clams collected from Chesapeake Bay. He 

 termed the condition "endemic" and reported 

 it in about 2 percent of clams collected from 

 certain b.iv areas. 



OTHER BIVALVE MOLLUSKS OF COMMERCIAL 

 IMPORTANCE 



There are two other groups of commercially 

 important bivalve mollusks — scallops and pearl 

 oysters — for which some information on dis- 

 eases and parasites is available. Mass mortali- 

 ties caused by disease have not been reported 

 in either Ki"oup. Scallops are infected by pro- 

 tozoan and trematode parasites, and occasion- 

 ally are affected by a shell disease. Pearl oysters 

 harbor a number of larval trematode parasites. 



Scallops 



Although major mortalities have occurred in 

 scallop populations (Dickie and Medcof, 1963; 

 Medcof and Bourne, 1964; Merrill and Po.sgay, 

 196-1; Sanders, 1966), none has been definitely 

 associated with disease. In fact, only a few dis- 

 eases and parasites are known, and their effects 

 on the hosts are slight. 



Two p.irasites of scallops have been recog- 

 nized. A coccidian, Pseiidoklossia pccti)iif<, was 

 described as a rare parasite in the kidney 

 tubules of the great scallop, Pecfoi maxiniiis, 

 at Roscofl", France, by Leger and Duboscq 

 (1915b), who found usually light infections 

 with no e.xtensive pathology. Sporocysts and 

 fork-tailed cercariae of a trematode (not fur- 

 ther identified) were found by Linton (1915) 

 in large bay scallops, Aequipecten irradianH, 

 from \\'oods Hole, Mass. Infections were rare. 



An abnormal brown discoloration of meats 

 was studied by Medcof (1949) in sea scallops, 

 Placnpectoi »iageUauicus (Gmelin), off the 

 south coast of Nova Scotia, Canada. He con- 

 sidered the condition to result from exten- 

 sive invasion of the shell by a boring sponge. 

 In advanced stages, the shell was completely 

 honeycombed, causing excessive inner shell 

 deposition and producing weak shrunken indi- 

 viduals which, Medcof assumed, died eventually 

 from effects of the shell disease. Meat yields 

 from heavily infected scallops were less than 

 half those of normal individuals, but only older 

 scallops (8 or 9 years old) were infected. 



Pearl Oysters 



Pearl-producing bivalves of the family 

 Pteriidae, called "pearl oy.sters" but actually 

 taxonomically remote from edible oysters of 

 the family Ostreidae, occur in many parts of 

 the world (Sivalingam, 1962). Interest in para- 

 sites of pearl oysters has naturally centered on 

 those larval worms considered responsible for 

 pearl formation (Jameson, 1902, 1912; Wright, 

 1966), but a few other parasites and diseases 

 have been recognized. 



Parasites of the pearl oysters of Ceylon, 

 Margaritifera (Pinctada) vulgaris Schum., 

 were studied by Shipley and Hornell (1904) 

 and Southwell (1911, 1912) particularly with 

 regard to the role of parasitic worms in pearl 

 formation. These authors described several 

 ■stages of cestode larvae, some clearly trypano- 

 rhynchid, from the digestive gland and gills of 

 the pearl oyster. The worms occurred, often in 

 great numbers, in fibrous capsules. Several larval 

 trematodes were also found, but only one, de- 

 -scribed as Midtua margaritiferae, occurred in 

 abundance. Metacercariae localized in the gills. 

 Other metacercariae, described as Mnsalia herd- 

 mani. were found in the muscles, mantle, and 

 foot. An aspidobothrid trematode, Aspidogas- 

 ter margaritiferae, occurred in the pericardial 

 cavity, and several species of encysted larval 

 nematodes were seen in the gonads, stomach 

 walls, and adductor muscles. 



Pearl oysters of Japan, Pinctada martoisii, 

 are commonly infested with sporocysts and 

 cercariae of a bucephalid trematode described 

 as Buccphahts margaritae by Ozaki and Ishi- 

 bashi (1934). Experimental infections of sev- 

 eral species of small fishes with cercariae from 

 pearl oysters (Sakaguchi. 1962, 1966a) pro- 

 duced metacercariae morphologically the same 

 as those identified as B. varicus by Manter 

 (1940). Adult trematodes were found in the 

 digestive tracts of carangid fishes, Caranx 

 sexfasciatus and C. igiiobilis. which were abun- 

 dant in the waters near oyster farms where 

 pearl oysters were heavily infested (Sakaguchi, 

 1966b). Marked decline in condition of pearl 

 oysters resulted from invasion by larval trema- 

 todes. Sporocysts were found to overwinter in 

 the host, and cercarial production began again 

 when water temperatures rose in spring (Saka- 

 guchi, 1965). Pearl oysters infected in the pre- 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



351 



