Bimodal distribution of coral bleaching prevalence is consistent with state transition dynamics in thermal stress response: five years of standardised monitoring in Japan

Coral bleaching is conventionally modelled as a continuous response to cumulative thermal stress, yet the distributional structure of site-level bleaching prevalence has rarely been examined. Here we analyse five years (fiscal years 2020-2024) of standardised bleaching surveys from Japans Monitoring Site 1000 program, encompassing 2,288 observations across 585 survey points at 26 sites in the Ryukyu Archipelago and adjacent waters. We document three principal findings. First, bleaching prevalence is bimodally distributed in all five years: the intermediate range (20-80%) remains stable at 21-27% of observations, while inter-annual variation is driven by redistribution between low and high domains -- confirmed by Hartigans dip test (all p 0%, p = 0.007), indicating that metric structure and the ecological severity threshold defining the outcome are inseparable design considerations.