Effects of gonadal neoplasms on oogenesis in softshell clams, Mya arenaria.

The average prevalence of gonadal neoplasms in softshell clams, Mya arenaria, from Whiting Bay, Washington County, Maine, was 19.4% in 1994. Monthly prevalences ranged from 10 to 26.7%. Neoplasms ranged in intensity from few, small foci of undifferentiated germ cells (Stage 1), to 50-100% of gonadal follicles being involved (Stage 2), to invasion and metastasis with loss of tissue architecture (Stage 3), indicating that the disease is progressive and lethal. There was no relationship (P > 0.05) between prevalence of neoplasms and clam size between 45.7 and 60.7 mm mean shell length. Clams of both sexes were affected, but females were significantly more likely (P </= 0.025) to have neoplasms than males. Female clams with neoplasms produced significantly fewer (P </= 0.001) gametes than healthy clams. Overall reduction of gamete number was 66%, resulting from direct displacement of gametes by tumor cells in affected gonadal follicles. Compared to healthy clams, clams with neoplasms exhibited a significantly lower (P </= 0.001) mean oocyte diameter before spawning and a significantly greater (P </= 0.001) mean oocyte diameter after spawning, as the result of a general (throughout the entire gonad) inhibition of normal oogenesis and spawning. It is concluded that gonadal neoplasms have a negative impact on the reproductive output of the Whiting Bay clam population.