Species-specific bleaching trajectories during the 4th global coral bleaching event in northeastern Peninsular Malaysia

Abstract During the 4th global coral bleaching event (2023–2025), over 80% of the world's coral reefs experienced bleaching-level heat stress above 4 °C-weeks (degree heating weeks, DHW). Inevitably, the large spatial extent of this event substantially impacted coral reefs worldwide, and data are needed to quantify bleaching trajectories and coral mortality. In northeastern Peninsular Malaysia, heat stress peaked at 9.6 °C-weeks (DHW) in June 2024, resulting in severe mass coral bleaching. Here, we quantified species-specific bleaching and mortality trajectories for 12 abundant reef-building coral species of the genera Acropora , Diploastrea , Echinopora , Heliopora , Montipora , and Porites , by tracking 1,850 tagged colonies across surveys conducted during peak heat stress in June 2024 and four months later in October 2024. We used a novel Colony Bleaching Response Index (CBRI) that considers scales of discoloration and colony surface extent. In June 2024, 92.9% of all surveyed corals were bleached, resulting in 40.6% mortality by October 2024. However, bleaching trajectories varied across species, whereby the least bleached species in June were not the first to recover nor the least affected by mortality, suggesting inherent species-specific bleaching trajectories and recovery pathways. Mortality of Porites cf. lobata and Diploastrea heliopora was ≤ 1%, whereas severe mortality was recorded for five Acropora species (range 46.3–95.4%), for Echinopora cf. horrida (44.7%), and for Montipora cf. aequituberculata (27.8%). Heliopora coerulea , a species commonly considered heat tolerant, was among the most bleaching-susceptible species (i.e., 86% of colonies fully bleached). In general, our findings do not support previous regional studies that reported a reversal of bleaching hierarchies, whereby historically ascribed heat tolerant species (i.e. slow-growing massive species) became susceptible, while historically susceptible fast-growing species became heat tolerant. Importantly, bleaching severity did not decrease with depth. These data represent the first regional account of species-specific coral bleaching mortality in Malaysia, highlighting distinctive ecological bleaching and recovery trajectories of coral taxa.