Corallium rubrum and Eunicella cavolini: distribution, population structure and co-occurrence in the deep Mediterranean Sea

The precious red coral, Corallium rubrum, and the yellow gorgonian, Eunicella cavolini, have been frequently reported as co-occurring species in the deep Mediterranean Sea. However, knowledge regarding spatial distribution of the two species in the deep sea is still fragmented, as well as potential drivers of their co-occurrence. This study aims to provide data on the status of deep-dwelling red coral populations, which are now the main target of commercial harvesting, study the demography of the yellow gorgonian in the deep sea, and finally analysing their variability when co-occur. Both species exhibited the highest population density at deeper depths and the presence of an inverse relationship between population density and colony height. We found a high percentage (40% on average) of red coral harvestable colonies (> 10 mm basal diameter), as well as of living colonies (77% as average). Our study evidenced that most of co-occurring colonies were found below 130 m depth. Red coral density did not significantly vary with the presence of E. cavolini, whereas the yellow gorgonian showed significant higher density when co-occur with red coral, even if with lower height. Distance-based linear modelling (DistLM) revealed that red coral density significantly influenced the presence of the yellow gorgonian, emphasizing the need for proper conservation strategies of red coral populations in order to preserve habitat complexity and associated diversity.