Breeding season of the sunray surf clam Mactra chinensis in Tomakomai, southwest Hokkaido [Japan]

Abstract

The annual reproductive cycle, breeding season and shell length at first sexual maturity of the sunray surf clam, Mactra chinensis, in Tomakomai, were examined histologically. Devel-opment of the germ cells was divided into five stages; in the female, they were oogonium, yolkless stage, early yolk-formation, late yolk-formation, and mature stage; in the male, they were spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid, and sper-matozoon. On the basis of cytological characteristics and the relative amount of germ cells, maturation of the gonad was classified into recovery, early growing, late growing, maturing, spawning, and spent stages. The early growing stage appeared in March, while the late grow-ing stage was found in April in the male and in May in the female. The maturing stage was observed from May to June in the male and in June in the female, while the spawning stage occurred from July to September. The spent and recovery stages appeared from Octo-ber to November and from December to February, respectively. The breeding season of the clam was estimated to be from July to September. Shell length at first sexual maturity was 25-30mm.