AGE AND GROWTH OF THE ARK SHELL SCAPHARCA BROUGHTONII (BIVALVIA, ARCIDAE) IN JAPANESE WATERS

Age and growth of the ark shell Scapharca broughtonii in 3 bays in Japan (Mutsu, where ark shells are cultured, and Nanao and Sendai, where they occur naturally) were examined using a shell sectioning and staining method. When a water-based marking pen was used to stain the opaque zones only in shell cross-sections, unstained translucent zones (i.e., growth rings) were clearly observed. Based on seasonal changes in marginal structures of shell cross-sections and marginal growth indices (MGIs), growth rings were estimated to be formed annually in summer when shell growth ceased. The estimated maximum ages for ark shells from Nanao and Sendai bays were 6 y and 20 y, respectively, whereas for Mutsu Bay the maximum age was not calculated because we had only young, cultured specimens. von Bertalanffy growth models were estimated as L-t = 80.1(1 - e(-0.365(t + 0.160)) for Mutsu Bay, L-t = 115.3(1 - e(-0.478(t -) (0.162)) for Nanao Bay, and L-t = 84.8(1 - e(-0.473(t + 0.222)) for Sendai Bay. There was no significant difference between von Bertalanffy growth models fitted for the northern and southern areas of Sendai Bay. Comparing among the 3 bays, faster shell growth may occur in the locality where water temperature is warmer.