Dramatic decline of Seriatopora corals in shallow reefs following the 2024 bleaching event in Okinawa Island, Japan

Mass coral bleaching driven by global warming has caused severe declines in shallow reef communities worldwide. Following the 1998 bleaching event, shallow population of the genus Seriatopora were presumed locally extinct around Sesoko Island, Okinawa, Japan. Recently, small shallow populations were rediscovered in northern Okinawa Island; however, the severe thermal anomaly in summer 2024 again caused widespread bleaching, particularly in shallow habitats, raising concerns about the long-term persistence of the genus. In this study, we assessed changes in Seriatopora cover during and after the 2024 bleaching event to evaluate its short-term impact and potential for recovery. Photoquadrat surveys were conducted before and after the bleaching events at two sites on Okinawa Island, "Sate" and "Ryugu," where a shallow population occurred across depths of 5-10 m. Overall, Seriatopora cover declined markedly following bleaching, with near-complete loss at ≤ 5m depth both sites. At deeper depth (≥ 10 m), several healthy colonies were discovered at Sate, whereas only a few surviving colonies were observed at Ryugu. These results indicate that shallow Seriatopora populations are highly vulnerable to acute thermal stress, while deeper colonies may function as refugia following bleaching events. Understanding the connectivity and recovery dynamics of these populations will be crucial for evaluating the feasibility of conservation and restoration strategies for shallow Seriatopora populations under ongoing ocean warming.