As oceans warm, high-temperature bleaching threatens reef-building corals, but some common reef corals may elevate their thermal tolerance by hosting more temperature-resistant algal symbionts. Because the environmental correlates of temperature-resistant symbionts remain unclear, judging a site’s likelihood of supporting such symbionts is difficult. Here we gather distribution data of symbiont types from 53 studies including 61 coral genera across 144 global localities and analyze these data in a niche-modeling framework.
We show that across the Indo-Pacific Ocean, the distribution of heat-resistant Symbiodinium Clade D is strongly correlated to local environmental variables. In particular, there appears to be a phase transition from more temperature-sensitive Clade C Symbiodinium to more temperature-tolerant Clade D occurring between maximum annual sea surface temperatures of 29 and 31°C. Multiple linear regression models show that high mean sea surface temperatures, high acidity, and low frequencies of high temperature anomalies account for 45% of the variation in proportion of Clade D at all examined sites. Models of future Indo-Pacific climate predict that areas now unsuitable for Clade D will rapidly change to favor Clade D dominance: the front of Clade D suitability is predicted to move across the subtropical South Pacific at rates of 20-49 km per year. Many of these areas currently host low levels of Symbiodinium D, raising the concern that environmental change will sweep over areas faster than dispersal and population growth can add Clade D symbionts to reefs.