Reproductive cycle of the Manila clam Ruditapes philippinarum in Japan

Histological observations of the adult gonad and monitoring condition factor of the Manila clam Ruditapes philippinarum were performed using samples collected from northern (Hokkaido), central (Kanagawa, Aichi, Mie) and southern (Kumamoto) areas in Japan from April 2010 to November 2012. Many clams could not be defined definitively by one reproductive stage. When more than one developmental stage was evident within a single individual, the clam was assigned to all the reproductive stages that were observed in the tissue section. The reproductive stages were expressed by frequency of the stages that was noted for each sample group. Histological examination revealed that the spawning period was in summer at two sites in Hokkaido, whereas spawning began in spring and continued throughout summer until autumn, with two peaks in spring and autumn at the other four sites. While maximum and minimum condition factors in Hokkaido were attained in August 2010 and October 2010, respectively, those in the other sites were achieved in April 2011 and December 2011, respectively.