The study was carried out along Tunisian coasts including the main islands. To present the information on the distribution of gorgonian species along Tunisian coasts, three sectors were considered on the basis of the hydrographic and geomorphological features of the bottoms (Fig. 1). In addition, to compile the information on the distribution, in each sector we screened the presence of gorgonian species in different localities, in which several sites were surveyed by different methods. Thus, the information encompassed different spatial scales: sector (10² km), locality (10 km) and site (1 km). A total of 27 localities and 87 sites were investigated (Table 1).

Sector 1 stretches from the Algerian border to Kélibia, which covers 393 km of coastline and is part of the western basin of the Mediterranean Sea (WMED). In sector 1, data from 11 localities and 49 sites were obtained (Table 1).

Sector 2 comprises the area between Kélibia and Ras Kapudia (261 km of coastline). In sector 2, data from 9 localities and 21 sites were obtained (Table 1).

Sector 3 extends from Ras Kapudia to the Libyan border, including the Gulf of Gabès (482 km coastline). In sector 3, data from 7 localities and 17 sites were obtained (Table 1).

Sector 1 in northern Tunisia is under the influence of Atlantic currents and is characterized by a unique morphology and a heterogeneous bottom type, making its benthic habitats diverse and patchy with a dominance of coralligenous assemblages (Azouz 1973). Accordingly, around Galite Island, the hard bottoms are rich, and there are coralligenous assemblages up to a depth of 100 m. At Zembra Island, the bottom is characterized by a series of parallel recesses bordered by rocks and coastal detrital bottoms (Lubet and Azouz 1969, Andromède 2010a, b). Like those of the Banc des Esquerquis and Banc Rezgui, the bottoms around Cani Island, which is situated in northern Tunisia, are dominated by rocky substrates (Azouz 1973).

Sectors 2 and 3 belong to the central Mediterranean. These sectors are dominated by sandy and muddy bottoms and harbour communities with subtropical species that are characteristic of the eastern basin of the Mediterranean Sea. Along the east coast, the sea bottom is fairly homogeneous, and the shelf is very wide, especially in the Gulf of Gabès (Bradai et al. 2004).

A literature survey was carried out using several bibliographic databases for the period from 1950 to the present. The research was done using different combinations of keywords, such as Mediterranean, gorgonians(s), cnidarian(s) species, coralligenous or benthic biocenoses, and Tunisia. In addition, information was obtained from technical reports published by Tunisian national institutions. Only seven references were retained from the more than 50 reports and scientific publications that were evaluated. The site coordinates were taken from either the references or the maps included in the publications. The literature survey noted that the white sea fan, Eunicella singularis, was recorded by Azouz (1973) in Northern Tunisia, as was Eunicella stricta (Bertoloni, 1810), which is generally regarded as a junior synonym of E. singularis, as reported by Weinberg (1976, 1978).

Species occurrences were also obtained from the LEK approach via interviews with local divers and fishermen. The LEK survey was applied at 23 localities along the Tunisian coasts: nine localities in sector 1, eight localities in sector 2 and six localities in sector 3 (Fig. 1 and Table 1). A total of 132 interviews were conducted, 19 diver interviews and 113 fishermen interviews. To ensure the quality of the information acquired, the interviews were conducted by showing samples and photographs of the gorgonian species. When the species were recorded by the interviews, the respondents sent photos with the corresponding coordinates. Nevertheless, when necessary the samples collected by divers and fishermen were identified in the laboratory. The LEK approach was successfully applied to compile information on the distribution and to track temporal changes of the presence and abundance of different marine species (Azzurro et al. 2011, Boughedir et al. 2015).

A total of 9 localities and 26 sites were surveyed for the presence of gorgonian species using scuba diving from May 2013 to December 2015. The sites were selected after analysing the literature and the results of the LEK surveys of fishermen and divers, and were based on information on the nature of the bottom. The surveys were conducted mainly in coastal areas but also included two insular areas (Fig. 1 and Table 1). A total of 20 and 6 sites were explored with scuba diving in sector 1 and sector, 2 respectively. Diving surveys were not conducted in sector 3 because the results from the interviews of both fishermen and divers indicated the absence of gorgonians in this sector (Fig. 1 and Table 1).

Diving surveys were carried out to a maximum depth of 40 m, and the surveys utilized a combination of both bathymetric transects and transects at fixed depths. The data collection was based on the presence or absence of gorgonian species. Species were identified in situ or after examining a sample in the laboratory when necessary. For each site, GPS coordinates were recorded. When gorgonians were present, semi-quantitative estimations of the abundance and the upper and lower depth limits were conducted when possible, and the estimations were specified for each species at each study site.

Species identification followed the descriptions provided by Carpine and Grasshoff (1975) and Bayer et al. (1983), using the morphological characteristics of the colonies, and identifications were made to the lowest possible taxonomic level. The nomenclature adopted in this work follows the World Register of Marine Species (WoRMS).

A total of eight gorgonian species were found in Tunisian coastal waters (Table 2). The reported species belong to four families and eight genera. The Gorgoniidae family is the most diverse in Tunisian waters with four species, followed by Plexauridae with two species and Coralliidae and Ellisellidae, both with a single species (Table 2). Only Paramuricea macrospina is considered endemic to the Mediterranean (Otero et al. 2017).

In this study, a total of 87 sites were investigated for the presence of gorgonian species, and the sites covered approximately 1300 km of coastline (Fig. 2). The presence of gorgonian species was confirmed at 54 sites (Table 1 and Fig. 2). At most sites, only one species was found (Table 3). Sector 1 constituted most of the observations of gorgonians in terms of both presence (11 localities and 44 sites) and the number of gorgonian species recorded (8 species) (Table 2, Figs 2 and 3). In sector 2, six localities encompassing nine sites were found to shelter populations of three gorgonian species: E. singularis, E. cavolini and Leptogorgia sarmentosa. In sector 3, only E. singularis was reported at only one site of the seven localities and 17 sites surveyed (Tables 1 and 3, Figs 2 and 3).

In general, the direct observations and the information reported by the LEK and literature surveys indicate that the substrate types on which gorgonian populations dwelt were rocky boulders, shoals, vertical walls or plunging cliffs. However, the white gorgonian E. singularis was observed on a sandy bottom at one site in the marine protected area (MPA) of Zembra locality (L9), but the colonies were settled and developed over small stones.

E. singularis, was the only species reported in all three sectors covering the Tunisian coast (Table 2, Fig. 3). Its populations displayed low to moderate abundance according to the LEK interviews with divers and scuba diving surveys. The bathymetric distribution of the populations ranged from 7 to 60 m depth.

In sector 1, E. singularis was present at most of the localities surveyed where the bottoms were characterized by rocky bottoms and coralligenous assemblages. For instance, in the western part of sector 1, the diving surveys in the Tabarka area reported populations developing at depths between 15 and 26 m with moderate abundances. Similar observations were reported around Galite Island, which is 60 km from the continental coast, where E. singularis was recorded at depths ranging from 23 to 48 m (Table 3). Likewise, in the eastern areas of sector 1, the information available from local divers and fishermen obtained in Cap Serrat (L3), Bizerte (L5), Fratelli Island (L4) and Cani Island (L6) indicated that E. singularis populations were abundant at depths between 20 and 40 m. The local divers and red coral fishermen further east in Banc des Esquerquis (L7) noted that the species was abundant at 40 to 60 m depth.

The dives conducted at two sites in the locality of La Marsa (L8) on rocky bottoms in the southeastern part of sector 1 indicated that E. singularis was also present in shallow waters (7-20 m depth). However, the colonies at these sites did not display dense patches. Further south, in Zembra Island National Park, E. singularis colonies were found at depths ranging from 30 to 40 m, according to the literature and diving surveys (Table 3). However, the presence of E. singularis at this locality is not widespread, because the diving surveys reported the presence of the species in only two of the five sites surveyed, though the dives were carried out on similar depth ranges and rocky substrata. The populations displayed low colony abundances at the two sites where E. singularis was observed. At the locality Haouaria (L11), at the limit with sector 2, scuba diving surveys at three sites recorded E. singularis at 18 to 26 m depth on rocky bottoms with relatively high abundances (Fig. 4A and Table 3).

In sector 2: Populations of E. singularis were reported at 6 of the 21 surveyed sites, and most of the populations corresponded to rocky banks (Table 3). At the Menzel Tmim locality (L12) in the northern part of sector 2, E. singularis populations were found between 25 and 35 m. At the Banc Korba (L13) and the Banc Maamoura (L14) localities, populations of E. singularis were reported between 25 and 35 m, while the populations were deeper (45 m) in Banc Hallouf (L19) (54 km from the coast) (Table 3). Finally, in the southern part of sector 2, E. singularis was reported to be present in the locality of Mahdia (L20) at 40-50 m depth (Table 3).

In sector 3, E. singularis was observed at only 1 site out of the 17 investigated (Fig. 3 and Table 3). In the central part of the Gulf of Gabès, its presence was reported at 40-55 m depth at the Fora locality (L27) (Table 3).

The yellow gorgonian, Eunicella cavolini (Fig. 3), was observed at four localities in sector 1 and two localities in sector 2; however, its populations displayed moderate to low abundances at all observed sites. In the northern part of sector 1, the diving survey highlighted the presence of E. cavolini Tabarka (L1) at depths ranging from 15 to 25 m, while at Galite Island (L2), the species was observed at 31 to 44 m depth (Fig. 4D). Populations were present at 30 to 40 m in the southeastern part of sector 1 in Zembra National Park (L9) and at 18 to 24 m in Haouaria (L11) (Table 3).

In sector 2, E. cavolini was reported at two localities in Menzel Tmim (L12), where the populations were reported to be between 25 and 35 m depth, and in Mahdia (L20), where the LEK survey indicated that its distribution could reach a depth of 50 m.

Eunicella verrucosa was reported at only two localities of sector 1 (Fig. 3). The diving surveys conducted in this study confirmed the presence of the species at 30 to 35 m depth on rocky bottoms around Galite Island (L2) and provided the first observation in Zembra National Park (L9) at 30 m depth. In both cases, a few colonies were observed on rocky boulders.

Leptogorgia sarmentosa was observed in the southeastern parts of sectors 1 and 2 (Fig. 3). However, this species was observed at only one locality in sector 1 and two localities in sector 2. The observation in sector 1 was from a scuba diving survey near the harbour of Haouaria (L11) at 5 m depth. The observations in sector 2 correspond to scuba diving surveys near the harbour of Hammamet (L16) at 32 to 45 m depth and on coralligenous falls in Banc Korba (L13) at 25 to 30 m depth (Fig. 4B). Moreover, in sector 2, technical reports indicated the presence of Leptogorgia sarmentosa in Banc Hallouf bank (L19) (approximately 54 km from Monastir), which is constituted by rocky zones formed by very extensive blocks and surrounded by coarse elements.

The LEK surveys in the Banc des Esquerquis (L7) indicated that both Paramuricea clavata and P. macrospina were present on rocky bottoms between depths of 60 and 80 m (Fig. 3). The diving surveys carried out up to a depth of 40 m on rocky cliffs and coralligenous formations in sectors 1 and 2 did not report the presence of these species.

Ellisella paraplexauroides is an Atlantic-Mediterranean gorgonian species that was reported at only one locality Sidi Daoud (L10) from sector 1 at a depth of 47 m (Table 3)

The precious red coral, Corallium rubrum, was present in only sector 1 (Fig. 4C). However, the species was observed at five localities in this sector, and the populations were still abundant despite the fact that they are still exploited by fishermen according to the LEK surveys. The populations of the red coral Corallium rubrum were mainly observed between 60 and 120 m at most of the localities (Table 3).