Marine heatwaves reshape survival of small corals, revealing size- and taxa-specific vulnerabilities

Abstract Despite decades of research, fundamental gaps remain in our understanding of coral vital rates across taxa. This is particularly true for the early life stages, which are critical for reef recovery, representing significant bottlenecks to population growth. We assessed the survival of 3334 small corals (0.3–10 cm maximum diameter) across four eastern Indian Ocean reefs through contrasting levels of heat stress. Under background conditions at multiple reefs, survival of small corals ranged between 0.43 and 0.92 yr −1 and increased with colony size. Following a major heat stress event (13 Degree Heating Weeks (°C-weeks)), which caused moderate bleaching at Ashmore Reef, survival was comparable to background levels (0.67 yr −1 HDI: 0.57–0.77). In contrast, major heat stress at Mermaid Reef (11 °C-weeks) in the same period caused severe bleaching and substantially reduced survival to 0.21 (6 mo ) −1 (HDI: 0.10–0.32). This was associated with a collapse of size-dependent survival for structurally complex, bleaching-susceptible taxa ( Acropora, Pocillopora ). Stress-tolerant taxa ( Goniastrea, Porites ) experienced smaller reductions in survival and retained size-specific survival patterns. Spanning multiple reefs under varying levels of heat stress, this study provides taxa- and size-specific insights for improving predictions of coral population trajectories and informing reef management and restoration strategies under future climate scenarios.