Evidence that reef-wide patterns of coral bleaching may be the result of the distribution of bleaching-susceptible clones

The hypothesis that intraspecific variation in coral bleaching is a result of the distribution of bleaching-susceptible clonal genotypes (genets) was addressed using photoquadrats recorded during the 1987 Caribbean bleaching event on a reef dominated by Montastraea annularis (Morphotype I), together with manipulative experiments with Porites porites. Nearest-neighbor analysis showed that bleached colonies (ramets) of M. annularis at 10 m depth had a high probability (0.80) of having a nearest bleached neighbor of the same genet rather than a bleached ramet of a different genet. Furthermore, the frequency distributions of bleached ramets of M. annularis in the photoquadrats was significantly different from a Poisson distribution, suggesting that bleached ramets were aggregated on the reef. Manipulative experiments with P. porites from 15 m depth showed that some genets were more susceptible to thermal bleaching than others, since three genets had significantly different rates of zooxanthellae loss when exposed to elevated temperatures in tanks receiving irradiances similar to those found in situ. These results suggest that the in situ patchy distribution of bleached ramets could correspond to the distribution of certain genets, and that adjacent genets can exhibit sufficiently different phenotypes to account for intraspecific variation in bleaching. Further studies of genet-specific coral bleaching may provide valuable insights into the causes and consequences of bleaching.