Differential response to thermal stress of shallow and deep dwelling colonies of Mediterranean red coral Corallium rubrum (L., 1758)

Over the last decades, the global climate change caused increase in seawater temperature, which have shown to be detrimental for Mediterranean red coral populations (Corallium rubrum). Recent researches on C. rubrum described how in shallow water habitats (i.e., up to 50 m depth), thermo-tolerance is influenced by local temperature fluctuations. Investigations throughout a wider bathymetric range are, however, still lacking. This work aimed at testing if C. rubrum colonies belonging to different populations (shallow versus deep water populations) do show different sensitivity to thermal stress. To do so, corals dwelling at different depths (32 m and 100 m) were collected, kept in aquaria, and their responses to thermal stress was evaluated through two lab experiments. Oxygen consumption at different temperature regimes (14°C, 21°C and 25°C) and coenosarc necrosis at 25°C (over a 9-day lasting experiment) were used as response variables. Shallow water samples showed significant lower oxygen consumption and coenosarc necrosis compared to deep samples, at all temperatures, revealing a potential higher survival rate in stress conditions. Significant differences in oxygen consumption between shallow and deep dwelling samples were detected in the 21°C treatment. Significant differences in necrosis were observed after 5 days, emphasizing a good tolerance to temperature increase for deep colonies. Implications of the obtained results for conservation of Mediterranean red corals are illustrated and discussed.