**1. Introduction**

The sea fan *Eunicella verrucosa* (Pallas, 1766) is a large, erected gorgonian, profusely branched, white to deep pink in color, with an Atlanto-Mediterranean distribution, ranging from Angola to Ireland, in the East Atlantic Ocean [1,2]. The species settles directly on bedrock, on large boulders and on artificial surfaces in areas with moderate water movement [3]. The depth range for the Mediterranean Sea goes from 26 to 215 m depth with most of the records located below 35 m depth [1,4–9]. The populations of this species provide structural complexity sustaining rich, associated biodiversity and aesthetic value to sublittoral communities [10,11]. The tridimensional morphology of *E. verrucosa* colonies observed in previous studies is complex and variable: the typical architecture is planar, but other growth forms, characterized by a high rate of branch overlapping, result from different environmental conditions, such as spatial constraints, current intensity, feeding ability and predation [12–18].

The growth rate of *E. verrucosa* was recorded as highly variable: demographic studies of different populations sugges<sup>t</sup> values of 0.6–3.5 cm year<sup>−</sup><sup>1</sup> for Mediterranean areas and of 1–4.5 cm year<sup>−</sup><sup>1</sup> for the English Channel [6,19–22]. The relatively slow growth rate of *E. verrucosa* coupled with its sensitivity to abrasion, mechanical disturbance by

**Citation:** Canessa, M.; Bavestrello, G.; Bo, M.; Enrichetti, F.; Trainito, E. Filling a Gap: A Population of *Eunicella verrucosa* (Pallas, 1766) (Anthozoa, Alcyonacea) in the Tavolara-Punta Coda Cavallo Marine Protected Area (NE Sardinia, Italy). *Diversity* **2022**, *14*, 405. https:// doi.org/10.3390/d14050405

Academic Editors: Simone Montano and Michael Wink

Received: 3 April 2022 Accepted: 8 May 2022 Published: 20 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

anchors, fishing gear and fin-stroke damage by scuba divers [23–26], as well as substratum loss [21,27], makes this species particularly sensitive to anthropogenic impacts and environmental stressors.

Thanks to recent Remotely Operated Vehicle (ROV) surveys [8] and SCUBA dives coupled with citizen science reports, the distribution of *E. verrucosa* on the Mediterranean scale has been recently updated [28]. This large amount of data confirms the occurrence of the species mainly along the coast of the western Mediterranean basin with a peak of records in the Ligurian Sea. In particular, a structured community dominated by this species was identified here in dozens of sites, mainly on sub-horizontal rocks characterized by heavy silting between 30 and 215 m (maximum occurrences 60–90 m) [8]. A lower number of records involve the North African coasts, the Sicily Channel, the North Adriatic Sea and the Aegean Sea. In this scenario, the most impressive gap sees the almost complete absence of the species from the central Tyrrhenian Sea, and in particular, from the Sardinian coast [28].

Very few data are available in the literature regarding NE Sardinia. The oldest records date back to 1990 when Bianchi et al. [29] recorded the species at a site in the Tavolara-Punta Coda Cavallo Marine Protected Area (TPCCMPA). Later, in the same area, field campaigns reported some colonies on the hull of the Klearchos wreck at 77 m depth [30], and another specimen settled on a granitic shoal (Tavolara2) at 55 m depth [31]. Recent surveys assessed the occurrence of this species, in association with large and erect sponge assemblages, on granite reliefs in the Tavolara Channel under 40 m depth [32,33].

This study aims to quantify the presence of *E. verrucosa* in the TPCCMPA, improve the knowledge of its Mediterranean distribution, and provide a morphometric description of the population. In addition, a study of the opportunistic fauna living on the colonies, used as an indicator of mechanical abrasion also of anthropic origin [34,35], was conducted to evaluate the impact of anthropogenic activities within the MPA.

#### **2. Materials and Methods**

Between 2015 and 2020, 110 scuba dives were carried out on 77 granite outcrops within the Tavolara Channel at depths between 12 m and 59 m (Figure 1). Each outcrop was georeferenced on a Geographic Information System (GIS) and the coordinates of the sites investigated were registered on the MPA web platform. All the sites where *Eunicella verrucosa* was recorded are listed in Table 1.

**Table 1.** Investigated sites with the occurrence of *Eunicella verrucosa* within the Tavolara–Punta Coda Cavallo Marine Protected Area and number of specimens in each one. Coordinates of the sites can be consulted upon request to the Marine Protected Area repository.



**Table 1.** *Cont.*

**Figure 1.** Location of the Tavolara–Punta Coda Cavallo Marine Protected Area and investigated sites (black polygons) with the indication of the number of *Eunicella verrucosa* colonies per site. Stars refer to ANDROMEDE [30,31] reports.

All the recorded colonies of *E. verrucosa* were photographed. The multi-zoom photographic approach [36] was used to characterize the site geomorphologies, the benthic assemblages, and in particular, the presence of *E. verrucosa*, and the occurrence of epibionts and damages on the sea fans (Figure 2).

**Figure 2.** Operative workflow for the characterization of the investigated sites by a multi-zoom approach. Preliminary Side Scan Sonar survey (**A**) graphic reconstruction and(**B**) final panoramic photographic rendering (**C**) of the site. Examples of the ecological context in which *Eunicella verrucosa* settles, mainly composed of large and erect sponges such as *Axinella* spp., *A. polypoides, Spongia lamella* (**D**), *Sarcotragus foetidus* (**E**) and other encrusting and massive species. Details of damages, (**F**) as entangled lines (b) and epibionts, (**G**,**H**) such as *Turbicellepora avicularis,* (c) *Crella elegans* and (e) other acrophilic species associated to the gorgonian (**H**) as *Astrospartus mediterraneus* (d).

Images were taken using a Sony A6000 camera (24 megapixels, two Inon S2000 strobes, color temperature 5000 K) with Sony 16–50 lens (focal length 19 mm), Nauticam WW1 wet wide lens (130◦ rectilinear field angle) and a Sea & Sea MDX-A6000 underwater case with a flat porthole. Panoramic renderings of the sites to localize the colonies were obtained with

multiple shots subsequently joined and optimized in postproduction using the Photoshop CS6 Merge tool.

Using a laser gauge as a reference (wheelbase 25 cm), the height was measured in each colony whereas the fan surface was evaluated for only 63 specimens with a suitable perspective. Photographic processing for measurements was carried out using ImageJ Software (Wayne Rasband and contributors, National Institutes of Health, Bethesda, MD, USA) [37].

As gorgonian colony height is considered a robust parameter leveraged for age estimation in this species [38], this datum was calculated according to the function proposed by Chimienti [39] for Mediterranean populations (Table 2):

$$\text{Age} = \varepsilon \frac{H + 18.39}{17.94} \tag{1}$$

**Table 2.** Morphometric parameters, percent portion of surface covered by epibionts, naked skeleton and damage of the specimens of *Eunicella verrucosa* investigated in the present study.


The size and age structure of the population were analysed in terms of size–frequency and distribution parameters (skewness and kurtosis) using Past 4.10 statistic (Øyvind Hammer, Natural History Museum, University of Oslo, Oslo, Norway).

Finally, photographs were analyzed to identify associated epibiont species to the lowest possible taxonomic level: when the identification was not possible, Operational Taxonomic Units (OTUs) were adopted. Species/OTUs were grouped into sessile opportunistic epibionts, predators/mucous feeders and vagile acrophilic species. The occurrence of each epibiont species and the percentage of colony surface covered were calculated (Table 2). Moreover, recent mechanical damages were also recorded as a percentage of the colony's naked skeleton portion. The total damage was estimated as the sum of the percent of naked portion and epibionted ones (Table 2).
