Adaptive bleaching: A general phenomenon

Laboratory and field data bearing on the adaptive bleaching hypothesis (ABH) are largely consistent with it; no data of which we are aware refute it. We generalize the ABH in light of these data and observations. The population of zooxanthellae within an organism is dynamic, the diversity of zooxanthellae is both surprising and difficult to ascertain, and field experiments demonstrate both turn-over in zooxanthella types and habitat-holobiont correlations. Dynamic change in symbiont communities, and the idea of an equilibrium or optimal community that matches the environment at a particular place and time, are concepts that underlie or emerge from much of the recent literature. The mechanism we proposed to explain responses to acute bleaching appears to operate continuously, thereby enabling the host-symbiont holobiont to track even subtle environmental changes and respond promptly to them. These findings enhance the potential importance of the ABH in the outcomes of acute bleaching, which can (1) accelerate this process of holobiont change, and (2) change the set of possible trajectories for how symbiont communities might recover.