Abstract: Thermally induced coral bleaching, or the breakdown in the partnership between the coral animal and its intracellular algal symbiont, threatens coral reefs globally. Why is coral-algal symbiosis so unstable? The influential Adaptive Bleaching Hypothesis (ABH) postulated that corals bleach to make room for better adapted algal symbionts. New symbionts taken up from the environment are predicted to be more heat tolerant, thus providing further resistance to bleaching. While the ABH provided a framework for more than a decade of coral biology research, unexplained patterns in coral-symbiont association patterns remain. Here, we propose a hypothesis connecting coral demography, bleaching, and symbiont transmission. It postulates that at least some level of bleaching is necessary to allow transmission of symbionts to the next generation of the coral host. We derived a system of difference equations to model two competing strains of symbionts, one more bleaching-tolerant than the other. We analytically determined the stable state of this system under different, ecologically-relevant parameter regimes.
From this reduced picture of coral biology we found that the winning symbiont is completely determined by the relative survivorship of bleached versus unbleached biomass and by whether coral growth in the system results predominantly from adult extension or from reproduction. Additionally, transitions to a more bleaching-tolerant symbiont may emerge from shifting coral survivorship and reproduction rates rather than the thermal tolerance advantage conferred. This is in opposition to the current paradigm which expects thermally tolerant symbionts to increase in abundance under climate change due to the bleaching resilience they confer to their host. The model also predicts a counter-intuitive positive correlation between bleaching severity and coral recruitment rate, which was supported (although not significantly) by the historical observations from the Great Barrier Reef. Our model predicts the previously hard to explain relationship between life history strategy of coral hosts and their disparity in bleaching tolerance. Overall, we argue that the connection between bleaching, coral reproduction, and symbiont transmission deserves future theoretical and experimental investigation and list some testable predictions. Broadly, these results solidify that changing patterns of association in multispecies partnerships can manifest as a side effect of demographic changes rather than imply adaptive shifts.
