For many shallow water reef-building Scleractinia, the association between the coral and its resident dinoflagellates is relatively unstable, collapsing after prolonged exposure to high seawater temperatures. The result of this collapse is departure of the dinoflagellate from the coral host, but at this time it is not clear which partner initiates the separation or what the trigger is. It is clear that thermal anomalies can cause bleaching and that this effect is exacerbated by high UV exposure. Whatever the trigger, the net result is “bleaching” of the coral colony; corals which have lost their dinoflagellate symbionts often have no pigmentation, and as the skeleton can usually be clearly seen, they appear as white ghosts of their former selves (Fig. 9.1). The bleached state is metastable—the coral can quickly recover if a compatible Symbiodinium strain can either be taken up from an external source or can grow back from a low background—but, if the coral fails to reestablish the photosymbiosis within a relatively short time, death follows. It has been suggested that coral bleaching might have adaptive value, in that it might enable “switching” or “shuffling” to a strain of Symbiodinium whose physiology might be more compatible to the host under changed environmental conditions. A major concern at present, however, is that as seawater temperatures increase, the threshold for bleaching will be reached more often, so frequent mass coral mortality will occur, perhaps resulting in complete demise of coral reefs.
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