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

Over the last decades, Global Climate Change (GCC) caused increase in seawater temperature, which have shown to be detrimental for Mediterranean red coral populations (Corallium rubrum). Recent researches described how responses to temperature increase can differ depending on location and previous stress history; however, investigations throughout a wide bathymetric range on the thermo-tolerance of specimens sharing the same thermally stable environment are still lacking. In order to test if C. rubrum colonies dwelling below the thermocline threshold have an intrinsic different sensitivity to thermal stress, corals at different depths (32 m and 100 m) were collected, kept in aquaria, and their responses to stress analysed in lab experiments. Oxygen consumption and necrosis dynamics were estimated at different temperature regimes (14°C; 21°C and 25°C). Shallow water samples showed a significant lower oxygen consumption and coenosarc necrosis at all temperatures, revealing a potential higher survival rate in stress conditions. On the other hand, significant differences in oxygen consumption between shallow and deep dwelling samples were detected in the 21°C treatment, and after 5 days in the necrosis experiment, underlining, however, a good capacity of tolerance to temperature increase also in deep colonies. Implications of the obtained results for conservation of Mediterranean red corals are illustrated and discussed.