Changes in zooxanthellae characteristics in the coral Stylophora pistillata during the 1998 bleaching event

This study investigated zooxanthellae densities, chlorophyll a and c2 concentrations, and zooxanthellae morphologies in replicate samples of Stylophora pistillata colonies before and during the 1998 coral-bleaching event in Okinawa, Japan. During the bleaching event sea surface temperatures reached 31 °C and Stylophora pistillata colonies supported significantly reduced mean chlorophyll a and c2 concentrations per cm2, and significantly more pale zooxanthellae cells in hospite than before the bleaching. Although the number of healthy zooxanthellae decreased during the coral-bleaching event, the total density of zooxanthellae in S. pistillata colonies (i. e., including the pale zooxanthellae) did not change significantly. Thus, the presence of pale zooxanthellae within colonies numerically compensated for the loss of healthy zooxanthellae. We suggest that pigment degradation in zooxanthellae and subsequent cell shrinkage, rather than zooxanthellae expulsion, was the mechanism involved in S. pistillata bleaching.