Microscopic observations of recovery in the reef-building scleractinian coral,Montastrea annularis,after bleaching on a Cayman reef

Living tissues of a single massive colony of the reef-building scleractinian coral, M. annularis, were sampled at one month intervals following the observation that areas of marked discoloration (bleaching) had developed by the beginning of the summer of 1988. Histological analysis of replicate plugs of tissue from bleached and unbleached regions of that colony allowed comparison of time-matched samples as well as zone-matched samples over a period of six months. During that interval, the entire colony was restored to a uniform and normal degree of pigmentation. Histology of the bleached zones documents the gradual accumulation of zooxanthellae and the thickening of the gastrodermal epithelium of the coral. Once reconstituted, the previously bleached gastrodermis, repopulated with algal symbionts, is indistinguishable from unbleached tissue in the coral colony.