Distributions of stress-resistant coral symbionts match environmental patterns at local but not regional scales

Distribution patterns of stress-tolerant coral symbionts suggest that maximum habitat temperatures can drive local scale adaptation of symbiont populations, but at regional scales other processes can dominate. We assayed clade membership for symbionts of 2 closely related corals from American Samoa, Fiji, the Philippines and Palmyra Atoll. Temperature stress-tolerant Clade D symbionts occur more frequently in American Samoa (83%) than in Palmyra, Fiji or the Philippines (<1%). In American Samoa, Clade D symbionts dominate habitats with higher maximum temperatures, while Clades C and D are both common under lower maximum temperatures. While corals in American Samoa show more stress-tolerant symbionts, this region does not exhibit higher sea surface temperatures, a greater record of heating anomalies or more bleaching than the other 3 regions. That these local patterns do not hold regionally suggests the importance of other factors, including host responses, other environmental correlates, within-clade physiological diversity and dispersal limitation, in driving the distribution of coral symbionts.