The reproductive cycle and shell growth of the Manila clam Ruditapes philippinarum, transplant-ed from Lake Akkeshi to Saroma Lagoon, Hokkaido, northern Japan, were studied from April 1991 to August 1992. Histological examination revealed that the spawning season is from July to Septem-ber, with a peak in August. The shell lengths at first maturity were 25mm (2 years) and 27mm (2 or 3 years) for males and females, respectively. Disturbance rings on the shells formed upon trans-plantation were used to estimate growth, which mainly occurred in May and June. Growth is described by the von Bertalanffy growth equation for both sexes Lt=49.71 (1-exp (-0.416 t)), where Lt is the shell length (mm) at age t (year). The clams in the lagoon had a shorter spawning and growing season, were larger and older at first maturity, and had a more moderate shell length growth rate than clams from southern Japan. Finally, some reproductive and growth-related responses to various habitats were discussed.
