Gross and microscopic lesions in corals from Micronesia

Abstract

We documented gross and microscopic morphology of lesions in corals on seven islands spanning western, southern, and eastern Micronesia. We sampled 76 colonies comprising 30 species of corals among 18 genera with Acropora, Porites, and Montipora dominating. Tissue loss comprised the majority (41%) of gross lesions sampled, followed by discoloration (30%), and growth anomaly (29%). Of 31 cases of tissue loss, most lesions were subacute (48%), followed by acute, and chronic (26% each). Of 23 samples with discoloration, most (40%) were dark discoloration, with bleaching and other discoloration each comprising 30%. Of 22 growth anomalies, umbonate growth anomalies comprised half with exophytic, nodular, and rugose growth anomalies comprising the remainder. On histopathology, for nine cases of dark discoloration, fungal infections predominated (77%), for seven bleached corals, depletion of zooxanthellae from the gastrodermis made up a majority of microscopic diagnoses (57%), and for growth anomalies other than umbonate, hyperplasia of the basal body wall was the most common microscopic finding (63%). For the remainder of the gross lesions, no single microscopic finding comprised over 50% of the total. Host response varied with the agent present on histology. Fragmentation of tissues was most often associated with algae (60%) whereas necrosis dominated (53%) for fungi. Two newly documented potentially symbiotic tissue-associated metazoans were seen in Porites and Montipora. Findings of multiple potential etiologies for a given gross lesion highlight the importance of incorporating histopathology in coral disease surveys. This study also expands the range of corals infected with cell associated microbial aggregates.