son (1938), who was apparently unaware of 

 Mori's report. Odlaiij? (1946) stated that Olym- 

 pia oysters infected with even small numbers 

 of M. orientalis had a lower condition index 

 than uninfected oysters. Chew, Sparks, and 

 Katkansky (1965) found a similar relation in 

 Pacific oysters, and Sparks (1962) demon- 

 strated pathological changes in gut epithelium 

 and underlying tissues of these oysters infected 

 with the copepod. Mori and Odlaug reported 

 M. orientalis as a parasite of mussels (Mijtihis 

 crassitesta Lischke and M. edidis L.) as well 

 as oysters, both in Japanese and United States 

 waters. Another species, Mijfilicola iiifeftfitiaUs, 

 first described by Steuer ( 1902, 1908) , has been 

 blamed by Korringa (1950, 1959) and others 

 for widespread mortalities of sea nnissels, Mii- 

 tilus ediilis, in Europe but is found only rarely 

 in European oysters (Baird, Bolster, and Cole, 

 1950; Hepper, 1953, 1956) and has not been 

 reported to cause significant mortalities. A 

 small specimen, identified as Mytilicola i)itcs- 

 tiiialif!, was found by Pearse and Wharton 

 ( 1938) in an American oyster from the Florida 

 coast. Humes (1954) believed that this copepod 

 was probably M. pomcfn, which he described 

 from nnissels and clams from Louisiana. 



Crabs of the genus Pimiotlicres occasionally 

 inhabit the shell cavity of oysters, where their 

 activities and eff'ects suggest that they are para- 

 sites rather than commensals (Christensen and 

 McDermott, 1958; Haven, 1959). Stauber 

 (1945) studied a sudden increase in abundance 

 of the oyst.;r crab, P. ostreum (Say), in Ameri- 

 can oysters of Delaware Bay in 1941. He ob- 

 served that the crabs robbed the oyster of food 

 and injured the gills, resulting in a weakened 

 condition of the host. Stauber found in 1941 

 that 90 percent of the oysters of Delaware Bay 

 harbored four to six crabs in the first parasitic 

 stage. He ascribed unusual mortalities in cer- 

 tain oyster populations to debilitation caused 

 by high abundance of crabs. In 1942 abundance 

 of crabs dropped to 25 to 30 percent, and con- 

 tinued to decline in following years. 



Another Pinnotheres was described from 

 Madagascar oysters, Ostrea vitrifacta Sowerby, 

 by Poisson (1946); infestation was accom- 

 panied by development of a characteristic irri- 

 tating flavor in the parasitized oyster. He spec- 

 ulated that this flavor might be traced in some 



way to the coelenterate Sertidaria. which often 

 grows on shells of oysters that contain Piii- 

 itothcrcs. Korringa (1952a) pointed out the 

 striking similarity of this observation with the 

 popular belief in Holland that eating mussels 

 parasitized by P. pisiini causes "nettle rash." 



Tumors 



Tumors in oysters have been reported. Be- 

 nign mesenchymal tumors of pericardial origin 

 have been described from oysters by Ryder 

 (1887) and Smith (1934). The nodular, poly- 

 poid, pediculated growth found by Smith was 

 over 3 cm. in greatest dimension. Sparks, 

 I'auloy, Bates, and Sayci' (1964a) described a 

 stalked mesenchymal tumor from the Pacific 

 oyster that ajjpeared to be histologically similar 

 to that found by Smith. Sjiarks, Pauley, Bates, 

 and Sayce (1964b) also reported a Pacific oys- 

 ter with a tuniorlike fecal impaction of un- 

 known origin, accompanied b.\' inflammation 

 and encapsulation. The literature on tumors of 

 other species of mollusks, as well as those of 

 other invertebrates, has been rexicwed by 

 Scharrer and Lochhead (1950). 



.MUSSELS 



Marine mussels, particularly Mutihis edidis. 

 are abundant and palatable. They are grown 

 by highly developed culture methods, especially 

 in Western Euroi)e. Culture of a single .specie.s, 

 often under crowded conditions, however, in- 

 creases vulnerability to disease. As with 

 cultivated oysters, extensive and repeated mor- 

 talities, presumably' caused by disease, have 

 occurred in mussel beds. The parasitic copepod, 

 MiifiUcnln, is the only specific pathogen that 

 has been shown to cause mortalities in Euro- 

 pean mussels, although other parasites, par- 

 ticularly haiilosporidan Protozoa and larval 

 trematodes, have been reported. 



Micrubial Diseases 



Eggs of the sea nuissel from the western 

 North Atlantic are occasionally infected with 

 a haplosporidan, Clnjfridiopnis nn/tilovum. 

 Field (1923) first described the parasite and 

 .several of its life history stages. R(:«.ently 

 Sprague (1965b) has redescribed life hi.story 

 stages in more detail. The parasite has a high 

 prevalence in some samples, although the pro- 

 j)ortion of infected eggs to normal eggs in any 



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U.S. FISH AND WILDLIFE SERVICE 



