LOW TEMPERATURE EVOKES RAPID EXOCYTOSIS OF SYMBIOTIC ALGAE BY A SEA ANEMONE

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Abstract
Darkness evokes expulsion of zooxanthellae from the sea anemone Aiptasia pulchella, but brief exposure to low temperature (4.degree.C, 4 h) increases the expulsion rate four-fold. Ninety-eight percent of the zooxanthellae are expelled within four days. Low temperature incubation had no detectable effect on host animal survival or behavior, but the effect on zooxanthellae was profound. Low temperature reduced the rate of photosynthesis, increased the rate of release of fixed carbon, reduced the number of viable cells and decreased the rate of cell division. Electron micrographs revealed that low temperature elicited the appearance of a thickened cell envelope and novel electron-dense inclusions, tentatively identified as crystallized lipoprotein. Immediately after low temperature incubation zooxanthellae move towards the apex of the host cells and are released to the coelenteron by exocytosis. The mechanisms by which low temperature may evoke exocytosis of zooxanthellae (e.g., disassembly of host cell microtubules; increased concentration of cytosolic calcium ions) are discussed.

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