Discharge pattern of symbiotic zooxanthellae from corals

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Corals harbor zooxanthellae (dinoflagellate genus Symbiodinium) as symbionts. Discharge of zooxanthellae from corals is closely linked to coral bleaching. Although discharge can take place under non-stress conditions, little is known about the timing of discharge or the quantity of cells discharged. We performed field and laboratory experiments to clarify the pattern of zooxanthellae discharge from corals. The results revealed that (1) the number of cells discharged varied among coral colonies; (2) the corals tended to release more zooxanthellae during the daytime than during the night; (3) more cells were discharged when the water temperature was high; and (4) high water temperatures also inhibited cell division of zooxanthellae within coral tissues. Thus, it is plausible that an increased number of cells discharged from corals and fewer cell divisions within corals are involved in coral bleaching under high water temperature conditions.