Field and laboratory comparisons of mortality in normal and neoplastic Mya arenaria

The results of a 6-month mark and recapture experiment involving approximately 900 adult Mya arenaria demonstrated that under natural conditions, significantly higher (P much less than .001, chi 2 test) mortality occurred among animals with neoplasia than those diagnosed as normal. Using a blood screening technique, the clams were diagnosed and placed in one of three diagnostic groups based on the severity of the disease (the percentage neoplastic cells per total number of blood cells): Nonneoplastic (NN), 0%; low severity neoplastic (LSN), less than 50%; and high severity neoplastic (HSN), greater than 50%. Fifty-one percent of those clams initially diagnosed as HSN died by the end of the test period as compared to 8% of the LSN clams and only 3% of the normals. Both progression and remission of the disease were also evident. Approximately 10% of the clams in the NN and LSN groups progressed to a LSN or HSN condition, whereas 16% of those clams initially identified as LSN, and that were recovered alive, underwent complete remission during the test period. Comparison of the field results with those of an 18-week laboratory study suggests that studies of mortality done under laboratory conditions may not provide useful data for the interpretation of the quantitative effects of a disease process, such as molluscan neoplasia, on the natural population of the animal studied.