FISHERY BULLETIN: VOL. 84, NO. 4 



Maryland have been documented in the past (post- 

 tropical storm Agnes in 1972) (S. V. Otto unpubl. 

 data). Antigenic similarity between neoplastic clams 

 in New England and Maryland suggests that target 

 cells in the disease are the same in both areas. Addi- 

 tionally, the sudden appearance of isolated occur- 

 rences of the disease in widespread areas of the Bay 

 and the apparent tenfold increase in frequencies 

 since its appearance in 1978 in populations occur- 

 ring over most of the geographic range of soft clams 

 in the Bay suggest an infectious etiology rather than 

 point source chemical oncogen activity or pollution 

 (Barry and Yevich 1975; Yevich and Barszcz 1977; 

 Cooper et al. 1982a; Reinisch et al. 1984) as has been 

 implied in some New England studies. 



hyperchromatic nuclei); and 5) clinical features such 

 as progression and malignancy. 



ACKNOWLEDGMENTS 



This study was supported in part by the U.S. 

 Department of Commerce, National Marine Fish- 

 eries Service (contract no. NA-82-FA-C-00048). We 

 thank Keith R. Cooper for the critical review of the 

 manuscript. We also thank the technical staffs of 

 the Oxford Biological Laboratory, NMFS— G. Roe, 

 C. Roney, D. Howard, M. Prettyman; DNR-R. 

 Scott; James B. Engle Scholarship at Washington 

 College, Chestertown, MD— E. D. Grogan; and 

 Tufts University— H. Sakamoto, P. Cronin. 



Classification 



Histologically, the clam sarcomas (Fig. 1A) con- 

 sist of diffusely disseminated round cells with a 

 large, 6-10 ^m, hyperchromatic, often lobed nucleus 

 containing one or more prominent nucleoli. Cyto- 

 plasm is sparse, mitosis is common, and nuclei are 

 more than twice as large as normal hemocyte nuclei. 

 Histocytologic preparations (Fig. 1B-G) reveal sar- 

 coma cells with identical characteristics and which 

 can be definitively recognized on the basis of their 

 morphology. 



Other authors (Yevich and Barszcz 1977; Brown 

 et al. 1977; Reinisch et al. 1983) have called this 

 disease a "hematopoietic neoplasm" because of the 

 general similarity of neoplastic cells and hemocytes, 

 and because of its occurrence in vascular spaces. 

 While this is the most probable origin for these cells, 

 previous studies in other species have shown that 

 these criteria can be misleading. The neoplasm in 

 Macoma balthica (Christensen et al. 1974), which 

 was characterized by anaplastic cells inhabiting the 

 vascular spaces, was shown ultrastructurally to be 

 of epithelial origin and, therefore, diagnosable as a 

 carcinoma (Farley 1976b). Since no specific identi- 

 fying organelles have been seen in the soft clam 

 neoplasm (Brown et al. 1977) and since some mono- 

 clonal antibodies developed against neoplastic cells 

 do not cross react with normal hemocytes, we prefer 

 the more conservative term "sarcoma" which iden- 

 tifies the disease by behavior and cytology but does 

 not imply a particular cell origin. These data indicate 

 disease irreversibility and satisfy most of the other 

 criteria for sarcoma or carcinoma, namely: 1) loss 

 of cell specialization (anaplasia); 2) cell proliferation; 

 3) invasiveness (diffuse infiltration of connective 

 tissue and muscle); 4) clonal alteration of genetic 

 material (probable polyploidy evidenced by enlarged, 



LITERATURE CITED 



Barry, M. M., and P. P. Yevich. 



1975. The ecological, chemical and histopathological evalua- 

 tion of an oil spill site: Part III. Histopathological studies. 



Mar. Pollut. Bull. 6:171-173. 

 Brown, R. S. 



1980. The value of the multidisciplinary approach to research 



on marine pollution effects as evidenced in a three-year study 



to determine the etiology and pathogenesis of neoplasia in 



the soft-shell clam, Mya arenaria. Rapp. P. -v. Reun. Cons. 



int. Explor. Mer 179:125-128. 

 Brown, R. S., R. E. Wolke, S. B. Saila, and C. W. Brown. 

 1977. Prevalence of neoplasia in 10 New England populations 



of the soft-shell clam (Mya arenaria). Ann. N. Y. Acad. Sci. 



298:522-534. 

 Brown, R. S., R. E. Wolke, C. W. Brown, and S. B. Saila. 

 1979. Hydrocarbon pollution and the prevalence of neoplasia 



in New England soft-shell clams (Mya arenaria). In 



Animals as monitors of environmental pollutants, p. 41-51. 



National Academy of Sciences, Washington, D.C. 

 Christensen, D. J., C. A. Farley, and F. G. Kern. 



1974. Epizootic neoplasms in the clam Macoma balthica (L.) 



from Chesapeake Bay. J. Natl. Cancer Inst. 52:1739-1749. 

 Cooper, K. R., R. S. Brown, and P. W. Chang. 



1982a. Accuracy of blood cytological screening techniques for 



the diagnosis of a possible hematopoietic neoplasm in the 



bivalve mollusc, Mya arenaria. J. Invertebr. Pathol. 39: 



281-289. 

 1982b. The course and mortality of a hematopoietic neoplasm 



in the soft-shell clam, Mya arenaria. J. Invertebr. Pathol. 



39:149-157. 

 Farley, C. A. 



1969. Probable neoplastic disease of the hematopoietic 



system in oysters, Crassostrea virginica and Crassostrea 



gigas. Natl. Cancer Inst. Monogr. 31:541-555. 

 1976a. Proliferative disorders in bivalve mollusks. Mar. 



Fish. Rev. 38(10):30-33. 

 1976b. Ultrastructural observations on epizootic neoplasia 



and lytic virus infection in bivalve mollusks. Prog. Exp. 



Tumor Res. 20:283-294. 

 Harshbarger, J. C, S. V. Otto, and S. C. Chang. 



1977. Proliferative disorders in Crassostrea virginica and 



Mya arenaria from the Chesapeake Bay and intranuclear 



virus-like inclusions in Mya arenaria with germinomas from 



a Maine oil spill site. Haliotis 8:243-248. 



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