EVIDENCE THAT REEF-WIDE PATTERNS OF CORAL BLEACHING MAY BE THE RESULTOF THE DISTRIBUTION OF BLEACHING SUSCEPTIBLE CLONES

The hypothesis that intraspecific variation in coral bleaching is a re sult of the distribution of bleaching-susceptible clonal genotypes (ge nets) was addressed using photoquadrats recorded during the 1987 Carib bean bleaching event on a reef dominated by Montastraea annularis (Mor photype I), together with manipulative experiments with Porites porite s. 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 ram et of a different genet. Furthermore, the frequency distributions of b leached 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 ther mal bleaching than others, since three genets had significantly differ ent rates of zooxanthellae loss when exposed to elevated temperatures in tanks receiving irradiances similar to those found in situ. These r esults suggest that the in situ patchy distribution of bleached ramets could correspond to the distribution of certain genets, and that adja cent genets can exhibit sufficiently different phenotypes to account f or intraspecific variation in bleaching. Further studies of genet-spec ific coral bleaching may provide valuable insights into the causes and consequences of bleaching.