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 and between 2018 and 2019 in the Ligurian Sea.

The same protocol was applied to estimate the population density of the five species in 585 stations. The 585 stations 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), and surrounding the ports of Toulon, La Spezia and Bastia (Figure 1). The spacing between stations was varied from 100m to 800 m according to bathymetry steepness. 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. In total, the sampling required 1500 dives.

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