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. Reef building corals form mutualistic symbiotic relationships with dinoflagellates (family Symbiodiniaceae), including those species which acquire their initial symbiont complement from their parents. Changes in the composition of symbiont communities, through the mechanisms of symbiont shuffling or switching, can modulate thermal limits. However, the role of shuffling in coral acclimatization is understudied and work to date 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 (32°C) and tracked changes in their growth, survival, photosynthetic efficiency, and symbiont community composition relative to controls. We found negative impacts with warming in all fitness-related traits, which varied significantly among larval families and life-stages. Surviving larvae exposed to heat exhibited changes in symbiont communities that favoured types that are canonically more stress tolerant. Compared to larvae, juveniles showed more rapid mortality under heat stress and their symbiont communities were largely fixed regardless of temperature treatment, suggesting an inability to alter their symbiont community as an acclimatory response to heat stress. Taken together, these findings suggest that capacity for symbiont shuffling may be modified through development, and that the juvenile life-stage is more at risk from climate warming.