“…The dinoflagellate genus Symbiodinium contains enormous genetic and functional diversity 71 , and communities associated with corals vary among species, environments and host PERSPECTIVE NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE3374 microhabitats 72 . The short generation time of Symbiodinium means that its rate of mutation is much faster than for the coral host 18 , and this, combined with its large within-host population sizes, potentially facilitates rapid responses to altered thermal environments, either through selection of existing genetic variants or through the evolution of novel adaptations 73,74 . Alternatively, the composition of host-associated Symbiodinium communities may vary temporally in response to environmental conditions or at different host lifehistory stages 75 , either through shuffling of existing symbionts 76 or through acquisition of new Symbiodinium types from the environment (that is, switching) 16 .…”