Database of sea fans spatial distribution in the northwestern Mediterranean sea

Sea fans (order : Alcyonacea) are the most remarkable and easily identifiable species of the subtidal hard substrate communities (Gili and Ros, 1985). From an ecological point of view, sea fans play an essential role (Gili and Coma, 1998) by increasing the biomass and the diversity of hard substrates (Mitchell et al, 1992; Ballesteros, 2006) via an umbrella species role providing habitat for small epifauna and a refuge for many fish (Ross and Quatrini, 2007). When their density is high enough, they form animal forests (sensu Rossi et al 2017) and become engineering species by modifying the level of turbulence and therefore of sedimentation of propagules in the benthic boundary layer (Chamberlain and Graus, 1975). In the shallow rocky habitats of the Mediterranean, five species of sea fans dominate: the white gorgonian Eunicella singularis (Esper, 1791), the orange gorgonian Leptogorgia sarmentosa (Esper, 1789), the yellow gorgonian Eunicella cavolinii (Koch, 1887), the red gorgonian Paramuricea clavata (Risso, 1826), and the red coral Corallium rubrum (Linnaeus, 1758). Some of these species, characterized by long life span, have been included in the management plans of the Gulf of Lion Marine Protected Areas, with surveys for the monitoring of the demographic structure of E. Singularis (Réserve Naturelle Marine de Cerbère-Banyuls, Aire Marine Protégée Agathoise) and C. rubrum (Réserve Naturelle Marine de Cerbère-Banyuls, Parc Marin de la Côte Bleue). In parallel with these management-related surveys, some scientific studies on the demographic structure and ecology of P. clavata, E. singularis and C. rubrum, with the aim of documenting mass mortality events (Cerrano et al. al., 2000, Garrabou et al., 2001) have been undertaken in the Parc Marin de la Côte Bleue, in the Parc National de Port-Cros, along the Côte Vermeille and Cap de Creus coast and in the Cinque Terre National Park (Garrabou and Harmelin, 2002, Torrents et al., 2005 ; Linares et al. 2008 Linares et al., 2010 ; Santangelo et al., 2011; Gori et al., 2011a ; Gori et al. 2011b; Rossi et al., 2008). The present database gathers two extensive inventory of sea fans populations performed between 2013 and 2015 in the Gulf of Lion, between 2018 and 2025 in the Ligurian Sea, with some stations revisited in 2022 in the Gulf of Lion.  A same protocol was applied to estimate the population density of the five species in 641 geo-referenced stations in the NortWest Mediterranean Sea (Figure 1). Each station was geo-referenced from the surface and located on the sea bed with a mooring. Counts of individuals of the five species were made by scuba divers trained to species identification in four quadrats (1m x 1m), positionned at 5 m from the mooring along the sea bed, in the four cardinal directions. The stations of the Gulf of Lion were defined a priori on a regular mesh mapping the main hard-bottom substrate units of the Gulf of Lion (Côte Bleue, Plateau des Aresquiers, Plateau du Cap d'Agde, Cap Leucate, Côte Vermeille). The spacing between stations was varied from 100m to 800m according to bathymetry steepness. The other sites surrounding the ports of Toulon, La Spezia, Bastia and in Liguria were identified from published litterature. In total, the sampling required 1700 dives. Author contributions Katell Guizien designed the study and organized the logistics of all field works. Lorenzo Bramanti supervised all scuba diving activities and trained all participants to gorgonian identification. Jean-Claude Roca, Bruno Hesse, Sylvain Blouet, Eric Charbonnel, Jérôme Payrot, Bruno Ferrari, Romain Bricout, Lyvia Lescure, Edouard Cheré, Virginie Hartmann, Ronan Rivoal, Jean-François Planque, Frédéric Cadène, Jean-François Laffon, Marie Monin, Olivier Bretton, Marc Dumontier, Céline Larbune, Sandrine Fanfard, and Florence Cornette performed species count in scuba diving.  References  Ballesteros E. 2006. 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