*3.2. Percentage Cover of Benthic Organisms and Density of Paramuricea Clavata*

In 2013, the majority of the substrate (65%) was covered by TSH and the remaining 34% by sessile macro-benthonic organism (occupied substrate). It was impossible to read the remaining 1% of the photo-sample (undetermined area, Table 1). *P. clavata* (50%) and Porifera (27.7%) were the dominant organisms. Hexacorallia (mainly *Parazoanthus axinellae* and *Leptosamnia pruvot*i), Phaeophyceae, Rodophyceae, Ascidiacea, Bryozoa, and Polychaeta covered the remaining surface of the occupied cliff (22.3%).


**Table 1.** Percent cover (%) of the whole Tinetto Islet area and two sites (Tinetto Island and Tinetto Shoal) in 2013 and 1998 [46]. The TSH substrate is composed by algal turf, sediment, and small hydrozoans; occupied substrate includes sessile macro-benthonic organisms listed below. \* = *p* < 0.05 level. TSH, occupied substrate, density of *P. clavata* adults and recruits (col m<sup>−</sup>2) in the whole Tinetto Islet area in 2013 are compared with those measured in 1998 [46].

A comparison with the data collected in the same area in 1998 on a similar number of samples highlighted some major changes occurring within the community over the pre-mortality/post mortality transition, showing about a twofold increase in TSH and halving of the cover of benthic organisms (Table 1). The *P. clavata* % cover was reduced threefold, and the adult density decreased twofold, while during the same time interval recruitment increased by four and a half times.

In 2013, no significant difference in TSH % cover or the % of the occupied substrate by sessile organisms between the two sample sites was found (*t* = −0.095, *p* > 0.05 and *t* = −0.07, *p* > 0.05), while the % cover of Porifera was significantly higher at Tinetto Island than at Tinetto Shoal (*t* = 2.56, *p* < 0.05, Table 1).

Overall 355 colonies were counted in 23 m<sup>2</sup> of the study area (130 at Tinetto Island and 225 at Tinetto Shoal), reaching an average density of about 15 colonies per m2. Recruits settled only on some of the plots (61.8%): overall, 64 recruits were found on 14 plots (6/12 plots at Tinetto Island and 8/11 plots at Tinetto Shoal), with an average density of 3.75 recruits per m2. A significantly higher recruit density was found at Tinetto Shoal (6.18 ± 2.14 col m<sup>−</sup>2) vs. at Tinetto Island (1.33 ± 0.48 col m<sup>−</sup>2; Mann–Whitney *<sup>w</sup>* <sup>=</sup> −2.06, *p* < 0.05, Table 1). Moreover, adult colony cover and density were also significantly higher in this latter area (14.0 ± 2.3 vs. 19.9 ± 2.4% cover: *<sup>t</sup>* <sup>=</sup> −1.80, *<sup>p</sup>* < 0.05; 12.07 ± 1.81 col m−<sup>2</sup> vs. 21.30 ± 4.17 col m<sup>−</sup>2: *<sup>t</sup>* <sup>=</sup> −2.08, *<sup>p</sup>* < 0.05).

No correlation emerged between larger/older colony and recruit densities (*n* = 23, *r* = 0.35, *p* > 0.05, Figure 6a), but a significant positive linear correlation was found between the density of recruits and that of smaller/younger colonies (*n* = 23, *r* = 0.78, *p* < 0.01, Figure 6b). Some linear trend was also found in the recruitment density measured in the same plot in two consecutive years (*n* = 24, Figure 6c). Both the last findings suggest that samples showing high recruitment in one year show similar values in the following years, as well.

No correlation was found between colony density and Porifera % cover (*r* = −0.34, *p* > 0.05) or between recruit density and TSH % cover measured in the same plots (*r* = 0.038, *p* > 0.05). These results suggest that neither the density of adult colonies nor the kind of substrate (at the gross-grain level examined) likely have any linear effect on *P. clavata* recruitment.

**Figure 6.** Trends of recruit and colony density: (**a**) density of recruits vs. that of larger/older colonies; no linear trend was found; (**b**) density of recruits vs. that of small/young colonies; a linear trend emerged; (**c**) recruit density measured in the same photo sample in two successive years (2007 vs. 2008 and 2009 vs. 2010) and the linear trend.
