Five-year study on the bleaching of anemonefish-hosting anemones (Cnidaria: Anthozoa: Actiniaria) in subtropical Okinawajima Island

Abstract

Since the 1980s, coral bleaching mainly due to rising seawater temperatures has become a serious problem for subtropical and tropical coral reefs around the world. Coral bleaching and the associated loss of corals have a negative impact on taxonomic richness of creatures. Bleaching is seen not only in hard corals (order Scleractinia), but also in other zooxanthellate animals, including anemonefish-hosting anemones (order Actiniaria), where it is known to significantly impact symbiotic anemonefish as well as other organisms. However, to date, there have been few continuous studies on bleaching and its effects on anemones and anemonefish. Therefore, in this study, we recorded the water temperature and the status of bleaching for individual host anemones and symbiotic anemonefish on the western coast of Okinawajima Island over five years from November 2014 to October 2019. In the summers of 2016 and 2017, when abnormally higher water temperatures were observed during ENSO events, five out of 30 host anemones bleached. All individuals of host anemone that bleached in 2016 again bleached in 2017, and these anemones took longer to recover in 2017 than in 2016. Anemonefish pairs inhabiting the five bleached host anemones survived these bleaching periods; however, one of the bleached anemones along with its associated anemonefish disappeared subsequent to the 2017 bleaching. The present study indicates that if abnormally high seawater temperatures continue in consecutive summers, recovery of host anemones from bleaching may become impaired, and associated mortality events may increase.