Abstract
Aim We investigate how susceptibility to mass bleaching varies between closely related coral species, and the validity of applying these susceptibility data (at the appropriate taxonomic level) across geographical regions. We also conduct a preliminary analysis of factors likely to affect geographical variability and examine which data are required for adequate assessment of the risk to species and regional populations from repeated mass bleaching events. Location Twenty‐two sites in the central Great Barrier Reef, Australia (GBR) and 30 sites in the central Maldives Archipelago (MA). Time period 2002 for the GBR and 2016 for the MA. Major taxa studied Corals (order Scleractinia). Methods After marine heat‐wave conditions, timed in situ surveys recorded bleaching responses of 7,368 coral colonies exposed to similar levels of temperature stress at a similar depth of occurrence and with similar subsequent mortality. In each region, a separate phylogenetic mixed model was used to estimate deep‐time phylogenetic‐, contemporary‐ (indicative of recent adaptation/acclimatization) and local‐scale effects. The results were compared between regions in relative terms as a hierarchy of species susceptibilities. Results Susceptibility varied widely between regions: 27 of 106 species analysed were in the same quartile for relative susceptibility in both regions. Closely related species varied widely in their individual susceptibilities. Phylogenetic effects were moderate in both regions, but greater contemporary effects in the MA suggested recent evolution and acclimatization, consistent with a stronger history of recent bleaching events. Main conclusions The high regional and intrageneric variation in susceptibility to bleaching described here suggests that there might be important differences in the extent to which these Indian and Pacific Ocean coral populations are exhibiting responses to deep‐time evolutionary changes, on the one hand, versus recent adaptation, on the other. We reveal a concerning lack of regional‐scale data identifying species most at risk from this, the greatest current threat to coral reefs.