Contrasting Patterns of Coral Bleaching Susceptibility in 2010 Suggest an Adaptive Response to Thermal Stress

TLDR

Coral bleaching severity varies, yet the hierarchy of susceptibility among taxa has remained consistent across regions and events. The study investigates spatial and temporal variation in thermal tolerance among scleractinian corals during the 2010 Southeast Asian bleaching event. Surveys of bleaching and mortality indices were conducted at three sites with differing thermal histories. Despite similar thermal stress, bleaching susceptibility differed markedly, with corals from historically variable, 1998‑bleached sites showing lower mortality, and taxa such as Acropora and Pocillopora being highly susceptible in some locations but tolerant in others, indicating adaptation to thermal stress and supporting the hypothesis that temperature variability confers resistance.

Abstract

Background Coral bleaching events vary in severity, however, to date, the hierarchy of susceptibility to bleaching among coral taxa has been consistent over a broad geographic range and among bleaching episodes. Here we examine the extent of spatial and temporal variation in thermal tolerance among scleractinian coral taxa and between locations during the 2010 thermally induced, large-scale bleaching event in South East Asia. Methodology/Principal Findings Surveys to estimate the bleaching and mortality indices of coral genera were carried out at three locations with contrasting thermal and bleaching histories. Despite the magnitude of thermal stress being similar among locations in 2010, there was a remarkable contrast in the patterns of bleaching susceptibility. Comparisons of bleaching susceptibility within coral taxa and among locations revealed no significant differences between locations with similar thermal histories, but significant differences between locations with contrasting thermal histories (Friedman = 34.97; p<0.001). Bleaching was much less severe at locations that bleached during 1998, that had greater historical temperature variability and lower rates of warming. Remarkably, Acropora and Pocillopora, taxa that are typically highly susceptible, although among the most susceptible in Pulau Weh (Sumatra, Indonesia) where respectively, 94% and 87% of colonies died, were among the least susceptible in Singapore, where only 5% and 12% of colonies died. Conclusions/Significance The pattern of susceptibility among coral genera documented here is unprecedented. A parsimonious explanation for these results is that coral populations that bleached during the last major warming event in 1998 have adapted and/or acclimatised to thermal stress. These data also lend support to the hypothesis that corals in regions subject to more variable temperature regimes are more resistant to thermal stress than those in less variable environments.