Expulsion of zooxanthellae (Symbiodinium) from several species of scleractinian corals: comparison under non-stress conditions and thermal stress conditions

Corals expel zooxanthellae (Symbiodinium) not only under thermal stress conditions, but also under non-stress conditions. To better understand the significance of this, we compared cell numbers, morphologies and photo-synthetic activities of the expelled Symbiodinium under different temperatures for six species of scleractinian corals; Acropora selago, Acropora muricata, Heliofungia actiniformis, Ctenactis echinata, Oxypora lacera and Pocil-lopora eydouxi, kept in aquariua. During the eight day aquarium experiment, every coral expelled two types of Symbiodinium morphologies, normal and degraded form, and the ratios of these forms differed depending on the temperature. Under a non-stress, initial temperature con-dition of 27°C, mainly degraded cells were expelled. In contrast, when the water temperature was increased and reached to 32°C, the expulsion rates drastically increased, except P. eydouxi, and almost all the expelled populations exhibited normal cell morphology. Under this thermal stress conditions, photosystem II maximum quantum yield (Fv/Fm) of expelled Symbiodinium from two Acro-poridae and two Fungiidae was 35% lower than those retained in the tissue, but was not significantly different in the cases of O. lacera and P. eydouxi. These results in-dicate that, under the non-stress conditions, the corals functionally regulate Symbiodinium density by expelling degraded cells. This function may collapse under high-temperature stress, resulting in the extensive loss of healthy-looking but photosynthetically damaged Symbio-dinium, probably because of host cell detachment. More-over, susceptibilities toward temperature increase might be different in coral species or symbiont types and result in the differences of expulsion amounts or photosynthesis damages.