Unique combinations of coral host and algal symbiont genotypes reflect intraspecific variation in heat stress responses among colonies of the reef-building coral, Montipora digitata

High temperatures disrupt coral-algal symbioses in multiple ways, with negative impacts on the physiology of the coral host, the algal symbiont, and the combined holobiont. Most heat stress studies on hard corals have understandably focused on species trends based on the combined observation of multiple individuals to account for phenotypic plasticity among colonies. As the average coral continues to decline while sea temperatures rise, the outlier colonies that exhibit neutral or positive responses to heat stress are coming to represent larger proportions of marginal coral populations. These colonies are those most likely to guide the future trajectory of reef ecosystems, but their dynamics are often obscured by aggregate analyses. To directly measure and analyze intraspecific variation in heat stress responses within a natural coral population, we performed aquarium experiments on sixteen colonies of the structurally important branching coral Montipora digitata from Okinawa Island, Japan. We resolved host and symbiont genotypes, exposed replicate coral fragments to ambient or elevated temperature, and monitored stress-driven differences in host calcification, symbiont photochemistry, and colony mortality. Over the 6-month experiment, six colonies appeared to tolerate stress (exhibiting no major physiological changes), seven were sensitive to stress (exhibiting reduced growth), and three expired. Both host and symbiont genotype contributed to this variation. These results demonstrate the degree to which unique M. digitata holobionts may differentially respond to thermal stress in warming oceans and highlight the important role of intraspecific variation in shaping future reef assemblages.