Bull. Jpn. Soc. Fish. Oceanogr. 79(3), Page 130-140, 2015   Condition for mass spawning of scleractinian coral Acropora in the Sekisei Lagoon, Ryukyu Islands Shuichi Fujiwara1†, Daisuke Kezuka1, Hidenobu Ishimizu1, Shigeo Tabata1 and Satoshi Nojima2 1 IDEA Consultants, Inc., 2-6-19 Aja, Naha, Okinawa 900-0003, Japan 2 1952-3 Tomioka, Reihoku-machi, Kumamoto 863-2507, Japan † E-mail: fjw20240@ideacon.co.jp To determine the accurate period for mass spawning of Acropora coral in the Sekisei Lagoon, Ryukyu Islands, the relationship between observations of spawning and slicks and lunar phase, as well as accumulated water temperature and solar radiation, was studied. Spawning was most often observed 3 days before to 2 days after the full moon in May. Most spawning occurred between 1 and 4 h after sunset and between −1.0 and 4.5 h of the high tide. All spawning occurred at a water temperature of 26℃ in May, 2 to 3 months before the peak. The conditions of mass spawning in the Sekisei Lagoon was estimated to occur at a water temperature and solar radiation of >1.2×103℃ and 0.6×103 MJ∙m−2, respectively. The most probable period of spawning is around and on the full moon in May. Key words: Acropora, mass spawning, lunar phase, accumulated water temperature, accumulated solar radiation