Despite global ecological relevance, surprisingly little is known about the cellular and molecular mechanisms of bleaching. This disruption in symbiosis has been described in five main ways – exocytosis (expulsion) of symbionts, in situ degradation of symbionts, host cell detachment, host cell apoptosis, and necrosis (Gates, Baghdasarian, & Muscatine, 1992; Oakley & Davy, 2018; V. M. Weis, 2008); however, the contributions of each during natural bleaching events are still unclear and they may act in combination. Expulsion of viable or dead symbionts alone or in an intact host cell occurs continuously and in many cnidarians, the rate of expulsion is usually similar to the growth rate of the algal intracellular population, suggesting exocytosis is the primary mechanism of symbiont turnover and regulation of symbiont density in coral tissues in environmentally stable conditions (reviewed in (Davy, Allemand, & Weis, 2012; Oakley & Davy, 2018). However, it is unclear to what extent exocytosis contributes to thermal bleaching. Viable but photosynthetically impaired symbionts have been observed expelled from corals under acute thermal stress (Fujise, Yamashita, Suzuki, & Koike, 2013) but another experiment showed that under increasing temperature, the percentage of expelled viable symbiont cells decreases in favor of apoptotic or necrotic cells in multiple corals (Strychar, Coates, Sammarco, & Piva, 2004).