REPRODUCTIVE-CYCLE AND SHELL GROWTH OF TR ANSPLANTED MANILA CLAM RUDITAPES-PHILIPPINARUM IN SAROMA LAGOON

The reproductive cycle and shell growth of the Manila clam Ruditapes p hilippinarum, transplanted from Lake Akkeshi to Saroma Lagoon, Hokkaid o, northern Japan, were studied from April 1991 to August 1992. Histol ogical examination revealed that the spawning season is from July to S eptember, with a peak in August. The shell lengths at first maturity w ere 25 mm (2 years) and 27 mm (2 or 3 years) for males and females, re spectively. Disturbance rings on the shells formed upon transplantatio n were used to estimate growth, which mainly occurred in May and June. Growth is described by the von Bertalanffy growth equation for both s exes L(t) = 49.71 (1-exp (-0.416 t)), where L(t) is the shell length ( mm) at age t (year). The clams in the lagoon had a shorter spawning an d growing season, were larger and older at first maturity, and had a m ore moderate shell length growth rate than clams from southern Japan. Finally, some reproductive and growth-related responses to various hab itats were discussed.