Larval thermal conditioning does not improve post-settlement thermal tolerance in the dominant reef-building coral, Montipora capitata

Abstract

Climate change-induced ocean warming has reshaped reef ecosystems as coral bleaching events continue to lead to mass coral die-offs globally. These thermal anomalies also negatively affect reef persistence and recovery when heat stress events continue to occur during critical reproduction and recruitment periods. As a result, recruitment (settlement and survival of early life stages) continues to decline on reefs globally, compromising the recovery of reef systems following mass bleaching and mortality events, which has led to the loss of ecological function on these reefs. One mitigation strategy within the active restoration framework includes propagated corals (i.e., adult fragments, larvae, or juveniles) produced in nurseries that are then directly out planted to the reef benthos. Restoration programs can greatly benefit from use of sexually derived coral propagules, which when optimized, increase the scalability and genetic diversity of outplanted stock. Thermal conditioning during larval rearing has the potential to be a cost-effective and scalable strategy to enhance the survival and stress tolerance of subsequent life stages. Here, we tested the potential for thermal conditioning during the larval stage to induce positive latent effects on the stress tolerance of Montipora capitata spat. We exposed larvae to cool (24.5 °C), ambient (27.2 °C), and warm (28.9 °C) conditions for four days and during this period we tracked larval survival. Following thermal conditioning during the larval stage, we measured settlement success and then exposed spat to a 47-day high temperature stress (+ 2 °C) and measured survival. We found that larval thermal conditioning did not provide survival benefits to spat under elevated temperature. Furthermore, larval exposure to temperatures outside of an ambient regime compromised survival, suggesting that although positive latent effects were not observed here in spat, optimizing thermal conditions is important for maximizing survival and settlement of coral larvae.