*2.1. Study Area and Sampling Design*

The study area is located at Tinetto Islet (Figure 1), part of a small archipelago of the Ligurian Sea at the western edge of the La Spezia Gulf (NW Mediterranean, Italy), an area included in the World Heritage list and under the protection of the United Nations Educational, Scientific and Cultural Organization (UNESCO). Oriented from northwest to southeast, the Gulf is about 5 km wide and 10 km long, and its circulation is affected by several factors: the Ligurian current, seasonal weather changes, and the strength of the socalled 'Sirocco' warm southerly wind. The area is characterized by high turbidity due to the Magra river plume influencing the water and its hydrodynamics, especially during rainy seasons [21,38]. The northwest and western sides of Tinetto Islet are composed of vertical calcareous walls while the southern and the eastern sides are characterized by medium slopes. A more detailed description of the area has been previously published [13,21].

The population of *Paramuricea clavata* living on a sub-vertical rocky cliff between 17 and 25 m depth was monitored between 1998 (pre-mortality) and 2010 by identification and measurement during SCUBA diving [13,21]. This paper deals with the results of the photographic sampling carried out via a housed X 100 G10 Canon (Canon Inc., Tokyo, Japan) digital camera at a 1:1 magnification on the same population in July 2013.

In 2013, 24 of the permanent polyvinyl chloride (PVC) quadrates (1 × 1 m, thus 1 square meter each) previously fixed at 2 sites a few hundred meters from one another were sampled for analysis. Twelve were on the northwestern side (Tinetto Island) of Tinetto Islet (44◦01 26 ' N, 09◦51 03 ' E), and 12 at the Tinetto Shoal, on the southern side of the islet (44◦01 23 ' N, 09◦51 05 ' E; unfortunately, one of the quadrates fixed at this site was lost). At each site, the squares were randomly arranged at a few meters' distance. Of the 23 quadrates sampled for this study, 12 corresponded to the same ones used for the previous monitoring study. Four photos of a 0.25 m<sup>2</sup> area in each one-square meter quadrate were taken vertically to the cliff. Overall, 92 photos of 23 quadrates in the whole area have been analyzed for the present study.

The main taxa of the sessile organisms in each photo-sample (Figure 2) were identified, and their percent cover was measured by the graphic program Image J. The "TSH" substrate (including algal turf, *sensu* Connell et al. [39], sediment, and small hydrozoans) was considered as "free surface", putatively suitable for the settlement of *P. clavata,* as recruits and young colonies of this gorgonian have been frequently found emerging from this

substrate [20]. The remaining substrate, hereafter named "occupied substrate" was covered by sessile macro-benthos.

**Figure 2.** *P. clavata*: (**a**) a large/old adult and (**b**) a small/young colony.
