Co-benefits not trade-offs associated with heat tolerance in a reef building coral

Abstract

Abstract As marine species adapt to climate change, their heat tolerance will likely be under strong selection. Yet trade-offs between heat tolerance and other life history traits could compromise natural adaptation or restorative assisted evolution. This is particularly important for ecosystem engineers, such as reef-building corals, which support biodiversity yet are vulnerable to heatwave-induced mass bleaching and mortality. Here, we exposed 70 colonies of the reef-building coral Acropora digitifera to a long-term marine heatwave emulation experiment. We tested for trade-offs between heat tolerance and three traits – colony growth (3D photogrammetry models), fecundity (oocyte counts), and symbiont community composition (ITS2 sequencing). Despite observing remarkable within-population variability in heat tolerance, all coral colonies were dominated by Cladocopium C40 symbionts. We found no evidence for trade-offs between heat tolerance and fecundity or growth. Contrary to expectations, positive associations emerged with growth, such that faster-growing colonies tended to bleach and die at higher levels of heat stress. Collectively, our results suggest that these corals exist on an energetic continuum where some individuals have co-benefits across multiple traits. Within populations, trade-offs between heat tolerance and either growth or fecundity may not be major barriers to natural adaptation or the success of restorative assisted evolution interventions.