage of oysters. Its rapid spread in 

 the years following 1930 was probably due to the 

 enormous quantities of old cockle shells, about 

 40,000 to 50,000 m.', scattered annually as spat 

 collectors. This gave the fungus a chance to pro- 

 liferate more rapidly and infect the oysters. Voi- 

 sin (1931) describes the disease in oysters imported 

 from Zeeland, Holland, for planting in the Ma- 

 rennes area on the west coast of France. He 

 states that more than 40 percent of these oysters 

 had shells infected by a fungus, probably belonging 

 to the genus Monilia. The identification is merely 

 a guess and cannot be verified. 

 Foot disease 



Foot disease or "maladie du pied" of French 

 oyster growers occurs in 0. edulis and C. angulata 

 in the waters of the western and southern coasts of 

 Europe. Korringa suggests that it is probably 

 identical with the shell disease. The name is an 

 obvious misnomer because the foot is lacking in all 

 adult oysters. 



"Foot disease" has existed in the Arcachon re- 

 gion smce 1877. Giard (1894) described its para- 

 sitic nature and attributed it to a schizomycete 

 fungus Myotomus osfrearum Giard, a genus not 

 listed in Johnson and Sparrow's treatise on fungi 

 (1961). 



The disease afifects the area of the attachment 

 of the adductor muscle, primarily on the lower, 

 concave (left) valve, and in certain cases the upper, 

 flat valve. The surface of the shell under the 

 muscle is covered with small, rough dark green 

 spots. In advanced cases the muscle becomes 

 detached from the valve and forms irregular cysts 

 of horny and slightly elastic material. Ijater on 

 when the cyst extends beyond the area of the 

 muscle attachment, the cyst walls become covered 

 with calcareous shell deposit. According to 

 Giard (1894) and Dollfus (1922), the parasitic 

 fungus grows by utilizing the conchiolin of the 

 shell and stmiulates its secretion by the mantle. 



418 



FISH AND WILDLIFE SERVICE 



