Submarine caves as potential climate microrefugia for symbiotic octocorals

Climatic refugia are receiving increasing attention in the context of global warming and the rising frequency and intensity of marine heatwaves (MHWs). Long-lived and slow-growing organisms, such as many octocorals, are highly threatened by the increase in seawater temperatures and the associated mass mortality events. Here, we evaluated the potential role of a submarine cave as climatic microrefugium for the symbiotic Mediterranean white gorgonian, Eunicella singularis. Environmental conditions (light and temperature) and biological responses (population structure, reproductive output, and physiological traits) were compared between colonies located inside and outside a submarine cave. Colonies outside the cave are exposed to high light and slightly increased temperatures compared to the inside of the cave, exhibiting high mortality (61% of dead colonies) and reduced reproductive output (65% of non-reproducing colonies and absence of mature oocytes). In contrast, colonies inside the cave showed lower signs of MHWs related stress (17 times lower mortality rate), higher reproductive output, and different photoadaptations to low-light conditions (chlorophyll content and pigment ratio). Although the observed differences are likely driven by lower light and temperature inside the cave, other environmental factors not considered in this study, such as hydrodynamic conditions and food availability, could also contribute. Our findings highlight the potential role of some submarine caves as climatic microrefugia, capable of buffering symbiotic octocorals from the combined effects of light and thermal stress.