Mechanisms driving symbiont shuffling in corals


The success of scleractinian corals in oligotrophic waters of the tropics is the result of an endosymbiotic association with unicellular photoautotrophs called zooxanthellae (the symbiont). By providing photosynthetic products, the symbionts satisfy most of the corals’ metabolic needs. Increasing sea temperature causes the breakdown of this association, the expulsion of the symbionts and, in severe cases, the death of corals. Observational evidence suggests that the shift in the relative abundance of resident symbiont populations within the coral host (symbiont shuffling) enables corals to transcend their thermal tolerance limits because some symbionts are more thermally tolerant than others. The mechanisms of symbiont shuffling, however, are poorly understood. I will present here a new trait-based, acclimation dynamics model with which we show that classic competition theory can explain symbiont shuffling when the competitive abilities of different symbionts are driven by their thermal tolerances. We also show that rapid symbiont shuffling can occur (1) with the presence of a positive feedback, according to which some of the symbiotic benefits received by corals are re-allocated to symbiont growth, thus yielding even higher benefits, or (2) by keeping a background population size of non-dominant symbionts, which enables them to become abundant once conditions become favourable. Our results narrow down the mechanisms that could help reconcile the different patterns observed in symbiont population dynamics and provide new model theories that can be tested with laboratory experiments