Over the past decade, the Mediterranean Sea has experienced frequent marine heat waves. Sea surface temperatures in the region have, at times, been 5° Celsius higher than normal which had widespread negative effects on numerous ecosystems and species, including slow-growing species like the red gorgonian. A study from 2021, for example, found that Mediterranean coral reefs were still recovering from a marine heat wave dating back to 2003 because heat events continued to affect them almost annually after that. 

During the summer of 2022, divers made an alarming observation in the Marseilles area. They witnessed the death of the vast majority of red sea fans, or gorgonians, at depths of up to 30 metres. On specific sites, they found that 100% of the gorgonians were decimated. Only greyish skeletons were left behind where colourful gorgonian colonies once dominated the scene. According to scientists, this mortality episode is most likely linked to the particularly high temperatures experienced in the region during the summer. 

But unlike hard corals that make up coral reefs, gorgonians are not directly affected by temperature variations, researchers have found. Global warming affects red sea fans for a very specific reason. It seems that the rise in water temperature has enabled the proliferation of vibrio cholerae, the bacterium responsible for cholera. As this bacterium is highly efficient at high temperatures, it multiplies rapidly and seems to have infected the gorgonians. 

As with all Mediterranean corals, red sea fans reproduce in summer. The large amount of energy invested in reproduction and the lack of nutrient supply during the season make gorgonians particularly vulnerable during the summer period. The increasing frequency of heat waves represents a further threat to gorgonian populations, which no longer have sufficient time to recover. The death of gorgonians raises a more global problem: the potential loss of all associated biodiversity. Above a certain population density, gorgonians create what are known as underwater animal forests. Like underwater trees, they form three-dimensional habitat structures, creating refuges for numerous species.