Global warming drives phenological shifts and hinders reproductive success in a temperate octocoral

Global warming is profoundly reshaping biodiversity. Until now, most research has focused on the impacts of extreme temperature events. However, in many ecosystems, it is becoming increasingly apparent that climate change is accelerating the onset of spring warming conditions. These advanced warming conditions can significantly disrupt critical biological processes such as reproduction, which is key for population persistence. While interest in phenological shifts has increased in recent years, their effects on marine foundation species, such as corals, remain poorly understood. Here, we combined observational and experimental approaches to assess the effects of advanced spring warming conditions driven by climate change on the reproduction of the Mediterranean octocoral Paramuricea clavata, a foundation species. Our findings reveal that a 2°C warming leads to a 2-week advancement in P. clavata spawning, as evidenced by both field observations, and ex-situ experiments. These results underscore the role of advanced spring warming as a significant driver of phenological shifts in coastal marine ecosystems. Furthermore, we show that this phenological shift lead to a reduction in the number of spawning events, as well as decreases in larval biomass, survival rates, and settlement success. These findings highlight the urgent necessity to monitor phenological changes in foundational marine species, as such shifts can undermine the long-term viability of coral populations and contribute to a substantial decline in associated biodiversity. Consequently, the increased vulnerability of species caused by phenological responses driven by seasonal changes may lead to more dramatic consequences of ocean warming than previously anticipated