Release of symbiotic algae by tropical sea anemones and corals after cold shock

When the tropical sea anemones Aiptasia pulchella and A. pallida are exposed briefly to subnormal temperatures (i.e. cold shocked), and then rewarmed to ambient temperatures, they release substantial numbers of zooxanthellae. Release consists of dissociation of zooxanthellae from the endoderm as a result of cold shock, and expulsion of zooxanthellae from the coelenteron after rewarming. Release varies with cold shock temperature and duration. A 4 h cold shock at 4 °C evokes release of 40 to 55 % of the zooxanthellae after 12 h, and in some cases up to 87 % in 72 h. As shock temperature increases to about 16 °C, fewer zooxanthellae are released. Above about 16 °C, and up to 28 °C, the response is similar to that of unshocked controls. Release is greater at 4 and 10 °C if the duration of exposure is extended up to about 7 to 8 h. Longer cold shock duration elicits no additional release. Release varies with rewarming temperature. Rewarming to at least 17.5 °C is essential for evoking maximum expulsion of zooxanthellae. Eleven species of scleractinian corals from the Seychelles Is., St. Croix, and Hawaii also released zooxanthellae after a 4 h cold shock between 12 and 18 °C. Release at 12 °C ranged from about 20 to 75 % after 12 h, depending on species, geographic location, and duration of cold shock. The ecological implications of cold shock on release of zooxanthellae are discussed in terms of upwelling and seasonal low temperatures. The mechanism by which low temperature evokes release is unknown, but could involve thermotropic effects on host cell membranes.