AbstractCorals of the genus Montastraea in the Caribbean are highly susceptible to disease, although not all neighboring colonies appear to be affected on the same time scales. Stressed corals are unable to function optimally and are less likely to be reproductively successful than those that are not stressed. Montastraea spp. corals, like all corals from the order Scleractinia, are adapted to the narrow ranges in temperature and irradiance found in tropical waters. These environmental factors play an important role in a coral colony's ability to maintain homeostasis and excursions from their normal ranges for extended periods of time can trigger coral stress responses. Experiments using specimens from two M. faveolata colonies exposed to Vibrio spp. bacteria, at normal and elevated temperatures, provided a means by which presumed physiological differences between these colonies could be identified and tested under acutely stressful conditions. Two strains of Vibrio harveyi and one strain of Vibrio parahaemolyticus were used separately and in combination to induce Caribbean yellow band disease (CYBD)-like lesions. At the conclusion of the challenge experiments (34 days), all specimens except one lacked lesions or other gross morphological changes generally recognized as signs of disease. The tissue sections, however, showed notable and often severe increase in pathological changes in all treated specimens. Pathological changes are defined as any alterations in the function, structure or appearance of tissues from normal that impair the coral. Severity scores corresponding to the frequency with which a pathological change was observed were assigned for 33 criteria (0% = no change). Exposed specimens scored consistently high for certain criteria, i.e., karyolysis of the nuclei in the gastrodermis, changes to nuclei of symbiotic algae (zooxanthellae), and loss of integrity of the epidermis (with average severity scores 3.9, 3.5, and 3.2, respectively). A severity score of 3.2 corresponds to approximately 55% frequency of the observed pathological change per section of tissue, and 3.9 corresponds to roughly 70% frequency, clearly suggesting coral stress. Kruskal-Wallis H test analyses of the histopathological indices failed to find statistically significant differences in responses of the two phenotypes and their overall responses to elevated temperature (27 vs. 32å¡C). Control specimens were generally found to be in better condition than the bacteria-exposed counterparts but the condition of the treated corals appeared to improve after the exposure period, while the controls' condition worsened. Notable morphological differences at the light microscopic level exist between the phenotypes but acute exposure to putative pathogenic bacteria resulted in similar pathological changes to the coral epithelia and mesenteries despite colony dissimilarities. Large numbers of degenerating zooxanthellae were observed associated with lysosomes in the deep gastrodermis of the basal body wall in what is believed to be the first reported case of symbiophagy in Montastraea spp. corals resulting in the digestion of the symbionts in situ. Another novel finding is the presence of clusters of suspect bacteria in the coral skeleton lining the calicodermis and in close association with the endolithic communities. Field-collected samples from CYBD-infected colonies were compared to the experimental specimens to determine whether the Vibrio spp. used as inocula caused similar pathology to that occurring in the natural environment. Histological analysis of these CYBD samples showed that the pathological signs were observably different from those experimentally induced and may result from chronic exposures that cause lesions when additional environmental factors provide conditions that favor the opportunistic microbes.