| Summary: | Ocean acidification and ocean warming constitute major threats to many calcifying reef organisms, including scleractinian corals. The combined effects of these two environmental stressors on the earliest life history stages of reef calcifiers remain poorly studied, particularly for Atlantic corals. Here, we investigate how acidification and warming influence the fertilization success, larval survivorship, and larval settlement of the threatened Atlantic coral, Orbicella faveolata. Gametes and larvae from O. faveolata were subjected to a factorial combination of warming (ambient versus + 1.5 °C) and acidification (ambient versus − 0.2 pH units) projected to occur by the year 2050. O. faveolata individuals were maintained in the same treatments throughout all early life history stages investigated. The fertilization success of O. faveolata was not affected by acidification, warming, or their combination. However, during larval development, warming caused complete mortality and prevented any subsequent settlement. Interestingly, these negative effects of warming were mitigated when combined with ocean acidification, such that both larval survivorship and settlement increased by 41% in the combined treatment relative to the isolated warming treatment. Our research suggests that temperature-induced increases in larval metabolism may be counterbalanced by acidification, which serves to reduce larval metabolism. Notwithstanding, larval survivorship and settlement were still reduced by 50% under combined acidification and warming relative to the ambient treatment, indicating that climate change will continue to serve as major stressor during the early life history stages of corals, jeopardizing the resilience of Caribbean reefs.
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