Sexual reproduction of Acropora pruinosa at its northern distributional limit

This study examined the possibility of sexual reproduction of Acropora pruinosa at Uchiura Bay, Shizuoka. Observations of spawning, early development, settlement, and post-settlement survival were conducted as follows. Mature A. pruinosa were transferred to flow-through tanks just before the expected spawning period in August 2021. Spawning was observed from 4–15 August, and egg–sperm bundles were collected and fertilized. Early development to the planula larval stage was observed every 12 hours. Numbers of settled individuals on two types of settlement plates, made of Amakusa pottery stone and of flexible board, were counted. Survival rates of settled individuals were also observed for four months in the sea. More than 50% of fertilized eggs developed into planula larvae after 61 hours. In total, 6,378 planulae were acquired, 41% of which settled on settlement plates. Settlement did not differ between the two types of plates. After transferring the plates to the sea, the number of polyps decreased drastically for the first 14 days. After 112 days, survival rates were 21.3% and 7.71% on Amakusa pottery stones and flexible boards, respectively. These results demonstrate the possibility of artificial fertilization and of seeding larvae of A. pruinosa in the field of its northern distributional limit. However, survival rates of acquired individuals as planulae were very low; therefore, further study is required to enhance survival rates by improving the settlement plates and/or by adding the symbiotic algae to planulae.