Heterotrophy in parental coral colonies enhances larval survival independently of heat stress

Recent studies underscore the critical role of heterotrophy in enhancing the resilience of symbiotic corals to global stressors, such as ocean warming. However, much remains unknown about the role of heterotrophy on coral reproduction, despite its key role in the persistence of coral populations and connectivity. In this study, we experimentally investigated how the trophic regime of parental colonies of the symbiotic gorgonian Eunicella singularis during gametogenesis may affect larval release, survival and settlement rates under both optimal and simulated marine heatwave temperatures. Eunicella singularis is widespread and abundant species in the Mediterranean Sea, being tolerant to a wide range of environmental conditions, and it has been proposed as a potential “winner” under future climatic conditions in the Mediterranean. Our results, however, suggest that predicted declines in marine primary production, zooplankton abundance, and particle flux could undermine their resilience. Notably, we observed a 1 week delay in larval release in absence of heterotrophic inputs, emphasizing heterotrophy’s significant contribution to gametogenesis. Moreover, heterotrophy also plays a crucial role in sustaining larval survival, since the absence of heterotrophic inputs lead to significantly higher mortality of the resulting larvae, regardless of temperature exposure. Overall, this study contributes to increase our understanding of the broader consequences of global change on coral populations under the globally forecasted reduction of primary production and zooplankton abundance.