Growth tradeoffs associated with thermotolerant symbionts in the coral Pocillopora damicornis are lost in warmer oceans

The growth and survival of reef corals are influenced by their symbiotic algal partners (Symbiodinium spp.), which may be flexible in space and time. Tradeoffs among partnerships exist such that corals with thermotol- erant symbionts (e.g., clade D) resist bleaching but grow more slowly, making the long-term ecosystem-level impacts of different host-symbiont associations uncertain. However, much of this uncertainty is due to limited data regarding these tradeoffs and particularly how they are mediated by the environment. To address this knowledge gap, we measured growth and survival of Pocillopora damicornis with thermally sensitive (clade C) or tolerant (clade D) symbionts at three temperatures over 18-55 weeks. Warming reduced coral growth overall, but altered the tradeoffs associated with symbiont type. While clade D corals grew 35-40 % slower than clade C corals at cooler temperatures (26 C), warming of 1.5-3 C reduced and eliminated this growth disadvantage. These results suggest that although warmer oceans will negatively impact corals, clade D may enhance survival at no cost to growth relative to clade C. Understanding these genotype- environment interactions can help improve modeling efforts and conservation strategies for reefs under global climate change.