Expulsion of zooxanthellae by symbiotic cnidarians from the Red Sea

The expulsion of zooxanthellae by octocorals (Xenia macrospiculata and Heteroxenia fuscescens), the scleractinian coral Stylophora pistillata, and the hydrocoral Millepora dichotoma, was measured in the field. The numbers expelled did not exceed 0.1% of the total standing stock of symbiotic algae per day, the rate of expulsion was less than 4% of the rate at which cells are added to symbiotic populations, and the carbon lost represented 0.01% of the total carbon fixed on a daily basis. Expulsion of zooxanthellae is therefore not a significant sink for fixed carbon in these symbiotic associations. In contrast to field populations, expulsion by X. macrospiculata increased 5-fold or more in the laboratory, suggesting that laboratory conditions may introduce stress.