Demographic census confirms a stable population of the critically-endangered caryophyllid coral Polycyathus chaishanensis (Scleractinia; Caryophyllidae) in the Datan Algal Reef, Taiwan

Polycyathus chaishanensis is a symbiotic caryophyllid coral described from a single population in a tidal pool off Chaishan, Kaohsiung, Taiwan. Due to its rarity, P. chaishanensis was declared a critically-endangered species under the Taiwan Wildlife Protection Act. In May 2017, a P. chaishanensis colony was discovered in the intertidal area of the Datan Algal Reef, Taoyuan, Taiwan. To determine whether this is a stable population in the algal reef, a demographic census—including data on occurrence, distribution, and colony size—was carried out in the algal reef in southern Taoyuan. Intertidal censuses and sediment collections were conducted at five different sections—Baiyu, Datan G1, Datan G2, Yongxing, and Yongan algal reefs—during the monthly spring low tide from July 2018 to January 2019. In total, 84 colonies—23 in Datan G1 and 61 in Datan G2—were recorded from a tidal range of − 160 to − 250 cm, according to the Taiwan Vertical Datum 2001 compiled by the Central Weather Bureau. No P. chaishanensis was found in Baiyu, Yongxing, or Yongan. The P. chaishanensis colony sizes ranged from 2.55 to 81.5 cm in diameter, with the larger P. chaishanensis present in the lower intertidal zone. Sediment was extremely high, with monthly site averages ranging from 3,818.26 to 29,166.88 mg cm(−2) day(−1), and there was a significant difference between sites and months, both of which affected the distribution of P. chaishanensis in the algal reef. Our study confirms the existence of a second population of P. chaishanensis in Taiwan, highlighting the importance of the Datan Algal Reef for the survival and protection of this critically-endangered caryophyllid coral and why it is so urgent that the reef should be conserved.