The role of gorgonians on the diversity of vagile benthic fauna in Mediterranean rocky habitats

Mediterranean gorgonian forests are threatened by several human activities and climatic anomalies that increase their susceptibility to pathogens and epibionts, potentially leading to mass mortality events. Although these phenomena have been largely documented, the effects of the loss of gorgonian colonies on temperate reef assemblages have been poorly evaluated until now, especially those that concern the vagile fauna. This is the first study comparing vagile fauna assemblages living in Mediterranean rocky bottoms between patches with and without the gorgonians Eunicella cavolini and Paramuricea clavata. Vagile fauna were analysed at the end of a field experiment where patches with and without gorgonians were simulated by transplanting gorgonian branches on to plastic recruitment panels. Panels were deployed for 4 months at two depths (24 m for E. cavolini and 40 m for P. clavata) at two sites in the north-western Mediterranean Sea (Tavolara Island and Portofino Promontory). Overall, 211 taxa belonging to the phyla Platyhelminthes, Nemertea, Mollusca, Brachiopoda, Nematoda, Anellida, Arthropoda and Echinodermata were found. Despite high local heterogeneity and large differences between sites, the abundance of some vagile species was related to the presence of gorgonians. For instance, the shrimp Periclimenes aegylios, absent at Tavolara, was facilitated by the presence of E. cavolini at Portofino, while the tanaid Leptochelia sp. was absent/scarce in the presence of gorgonians at both sites. Overall, at 40 m depth, vagile community structures did not differ significantly between experimental patches with and without P. clavata; at 24 m depth, vagile assemblages in the presence of E. cavolini were different and less heterogeneous compared to those found in patches without transplanted gorgonians. Ultimately, gorgonians could directly and indirectly affect vagile assemblages.