Hillman (1963) and Cheng (1964) made a 

 similar speculation about haplosporidan hyper- 

 parasites of oysters. 



Other larval trematodes occur on and in 

 oysters. Fujita (1925, 1943) described Gymno- 

 phalloides tokieiisis, a metacercaria which en- 

 cysts, often in great numbers, on the mantle 

 and gills. The host's physiology is disturbed, 

 growth is halted, and reproduction is inhibited. 

 Marine birds are definitive hosts for the para- 

 site. Metacercariae of Proctoeces ostrea Fujita 

 are also found in Japanese oysters. About 10 

 percent of the oysters in Hiroshima Bay were 

 infected by the larval trematode, which local- 

 izes in gonad tissue. European oysters harbor 

 the related Proctoeces macultitus Looss. De- 

 finitive hosts are labrid fishes in Europe, and 

 snappers and red groupers, Pagrosomus major 

 and Epivephetiis akaara, in Japan. 



Massive invasion by metacercariae in Ameri- 

 c;m oysters from the Texas coast was reported 

 recently by Little, Hopkins, and Schlicht 

 (1966). Feeding experiments showed that the 

 trematodes were Acaiithopariiphiiim spiniilo- 

 svm (Johnston), which matures in the intes- 

 tine of shore birds. Most of the oysters 

 examined had metacercariae in the mantle; the 

 number averaged 45 worms per oyster. 



Cestodes. — Oysters in several regions of the 

 world are parasitized by larval cestodes of the 

 genus Tjilocephalum. These parasites are Lec- 

 anicephaloidea that occur as adults in the diges- 

 tive tracts of elasmobranchs. Sparks (1963) 

 reported heavy infections of Tyloceplialum in 

 American oysters introduced in Hawaii. In an 

 addendum. Sparks noted that oysters from 

 Florida had been reported by the BCF Biologi- 

 c.il Laboratory, Oxford, Md., to harbor similar 

 larval cestodes (fig. 6). We have found similar 

 larvae in oysters from Georgia and North Caro- 

 lina. The coracidium of T ylocephalum was re- 

 cently reported in the stomach and gills of 

 American oysters collected at Pearl Harbor, 

 Hawaii (Cheng, 1966). Penetration of gill or 

 digestive epithelium was postulated from study 

 of histologic sections. The pronounced cellular 

 reaction in the subepithelial tissues — including 

 encapsulation of the larvae — was described. 

 Larval cestodes, probably T ylocephalum, have 

 been found in Pacific oysters from Japan and 

 Taiwan by staff members of the BCF Biological 



Figure 6. — Tylocephalum in American oyster from 

 Apalachicola Bay, Fla. (x 70). 



Laboratory, Oxford, Md. Members of this tape- 

 worm group are known as parasites of pearl 

 oysters in the Far East and were held to be 

 the cause of pearl formation (Herdman, 1904; 

 Herdman and Hornell, 1906; Shipley and Hor- 

 nell, 1904, 1906; Southwell, 1924). Jameson 

 (1912) presented convincing evidence, how- 

 ever, that invasion by larval trematodes, rather 

 than larval cestodes, was more important in 

 pearl development. Pearl formation around 

 trematode and cestode larvae which invade the 

 mantle and die appears to follow a similar pat- 

 tern of host responses in bivalve mollusks 

 (Wright, 1966). 



Diseases Caused by Parasitic Crustaceans 



A parasitic copepod, Mytilicola orientalis, 

 was described from the digestive tract of the 

 Pacific oyster from Japan by Mori (1935). The 

 parasite was transferred to the United States 

 west coast with imports of seed oysters from 

 Japan, and was described as M. ostrea by Wjl- 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



345 



