Symbiont community changes confer fitness benefits for larvae, but not juveniles, in a vertically transmitting coral

Coral reefs worldwide are threatened by increasing ocean temperatures because of the sensitivity of the coral-algal symbiosis to thermal stress. Most reef building corals form mutualistic symbiotic relationships with dinoflagellates (family Symbiodiniaceae), including those species which acquire their initial symbiont complement from their parents. Corals can modulate their stress limits through changes in the composition of their symbiont communities, a response referred to as symbiont shuffling. However, the role of shuffling in coral acclimatization is understudied and has largely focused on adults. To quantify the fitness consequences of changes in symbiont communities under a simulated heatwave in early life-history stages, we exposed the larvae and juveniles of the widespread, vertically transmitting coral Montipora digitata to heat stress (32C) and tracked changes in their growth, survival, photosynthesis, and symbiont community composition relative to controls.