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Ophiopholis mirabilis is a common species with a high population density on the western coasts of the northern Pacific Ocean.The number of O.mirabilis has been increasing recently in the scallop aquaculture zone(the Zhangzi Island area,northern Yellow Sea)in China.To explore the mechanism of its population variation,the reproductive cycle of O.mirabilis was investigated in this area(39°04'N;122°51'E)from February 2017 through January 2018 and determined by the monthly gonad index(GI),histological examinations of the gonads and the oocyte size-frequency distribution.O.mirabilis had a clear annual reproductive cycle that was synchronous between males and females.Sea temperature and food availability played important roles in O.mirabilis reproduction.The GI value was less reliable for determining reproductive activity in O.mirabilis because the nutritive tissues within the gonads may be utilized to synthesize gametes,leading to a decrease in GI during maturation.The histological results also show that abundant nutritive phagocytes were present in the gonads of O.mirabilis,which,together with the germ cells,affected the weight of the gonads.In addition,the mature oocytes of O.mirabilis were relatively small(75-150 pm),indicating that the larval development was planktotrophic.This study provided insights into the reproductive patterns and biology of O.mirabilis and is an essential basis for the quantity control of this species in aquaculture areas.