Hc.rcuiiita sp., a flagellate protozoan, occurs 

 frequently in the digestive tract of oysters, but 

 its parasitic or saprozoie role has not been ade- 

 quately determined. First described by Certes 

 (1882) as a commensal in European oysters, 

 the flagellate was later held responsible for 

 oyster mortalities from "pit disease" in Holland 

 (Mackin, Korringa, and Hopkins, 1952). Clear 

 evidence of pathological eff'ects was not ob- 

 tained, and heavy bacterial infections further 

 complicated the study. Hexamita was also 

 blamed for mortalities of Olympia oysters in 

 the State of \\'ashington (Stein, Denison, and 

 Mackin, 1961), but again clear evidence of 

 pathogenicity was not presented. Scheltema 

 (1962), who examined the relation between 

 Hexamita and American oysters from Dela- 

 ware Bay, concluded that the organism did not 

 contribute significantly to deaths of oysters. 

 He suggested, as did Stein et al. (1961), that 

 Hexamita may act as a pathogen during pe- 

 riods of low environmental temperatures and 

 low host metabolism but that prevalence de- 

 clines at higher temperatures because the proc- 

 esses in oysters which act to remove the 

 trophozoites exceed the reproductive rate of the 

 flagellate. We have recently seen Hexamita in 

 Pacific oysters from Korea and Taiwan. 



Several ciliate parasites have been described 

 from American oysters. A member of the genus 

 Spheiiiiphnja was reported by the BCF Biologi- 

 cal Laboratory, Oxford, Md. (Anonymous, 

 1965), as the cause of an oyster disease char- 

 acterized by formation of large cysts on the 

 gills. Richardson 2 and Laird (1961) identified 

 ciliates in the gut of C. virginica from Prince 

 Edward Island, Canada, as Orchitophriia stel- 

 laj-um Cepede. Prevalence was low, but infec- 

 tions were heavy and the intestinal epithelium 

 had been invaded. 0. stellanim is known as a 

 serious pathogen of starfish, in which it causes 

 gonad destruction (Cepede, 1911; Smith, 1936; 

 Vevers, 1951). Laird speculated that the organ- 

 ism may be a regular and possibly harmful 

 parasite of oysters and that starfish may be- 

 come infected from them. 



Mackin (1962) mentioned a ciliate parasite 



2 Data pidvided in Fisheries Research Board of Canada. Manu- 

 script Report Series (Biology), mimeographed, unnumbered. I'J^i'.t. 

 "Report on the studies of eastern coast oysters during the season 

 of 1939," by L. R. Richardson. 



of oysters from the Atlantic and Gulf coasts 

 of the United States, which he considered to be 

 Aiicistrocoma peltoieeii. a well-known parasite 

 of mussels. The ciliates were abundant in the 

 digestive tracts of oysters infected with Dermo- 

 cystidium mnrixum. but Mackin did not believe 

 that they were pathogenic to the oyster host. 



Two amebae are known from American oys- 

 ters. Hogue (1914, 1921) described Valdkamp- 

 fia calkciisi and V. patiixeiit, which are parasitic 

 in the digestive tract. She distinguished the 

 two species on the basis of differences in the 

 cyst wall. No evidence of pathogenicity was 

 found, nor were these forms demonstrated to 

 be other than saprozoie. Additional ameboid 

 organisms isolated from American oysters were 

 reported briefly by Sawyer (1966). 



Diseases caused h\ Helminths 



Both trematodes and cestodes parasitize 

 oysters. 



Trematndts. — European and American oys- 

 ters are parasitized by larval trematodes of the 

 genus Bucephahm. B. haimcainm w;is first re- 

 ported by Lacaze-Duthiers (1854) from Euro- 

 pean oysters in the Mediterranean Sea, and B. 

 ciiciihis was described by McCrady (1874) in 

 American oysters from South Carolina. Sporo- 

 cysts occur in the gonad and digestive gland 

 of the oyster (fig. 5), and sterilize the host. 

 The tentative life cycle of the parasite (Ten- 

 nent, 1906) includes minnows (Cyprinidae) or 

 mullets (Mugilidae) as second intermediate 

 hosts, and gars (Lepisosteidae) as definitive 

 hosts. Hopkins (1954, 1957b) reported para- 

 sitization of more than one-third of the oyster 

 population in localized areas of the United 

 States, although prevalence generally was much 

 lower, particularly in open waters. Menzel and 

 Hopkins (1955a, 1955b) sugge.sted that early 

 infections temporarily stimulate growth of the 

 oyster, but that older infections retard growth. 

 Hopkins (1957b) made the interesting, if some- 

 what facetious, observation that BucephahiR 

 might be considered a gastronomically beneficial 

 parasite in southern waters, since infected 

 oysters have an excellent flavor and are fat- 

 looking and glycogen-rich throughout the year, 

 whereas normal oy-sters are spawned out, thin, 

 and relatively tasteless during part of the year. 



DISEASES OF THE MARINE BIVALVE MOLLUSCA AND CRUSTACEA 



343 



