Changes in the Ultrastructures of the Coral Pocillopora damicornis after Exposure to High Temperature, Ultraviolet and Far-Red Radiation

Ultrastructural changes in the coral tissues and symbiotic zooxanthellae of the scleractinian coral, Pocillopora damicornis, after exposure to high temperature (HT), ultraviolet (UV) and far-red (FR) radiation were examined by transmission electron microscopy. In normal environmental conditions, the coral gastrodermis contained zero to two zooxanthellae and maintained well-defined cortical and intracellular structures. When corals were exposed to HT, UV and FR, the gastrodermal features changed remarkably depending on the duration of exposure. Treatment with HT for 15 min or exposure to UV and FR for 6 h induced the appearance of numerous, small, electron-dense granules in the gastrodermis. Autophagosomes and lysosomes in the gastrodermal cells also increased in number. The gastrodermal cells containing no zooxanthellae began to degrade and became vacuolated through the destruction and fragmentation of their cytoplasmic contents. Prolonged exposure to these stressors brought about the disintegration of the cells without zooxanthellae and led to the collapse of the gastrodermis. This resulted in the release of the other gastrodermal cells containing zooxanthellae from the gastrodermis to gastrovascular cavities. HT, UV and FR affected the gastrodermal cells without zooxanthellae more severely than the zooxanthellae themselves, although the thylakoids of the zooxanthellae were often destroyed when the coral was exposed to UV and FR. These results suggest that the coral receives the distinct stressors, HT, UV and FR, but has a common mechanism that promotes autolysis activities in the gastrodermis and discharges cells containing zooxanthellae.