Record of Ellisella paraplexauroides (Anthozoa: Alcyonacea: Ellisellidae) in Italian waters (Mediterranean Sea)

Abstract

The occurrence of the candelabrum coral Ellisella paraplexauroides in Italian waters (Sicily Strait, Mediterranean Sea), was documented during a remotely operated vehicle cruise, carried out by ISPRA in May 2010 on-board the RV ‘Astrea’. Five specimens were photographed and sampled from 80 to 94 m depth on the north-east coasts of Pantelleria Island, confirming the distribution of this species in the central Mediterranean Sea. A description of the living colonies and other taxonomic and ecological characteristics is provided.

INTRODUCTION

The gorgonians of the genus Ellisella Gray, 1958 are widely distributed from tropical to temperate waters (Fabricius & Alderslade, 2001; Weinberg & Grasshoff, 2005). In the Mediterranean Sea, this genus is represented only by one species, Ellisella paraplexauroides Stiasny, 1936 (family Ellisellidae), forming brick-red candlestick colonies, up to 2 m high (Carpine & Grasshoff, 1975). The branched colonies have thin, whip-like ramifications arising from a common short stem, heading parallel upwards and dividing dichotomously (Fabricius & Alderslade, 2001). The unbranched or sparsely branched specimens of this species can be confused with Viminella flagellum (Johnson, 1863) (Carpine & Grasshoff, 1975; Fabricius & Alderslade, 2001; Giusti et al., in press), the second ellisellid coral found in the Mediterranean basin, but are easily distinguishable by the sclerites shape and for the red colour, contrasting the white-yellow appearance of V. flagellum.

Ellisella paraplexauroides, similar to V. flagellum, has an Atlanto-Mediterranean distribution (Vafidis et al., 1994), however, in contrast with this species, it has been reported in southernmost warm temperate oceanic waters along the western coasts of Africa (Table 1). The first record of E. paraplexauroides was made by Stiasny (1936) from Cap Blanco (Mauritania). Following this, the species was reported in several other Atlantic localities including the coasts of Portugal (Stiasny, 1939a), the Gulf of Cádiz (Arroyo Tenorio et al., 2008; Aguiliar et al., 2010), several sites along the Moroccan coast (Stiasny, 1939b; Grasshoff, 1972, 1992) and in the whole Canary Archipelago (Brito et al., 1984; Brito, 1985; Arístegui et al., 1987; Grasshoff, 1992; Ocaña et al., 1992; Bianchi et al., 2000; Brito & Ocaña, 2004). It also occurs further south along the African coast (apart from the type locality): in Senegal (Grasshoff, 1992; Miralles et al., 1992), Ivory Coast, Ghana, Nigeria and Angola (Grasshoff, 1992). No records have been given so far for the Azores, Madeira Islands, Great Meteor and Josephine Seamounts and Cape Verde Archipelago, where, on the contrary, V. flagellum was found. This led Grasshoff (1972) to suppose that E. paraplexauroides was mainly distributed along the continental coasts, excluding the Canary Islands.

Table 1. Biogeographical records for Ellisella paraplexauroides.

Ellisella paraplexauroides has been firstly reported in the Mediterranean Sea by Stiasny (1940), examining fragments collected along the Algerian and Tunisian coasts (Grasshoff, 1992). Most of the following records within the Mediterranean Sea came from the Gibraltar Strait (Ocaña et al., 2000; Brito & Ocaña, 2004; Arroyo Tenorio et al., 2008; Torres Gavilá, 2008), Alborán Island (Templado et al., 1986, 2006; Aguiliar et al., 2008), Alborán Sea (Aguiliar et al., 2008) and along the Moroccan coast, in the Chafarinas Islands, where several biological observations were made by SCUBA diving on a rich shallow water population of E. paraplexauroides (López et al., 1996; Peña Cantero & García Carrascosa, 2002; Castellanos et al., 2003; González García et al., 2005; Aguiliar et al., 2008; Torres Gavilá, 2008; Tocino et al., 2009). A further record was documented from the central Mediterranean Sea, at Sidi Daoud (Tunisia) (Mustapha et al., 2004). Finally, Stiasny (1942) reported E. paraplexauroides also from the Bay of Naples (representing the northernmost record so far) even if, on the basis of the known distribution of the species, this record was generally considered doubtful (Grasshoff, 1972) (Figure 1).

Fig. 1. Map of the geographical distribution of Ellisella paraplexauroides (black dots) in the Mediterranean Sea. Black triangle represents the present record.

Few data are available concerning biology and ecology of E. paraplexauroides. This has probably resulted from the analysis of small fragments only (Carpine & Grasshoff, 1975; Ocaña et al., 1992) or from investigations focused on the associated fauna (López et al., 1996; Peña Cantero & García Carrascosa, 2002; Aguiliar et al., 2008; Torres Gavilá, 2008), while in situ observations have been rarely dedicated to this species (Bianchi et al., 2000; Mustapha et al., 2004; Aguiliar et al., 2006, 2008, 2010).

Typical of the circalittoral plain, E. paraplexauroides may show a wide bathymetric distribution ranging from 15 to 690 m depth, with the shallowest records found mainly in the Mediterranean Sea and the deepest specimens found in the Atlantic Ocean. In particular, it has been reported on the shelf's edge at mesophotic depths between 50 and 150 m (Templado et al., 1986, 2006; Tocino et al., 2009), but it can also live at bathyal depths, like in the Canary Islands, where it was observed from 63 to 690 m, mainly associated with the assemblages of the scleractinian Dendrophyllia ramea (Linnaeus, 1758) (Brito & Ocaña, 2004; Arroyo Tenorio et al., 2008). In the Canary Islands it has also been observed that the complexity of the branching pattern and the diameter of the ramifications depends on the type of substrate, as a result the colonies are thinner and sparsely branched when they live on unstable bottoms (Brito & Ocaña, 2004).

In the Gulf of Cádiz, Gibraltar Strait and Alborán Sea there are some sites where E. paraplexauroides was observed in very shallow waters, respectively 15–30 m along the coastline of Cádiz (Aguiliar et al., 2010), 15–35 m depth between the Posidonia oceanica ((L.) Delile, 1813) mattes, at Chafarinas Islands (González García et al., 2005) and 20–25 m depth in Ceuta, along the Moroccan coast (Ocaña et al., 2000; Brito & Ocaña, 2004).

Quantitative data on the populations of E. paraplexauroides have never been reported in the literature. This species has always been considered very rare and occasional (Grasshoff, 1992; Arroyo Tenorio et al., 2008) and never forming dense meadows, as in the case of V. flagellum (Grasshoff, 1971, 1972, 1977). The richest population known so far is the one at Chafarinas Islands (González García et al., 2005). The tallest colonies reported so far, are from the Mediterranean Sea (Grasshoff, 1992).

The colonies may host numerous epibionts, especially in their basal part and on larger ramifications. The associated fauna, occupying dead portions of the colonies, is mainly composed of encrusting algae, bryozoans (González García et al., 2005), hydroids (Peña Cantero & García Carrascosa, 2002), stoloniferans such as Sarcodictyon canariensis Ocaña, Brito & Nunez, 1992 (Ocaña et al., 1992), and ascidians such as Pycnoclavella taureanensis Brunetti, 1991 and Clavelina nana Lahille, 1890 (Aguiliar, 2004; Tocino et al., 2009). Crustacean isopods of the genus Paranthura (Castellanos et al., 2003) and numerous species of polychaetes (López et al., 1996; Torres Gavilá, 2008) are the main components of the vagile fauna. The number of colonies with epibionts or dead colonies of E. paraplexauroides tends to decrease depending on the depth (Tocino et al., 2009).

The aim of this paper is to provide the first detailed record of E. paraplexauroides for the Italian waters. Moreover, some remarks on its distribution, biology and ecology are given on the basis of remotely operated vehicle (ROV) video and photographic samplings and specimens collection.

MATERIALS AND METHODS

The Island of Pantelleria lies along the continental rift system of the Sicily Strait (central Mediterranean Sea). It consists exclusively of volcanic rocks (Villari, 1974) and the entire area is subjected to intensive volcano–tectonic activity. The structural setting of the island is specified by faults and fractures that follow the regional north-west–south-east and north-east–south-west trends (Civetta et al., 1988).

The study site, named Cala Caruscia (locality Punta Spadillo) (36°49.139′N 12°01.537′E), is located on the north-east side of the island (Figure 2) and was explored through a ROV survey conducted in May 2010 on-board the RV ‘Astrea’. The site is characterized by inclined rocky walls down to about 200 m depth with the bottom first turning into heavily silted rocky boulders and then ending in a gently inclined soft-bottom slope.

Fig. 2. Map of the sampling area (black dot) on the north-east coast of Pantelleria Island (Sicily Strait).

The ROV ‘Polluce’ was equipped with a digital camera (Nikon D80, 10 megapixel), an underwater strobe (Nikon SB 400), a high definition video camera (Sony HDR-HC7) and a 3-jaw grabber (SeaBotix Inc.) to take samples. Additionally, it had a depth sensor, a compass, and three parallel laser beams 10 cm apart used as a reference scale to assist the observers in estimating: frame area, sizes (height and width) and branching pattern of what was photographed using the software ImageJ, (Brazeau & Lasker, 1988). Colonies of Ellisella paraplexauroides were photographed in the mesophotic zone, between 80 and 94 m depth. Photographs were then analysed to describe the general morphology of the coral colonies and their in vivo appearance. Direct sampling of one specimen was carried out for the species description. Sclerites were analysed with scanning electron microscopy (SEM) after dissolving the coenenchyme in sodium hypochlorite and lastly, a fragment fixed in 95% ethanol was used for the morphological analysis of polyps carried out with SEM and stereomicroscope.

RESULTS

The coral community of Pantelleria Island is mainly composed of a mixed assemblage of three-dimensional anthozoans. The dominant species up to 80 m depth are the gorgonians Paramuricea clavata (Risso, 1826), Eunicella cavolinii (Koch, 1887), Corallium rubrum (Linnaeus, 1758) and, occasionally, the zoanthid Savalia savaglia (Bertoloni, 1819). Sparse colonies of the black coral Antipathella subpinnata (Ellis & Solander, 1786) were observed around 100 m depth, while the gorgonians Viminella flagellum and Swiftia pallida Madsen, 1970 were typically distributed in the deepest depth range, below 130 m depth (Giusti et al., in press). In total, five red brick colonies of Ellisella paraplexauroides were found scattered on the explored hard substrate (Figure 3A–D), between 80 and 94 m depth.

Colonies were anchored to the substrata with a large basal plate, about 10 cm wide, sometimes covered by fine sediments and other organisms such as sponges or encrusting algae (Figure 3A). The common stem arising from the plate was about 15 cm long and showed a 2–3 cm wide basal diameter, not bearing polyps. Observed colonies reached considerable dimensions, from 83 to 173 cm in height (130 ± 35 cm average) and from 12 and 42 cm in width (25 ± 12 cm average), being taller than larger because of the typical upward arrangement of their branches (Figure 3A–D). The colonies could show up to 30 terminal branches (basal diameter around 3 mm).

Fig. 3. Ellisella paraplexauroides from Pantelleria Island. (A–D) Underwater photographs of E. paraplexauroides. Occasionally, colonies can be colonized by sessile organisms (Figure D); (E) expanded polyps of a living colony; (F) scanning electron microscopy image of the arrangement of calyxes on the apical portion of a branch; (G) stereomicroscope photograph of the sclerites, both yellow coloured and colourless; (H) sclerites of E. paraplexauroides composed of: a, double head from the superficial layer of coenenchyme; b, thick capstan; c, thick capstans with high tubercles; d, girdle spindles; e, slender girdle spindle; f, long rods from the deeper layer of the coenenchyme and from calyxes; g, rods from the pharynx. Scale bars: A–E, 10 cm; F, 1 mm; G, 50 µm.

Only one colony showed the basal portion of some branches partially dead (Figure 3D). The naked skeleton was covered by numerous hydroids (unidentified sertularids and eudendrids entrapping a notable quantity of sediment), a colony of Paramuricea clavata, the polychaete Filograna sp. and various encrusting sponges and bryozoans.

During the ROV survey, a sample 35 cm long was collected from a sparsely branched colony 110 cm high. Coenenchyme was red, while polyps, monomorphic and highly contractile, were crowded, cylindrical and whitish. Underwater images showed extended polyps characterized by tentacles almost half the length of the calyxes (Figure 3E). Along the studied apical portion, polyps were arranged in a few longitudinal rows. However, photographs showed multiple rows of calyxes in the most basal parts of the colonies, as already reported in previous descriptions (Grasshoff, 1972; Carpine & Grasshoff, 1975). Polyps emerged from the branches with a sharp inclination and could show a different orientation when compared to the main stem (Figure 3F). Calyxes (up to 1.5 mm high) (Figure 3E, F) were characterized by a thin calyx wall. When contracted, the polyps formed knobs on the surface of the branches.

Surface sclerites were orange-yellow, while subsurface ones were almost colourless (Figure 3G). Sclerites included several types (Figure 3H): symmetrical double heads and thick capstans (up to 50 µm high) (Figure 3Ha-b) were densely packed on the surface of the coenenchyme; thick highly tuberculated capstans (between 60 and 70 µm high) (Figure 3Hc) were organized in the subsurface of the coenenchyme; spindles (between 60 and 80 µm high) were found in the wall of calyxes (Figure 3Hd-e); and 50 µm high rods with few tubercles were found in the pharynx of the anthocodiae (Figure 3Hf-g). Sizes of the sclerites were slightly smaller than what has already been reported for some Atlantic specimens (Carpine & Grasshoff, 1975), but quite similar to that reported for the Canary Islands (Brito & Ocaña, 2004).

DISCUSSION

The present record of Ellisella paraplexauroides adds a new species of alcyonacean to the Italian fauna (Morri et al., 2008) (which now accounts for 27 species) and confirms the presence of this species in the central Mediterranean Sea. This record represents also the most eastern report of E. paraplexauroides in the Mediterranean basin. The ROV observations on the small population found in the mesophotic zone of Pantelleria Island confirm previous data regarding size, colour, branching pattern and associated fauna (Grasshoff, 1972; Carpine & Grasshoff, 1975; Arroyo Tenorio et al., 2008). The colonies observed in Pantelleria are also some of the tallest reported in the literature.

Pantelleria Island is situated in a unique area of the Mediterranean Sea, both from the oceanographic and topographic point of view, leading ultimately to an important role in the composition of the benthic fauna (Manzella et al., 1988; Greenpeace International, 2009). In the Sicily Strait, representing the connection between the eastern and western basins of the Mediterranean Sea, two water layers have been observed. The most superficial is composed of the surface Atlantic Waters (AW), running eastward, while the deepest one by the Levantine Intermediate Waters (LIW) flowing in the opposite direction, down to about 500 m (Napolitano et al., 2003). The particular hydrographic conditions of this area probably favour the presence of rich benthic communities of filter-feeding organisms (Bianchi & Morri, 2000).

Ellisella paraplexauroides is a typical Atlanto-Mediterranean species with a wide bathymetric distribution range. The superficial variations in temperature of the Mediterranean waters probably confine this species below 50 m depth in the Sicily Strait, although some populations were locally recorded at very shallow depths (15–35 m in the Gulf of Cádiz, Strait of Gibraltar and the Alborán Sea) (Arroyo Tenorio et al., 2008).

The scattered distribution of E. paraplexauroides in the explored area was observed in a limited depth range, being the species confined below the distribution limit of the most common gorgonians (Paramuricea clavata and Eunicella cavolinii) and above the distribution range of Viminella flagellum (Giusti et al., in press). Although this species is occasionally considered as characterizing a facies (Gulf of Cádiz, Atlantic Ocean) (Aguiliar et al., 2010), it never occurs in dense monospecific meadows and can be considered an occasional element in the lower fringe of the circalittoral assemblage.

The recorded susceptibility of E. paraplexauroides to epibiosis may be enhanced by mechanical injuries of the coenenchyme, as already suggested for other deep Mediterranean corals (Tocino et al., 2009). Commercial fishing activities, for example through long-line fishing, directly damage these corals that are particularly vulnerable because of their arborescent morphology, fragile construction and the general slow growth rate (Bavestrello et al., 1997; Mortensen & Buhl-Mortensen, 2004; Bo et al., 2009).

It is well known that assemblages of colonial anthozoans represent biodiversity hotspots (Mortensen & Buhl-Mortensen, 2004; Ballesteros, 2006; Bo et al., 2009; Cerrano et al., 2010), adding three-dimensional complexity to the habitat and commonly housing a rich associated fauna as well as a great abundance of fish (Tsounis at al., 2006; Mortensen et al., 2008). In the Mediterranean Sea, these communities have been studied mainly in shallow waters, and few data are available for deep environments (Bo et al., 2009, 2010, 2011; Gori et al., 2010).

The complex structure and richness of species of the assemblages hosting E. paraplexauroides, together with their vulnerability to human activities, should motivate the decision of including them within the Mediterranean species and habitat list of conservational relevance.