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

During the 4th global coral bleaching event (2023-2025), over 80% of the worlds coral reefs experienced bleaching-level heat stress above 4 {degrees}C-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 accumulated to record levels by June 2024 (DHW=9.6 {degrees}C-weeks), 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 1850 tagged colonies from September 2023 to October 2024 using 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 Acropora species (five species, range 46.3-95.4%), Echinopora cf. horrida (44.7%), and Montipora cf. aequituberculata (27.8%). Among susceptible species was Heliopora coerulea, a species commonly considered heat tolerant. Our findings challenge previous regional studies that concluded a reversal of bleaching hierarchies of species over time, whereby historically ascribed heat tolerant species (i.e., slow-growing massive species) became susceptible and historically susceptible fast-growing species became heat tolerant. Importantly, bleaching severity did not decrease with depth. These data represent the first regional accounts of species-specific coral bleaching mortality in Malaysia, highlighting distinctive ecological bleaching and recovery trajectories of species.