Isidella elongata (Cnidaria: Alcyonacea) facies in the western Mediterranean Sea: visual surveys and descriptions of its ecological role

Abstract Isidella elongata is a candelabrum-shaped alcyonacean forming important facies on the bathyal muddy bottoms of the Mediterranean Sea, currently considered a sensitive habitat and heavily impacted by deep-sea fisheries. Until a few decades ago, this facies was a widespread habitat of the deep Mediterranean seabed and I. elongata was a common species in the trawling fishery’s bycatch. Despite its current persistence in dense aggregations being very scarce, a dense facies of I. elongata was revealed during several ROV (Remotely Operated Vehicle) surveys carried out from 2010 to 2014 on the muddy bottoms between two seamounts east of Ibiza (Balearic Sea). The facies developed in an area between 480 and 615 m in depth where trawling is forbidden, with an extraordinary density of about 2300–2683 colonies/ha, representing one of the biggest facies of I. elongata currently known for the Mediterranean Sea considering the surface covered and the colonies’ density. The associated community was surveyed, with 50 taxa identified. Moreover, a canyon southwest of Formentera characterised by the presence of I. elongata together with a high trawling impact was investigated. The density of the colonies was 53–62 colonies/ha and only 19 taxa of associated fauna were observed. The results of the two areas are compared and discussed in the framework of the protection of such an important habitat.


Introduction
Isidella elongata (Esper, 1788), also known as bamboo coral, is a deep-sea alcyonacean belonging to the Isididae family. In the Mediterranean Sea, this species can characterise a facies on bathyal compact mud between 500 and 1200 m depth (occasionally shallower, up to 210 m in depth) on relatively flat bottoms with a slope of less than 5% (Pérès & Picard 1964;Pérès 1967;Bellan-Santini 1985;Laubier & Emig 1993;Bo et al. 2015). Grasshoff (1989) defined I. elongata as a near-endemic species in the Mediterranean Sea, although it has also been collected in the Ibero-Moroccan Gulf (Atlantic Ocean). Pérès and Picard in their Nouveau manuel de Bionomie benthique de la Mer Mediterranee (1964) wrote about a facies of the biocoenosis of the bathyal mud (VP) characterised by the branched gorgonian I. elongata settled on firm and compact mud, but Pérès himself (1967) later regretted not having any images showing such facies. Carpine (1970) confirms that I. elongata is (or maybe was) the most common gorgonian species in the middle horizon of the bathyal zone of the Western Mediterranean Sea, and Laubier and Emig (1993) emphasised the ecological role of such deepsea species not only as a habitat former, but also as a secondary biological hard substratum for other species such as epibiotic species or as a spawning substratum for cephalopods and sharks.
The real exploitation of deep-sea habitats in the Mediterranean Sea only started in the first few decades of the last century with the relentless development of trawl fishing activities mainly to capture red shrimps Aristeus antennatus and Aristaemorpha foliacea as well as the Norway lobster Nephrops norvegicus (Sardà et al. 2004). However, with the advance in fishing gear technology, trawling activities began what became the most devastating impact on these particular deep-sea habitats. Among the habitats affected by the deep-water fisheries in the Mediterranean Sea, the biocoenosis of bathyal mud and in particular the viscous mud with a very fluid superficial layer (250-500 m in depth) as well as the firm and compact mud (500-1000 m in depth), sensu Pérès and Picard (1964), proved to be the most damaged. The benthic facies of the sea pen Funiculina quadrangularis and of the alcyonacean I. elongata, that both develop on the above-cited biocoenosis, almost completely disappeared from most of the trawlable bottoms of the Mediterranean Sea (D'Onghia et al. 2003;Sardà et al. 2006;Mastrototaro et al. 2013Mastrototaro et al. , 2015. For this reason, coral forests sensu lato (including sea pens and alcyonacean gardens) were identified as Vulnerable Marine Ecosystems in the FAO's (Food and Agriculture Organization) International guideline for the management of the deep sea fisheries in the high seas (FAO 2009(FAO , 2011. At the same time the General Fisheries Commission for the Mediterranean (GFCM) included I. elongata facies (IF) on the list of Sensitive Habitats (GFCM 2009). Mapping vulnerable marine ecosystems is considered the first and an indispensable step in the framework of environmental protection, as declared in the European Marine Strategy Framework Directive (2008/56/EC), aiming to preserve and restore marine biodiversity and reach good environmental status of the marine environment in 2020 (the so-called Horizon 2020).
In spite of the widespread decline of Mediterranean muddy-bottom facies characterised by sessile organisms such as sea pens and alcyonaceans, these peculiar and fragile bathyal facies have been accidentally preserved in some areasfor instance, where trawling activities are not possible such as in areas near deepsea coral banks (Mastrototaro et al. 2013(Mastrototaro et al. , 2015 or canyon flanks (Fabri et al. 2014), as well as where trawling is forbidden due to the presence of submarine cables or pipelines. This is the case of the muddy bottoms located 20 nautical miles east off Ibiza (Balearic Sea, Western Mediterranean) where trawling activities are forbidden because of submarine electricity cables (Acosta et al. 2004). Several ROV dives on these accidentally protected areas revealed the presence of a wide IF between 480 and 615 m in depth. ROV (Remotely Operated Vehicle) visual surveys provided data on the conservation status and the density of the I. elongata population, and allowed observation of the associated biodiversity and the real distribution of the species in this particular facies. Moreover, some peculiar behaviours of uncommon deep-sea species, most of them considered rare and very scarcely known (Fabri et al. 2014), were observed. The density of the colonies and the associated animal community in the untrawled area was then compared with the ones observed in a nearby area, with comparable depth and environmental features, butunlike the first oneaffected by trawling activities.

Study area
The study area is located in the Balearic Promontory, in the block formed by the islands of Ibiza and Formentera. The eastern flank of this block is characterised by two seamounts, located on the upper slope east of Ibiza, Ausias March and Oliva Bank (Canals et al. 1982). Both seamounts seem to be affected by a NE-SW oriented fault system (Acosta et al. 2001a). The flat summits of the seamounts appear to rest at two different bathymetric levels, bounded by faults, which could indicate relative displacements between the seamounts (Acosta et al. 2001b). In particular, the investigated area lies in the Mallorca Channel, between Ausias March and Oliva Bank seamounts (Figure 1), in a muddy area covering approximately 80 km 2 , where trawling activity is forbidden because of the presence of marine cables. This area is characterised by large pockmarks from 400 to 750 m in depth (Acosta et al. 2004) and it was identified as a gas seepage zone by the Spanish Institute of Oceanography (IEO 2005). Seabed features such as the consistency and compactness of the mud make this area feasible for the settling of I. elongata.
A second area, located in a canyon southwest of Formentera (close to Emile Baudot escarpment, about 20 nautical miles south-west of the main surveyed area) was also investigated ( Figure 1). This area proved of comparable depth and edaphic features with respect to the first one, as well as being characterised by the presence of I. elongata. Differently from the first area, this second one was affected by trawling impacts, and it was investigated in order to compare its I. elongata population and benthic community with those found in the untrawled area.

Material and methods
The morpho-bathymetric data were collected using Simrad EM-12S and EM-1000 multibeam systems operating at frequencies of 12 and 100 kHz, respectively. The survey was designed in order to insonify 100% of the seafloor by overlapping the outer beams between tracks. Positioning was obtained via a double-frequency differential correction GPS (Global Positioning System) system using satellite corrections from the Skyfix and Omnistar systems integrated in a Konmap navigation software package.
Multibeam data were processed using Neptune from Kongsber. Fledermaus 7.0 from IVS and ArcGis 10.2 from ESRI were used for analysis and editing. The digital elevation model (DEM) used in the morphometric analysis is shown in colour in Figure 1(b) and (c). The extension of this DEM and subsequent digital terrain models (DTM), shown in Figure 2, was 825 km 2 . The final data resolution was 50 × 50 m in the geographic system WGS84 UTM31 N.
Video footage was gathered between 2006 and 2014, when 58 ROV dives were performed on and around the Mallorca Channel seamounts. IF was found during five of these dives (Figure 1), carried out within the research cruises BALSEA 2010, BALBAU 2013 and BALSEA 2014. In particular, four ROV dives were carried out in the area between the Ausias March and Ses Olives seamounts (untrawled area), for a total of 6 hours and 8 minutes of underwater video, while only one ROV dive was carried out in a canyon southeast of Formentera (trawled area), with 1 hour and 15 minutes of recording (Table I). All the ROV surveys were performed from the Ketch Catamaran Ranger using a Saab Seaeye Falcon DR ROV equipped with an HDV (High Definition Video) camera of 480 TVL (Tele Vision Lines) with Minimum Scene Illumination of 2.0 lux (F1.4), a ½″ CCD (Charge-Coupled Device) pick-up device, an image sensor, and a 3.8-mm spherical and wide angle lenses. The ROV performed transects from 1 to 2 km in length with an average speed of 0.2-0.3 knots, by recording at the same time in both highdefinition (HD) and low-resolution modes. The average speed and wide angle of the camera filming an aisle of approximately 1.5-1.75 m allowed the observation of approximately 1500-1750 m 2 in each kilometre of transect. The position of the ROV was continuously recorded using a LinkQuest Tracklink USBL Transponder with up to 0.25°accuracy.
All the colonies of I. elongata observed were counted. Considering the ROV track length and the camera objective width, the density of the colonies was estimated in colonies per hectare by considering a range of values based on the minimum and maximum of the wide angle of the camera. Moreover, every specimen belonging to other taxa was identified at the lowest possible taxonomic level, and one or more frames were taken from the video to provide a visual identification of each taxa. Specific identification by images was driven by expert opinions (see Acknowledgements).
Considering the different sampling efforts between the two surveyed areas, a quantitative comparison of I. elongata density and only a qualitative comparison of the associated fauna were performed.

Results
On the muddy seabed of the Mallorca Channel, between Ausias March and Oliva Bank seamounts (untrawled area), a wide IF was observed on an area of about 0.72-0.84 ha, from 485 to 616 m depth (Table I; Figure 3(a)). A total of 1932 I. elongata colonies were counted, estimating a mean density of 2300-2683 colonies per hectare (Table II). Most of the colonies observed were in good development    conditions, large and tall (up to 40 cm), with the typical candelabrum-shaped morphology, numerous branches and open polyps (Figure 3(b)). The alternation of white carbonatic internodes and brown organic nodes was evident along the ramifications (Carpine & Grasshoff 1973). The high number of living polyps confirmed the healthy condition of the colonies (Figure 3(c)). Although fishing activity is forbidden, some trawl marks and longlines were observed, together with some plastic litter ( Figure 4). One of the submarine cables present in the area, colonised by some Caryophylliidae corals, was observed (Figure 3(d) and (e)). Close to the cable, a trawl mark was present too (Figure 3(d)).
Isidella elongata colonies also occurred in the canyon southwest of Formentera (trawled area), with a density of about 53-62 colonies per hectare (Table II). In this area the colonies were mostly small (up to 15-20 cm), and young or damaged, with a low number of branches (Figure 3(f)).
Considering the associate community, a total of 50 taxa (one Foraminifera, three Porifera, seven Cnidaria, one Sipuncula, two Mollusca, two Annelida, 14 Crustacea, two Bryozoa, four Echinodermata and 15 Pisces) were observed living on the I. elongata colonies as epibionts, near the colonies or around them in the untrawled area of the Mallorca Channel, while only 19 taxa (one Foraminifera, three Porifera, five Cnidaria, one Mollusca, three Crustacea, one Echinodermata and five Pisces) were observed in the trawled area, 16 of them in common with the untrawled area (Table III).

Foraminifera
Arborescent colonial Foraminifera, which seems to belong to the genus Pelosina (courtesy D. Violante), was found in both areas (untrawled and trawled) ( Figure 5(a)). Although we were not able to collect a sample, it was tentatively identified as Pelosina arborescens Pearcey, 1914, considering that this species is widespread along the North-East Atlantic coasts on soft muddy bottoms in both shallow and deep water (Cedhagen 1993;Alve 2009).

Porifera
Several colonies of the bathyal Demospongiae Thenea muricata (Bowerbank, 1858) (Figure 5(d) and (e)), a peculiar mushroom-shaped sponge considered one of the characteristic species of the Mediterranean bathyal mud (sensu Pérès & Picard 1964), were observed around Isidella colonies in both trawled and untrawled areas. This sponge is often stalked or rooted, as it lives on muddy bottoms in deep water (Pérès 1967) in which it can make a facies of the bathyal sandy muds (Laubier & Emig 1993). The carnivorous sponge Cladorhiza abyssicola Sars, 1872 (courtesy C. Longo; see Figure 5(b) and (c)) was also recorded in both areas, with several colonies in the IF of the pockmarks' field (untrawled area). Moreover, an unidentified massive whitish Demospongiae was also observed (Figure 5(f)).

Sipuncula
One peanut worm was observed at the base of a colony of I. elongata in the untrawled area (Figure 6 (a)). Considering its size of about 30-40 mm, the specimen seems to belong to the Sipunculus genus, a widespread peanut worm genus of the bathyal seabed (Southern 1913;Cutler 1977). 1798, were observed in the untrawled area (Figure 6(c) and (d)). The first species was documented using the bioturbated seabed and the base of the felled colonies as a refuge, while L. vulgaris was recorded swimming among the colonies. In contrast, one nudibranch belonging to the Facelinidae family was observed moving on the muddy seabed of the Formentera canyon (trawled area; Figure 6(b)). This species showed a Facelina genus' morphology and in particular it is likely to be Facelina bostoniensis (Couthouy, 1838) (courtesy M. Doneddu and E. Trainito) even though this species is usually observed in shallower waters (Lipej et al. 2008).

Annelida
Sessile polychaetes were observed in the untrawled area, but their specific identification was not possible with visual surveys alone. In particular, several specimens of Filogranula sp. (courtesy R. Sanfilippo) were observed on I. elongata branches (Figure 7(g)) and small Serpulidae in the central part of some colonies of the bryozoan Kinetoskias sp. colonies (Figure 7(a) and (b)).

Crustacea
Nine Decapoda species were identified in the untrawled area: the crabs Macropipus tuberculatus  (Figure 6(p)) were mainly observed recovering in proximity of the small holes in the bioturbated muddy seabed as well as near the root-shaped bases of felled I. elongata. Occasionally, G. longipes, M. tuberculatus and the lobster P. mauritanicus were also observed in proximity to seabed small holes and felled Isidella colonies ( Figure 6(e-g)). The digger Norway lobster N. norvegicus (Figure 6(h)) was common along the whole IF, as confirmed by the high number of its characteristic burrows. The particular trophic behaviour of the crab A. rissoana, climbing on the I. elongata colonies to catch its small preys, was also observed ( Figure 6(j-n)).
Specimens of Pasiphaeidae (Figure 6(q)), probably Pasiphaea sivado (Risso, 1816), were observed swimming around I. elongata colonies together with many other small crustaceans. Moreover, a great number of small crustaceans were observed around the Isidella colonies, but due to their small size and their frenetic activity, specific identification was possible only to the orders Mysida (Figure 6(s)) and Euphausiacea, and subclass Copepoda. Several Ampeliscidae tubes were observed on the muddy seabed ( Figure 6(t)).
No crustaceans were observed in the trawled area, except for one specimen of Munida sp. and some swimming Mysida and Pasiphaeaidae.

Echinodermata
Four echinoderms were observed within the untrawled area: the crinoid Antedon mediterranea (Lamarck, 1816); a small Ophiurida, likely to be Ophiura ophiura (Linnaeus, 1758) (Figure 7(f)); an Elasipodida species, tentatively identified as Penilpidia ludwigi (von Marenzeller, 1893) (courtesy A. Gebruk); and another undetermined holothurian species only detected by the observation of its tentacles arising from the mud (Figure 7(e)). In particular, A. mediterranea used Isidella branches to arise from the muddy bottom (Figure 7(g)), while several specimens of P. ludwigi were observed moving on the muddy bottom and sometimes swimming with a characteristic "S" movement (Figure 7(h-n)). In some of these specimens, the two anteriormost pairs of dorsal papillae forming a small velum (lobe) were particularly evident (Figure 7(h-n)). On the contrary, only one specimen of A. mediterranea was observed in the trawled area south-west of Formentera. Figure 7. Bryozoans, polychaetes and echinoderms observed. In particular, (a,b) Kinetoskias sp. with Serpulidae in the central part of the colony with (c) scheme of the genus Kinetoskias, (d) unidentified branched bryozoan at the base of a Isidella elongata colony, (e) undetermined holothurian tentacles, (f) Ophiurae species, probably Ophiura ophiura, (g) Antedon mediterranea with Filogranula sp., (h-j) Penilpidia ludwigi with (k-n) swimming sequences.

Pisces
A total of 15 fishes were observed in the untrawled area: two Chondrichthyes and 13 Osteichthyes.
Considering  (Figure 8(d-q)). In particular, C. agassizi and H. mediterraneus mediterraneus were observed in small schools of about 3-4 specimens near the seabed within the IF (Figure 8(e)). Solitary specimens of P. blennoides, S. phaeton, L. boscii and H. dactylopterus were also observed standing on the muddy bottom, while P. blennoides was mainly present near the base of I. elongata and occasionally near some plastic or other marine litter acting as possible refuge (Figure 4  (a-h)). On the contrary, G. argenteus, M. merluccius and M. potassou were seen swimming among the colonies of I. elongata. The observation of the behaviour of B. robustus and S. boa boa was particularly interesting: the former camouflaged itself swimming vertically behind the branches of Isidella (Figure 8(p) and (q)), and the latter stood vertically in the water column (Figure 8(o)). Only four of these species were also observed in the trawled area (Table III).
Dense groups of Myctophidae were also recorded, likely to be Benthosema glaciale (Reinhardt, 1837) and Ceratoscopelus maderensis (Lowe, 1839) in the untrawled and trawled areas, respectively, even if their specific identification is uncertain.

Discussion
The seabed morphology of the study area is the result of a geological setting where tectonics controls an important fluid seepage, as can be deduced by the presence of numerous pockmark lineations. Both seamounts seem to be affected by a NE-SW-oriented fault system, and a NW-SE gully system (Figure 2). With the exception of the seamount flanks, slope values are low (Figure 2), as is the sediment input (Zúñiga et al. 2007).
The deep-sea environment of this particular area of the Mediterranean Sea has been explored, for the first time, using an ROV. The most relevant outcome is the discovery of a huge IF located on the muddy seabed between the Ausias March and Ses Olives seamounts. The seabed between the two seamounts proved to be the preferential location for I. elongata (Figure 1). This facies was characterised by an extraordinarily high density of colonies, representing one of the last and most well-developed IF in the Mediterranean Sea and the largest one known so far in this basin, considering its extension and density of colonies (Bo et al. 2015).
Studies on bamboo coral densities are scarce. Buhl-Mortensen and Buhl-Mortensen (2014) estimated up to 16,700 colonies per hectare for a congeneric species, Isidella lofotensis, in specific places of Hardangerfjord in the North-East Atlantic. Farther north, the estimations are even higher, to around 57,000 colonies per hectare in the upper continental slope of East Greenland (Mayer & Piepenburg 1996). Dense bamboo coral forests on muddy bottoms have also been reported for other species of Isididae, such as Keratoisis sp. in Baffin Bay (NW Atlantic; Neves et al. 2015).
The high densities that I. lofotensis and other bamboo corals can reach in some areas of the Atlantic are very far from those of the Mediterranean facies, this latter being considerably lower in comparison. Cartes et al. (2013) described a dense coral forest of I. elongata at 620 m depth along the Catalonian coasts, some 110 nautical miles away from the one described here, with a highest density of 225 colonies per hectare. This value is about 10 times lower than the average density found in the pockmarked field described here (untrawled area).
Recently, a well-conserved IF has also been found in surprisingly "shallow" waters, at about 200-210 m in depth, in south-west Sardinia (Italy, Western Mediterranean) (Bo et al. 2015), with an average density of 5000 colonies/ha. The colonies were healthy and with an average size of 36 cm tall and 22 cm wide, highly branched, very similar to the ones observed in the Balearic pockmarked field, even if it seems to occur in a smaller area and the colonies' density was about 2 times higher than the one here reported.
Visual surveys allowed us to deepen the ecological role of I. elongata from different points of view, considering that Isidella gardens are more complex and heterogeneous compared to the flat monotonous muddy bottoms on which they are settled. In fact, the arborescent morphology of I. elongata and its tendency to form dense facies makes this alcyonacean a true habitat-forming species.
On the contrary, the density in the Formentera canyon, where trawling is allowed, was considerably lower (about 38 times lower). This was confirmed by the unhealthy conditions of most of the colonies, their low number of branches, the presence of mainly young colonies and their small size due to mechanical impacts of fishing activities.
As a habitat former (Buhl-Mortensen et al. 2010), I. elongata contributes to three-dimensionally structuring the bathyal environment, enhancing its ecological functionality and providing new ecological niches for several species, playing many different roles in the ecology of the bathyal zone. First of all, the branched colonies act as a secondary substratum for the settling of several epibionthic species such as the actinian A. dohrnii (Figure 5(g) and (h)) and the polychaetes Filogranula sp. (Figure  7(g)). Furthermore, I. elongata branches are also used as support to lay eggs by small oviparous sharks such as G. melastomus (Figure 8(a) and (b)), representing a spawning and nursery area for such species. Other species, such as the squid Loligo forbesi, have also been seen using I. elongata for spawning (Orsi-Relini et al. 2009). Bamboo coral forests also act as trophic areas where many species find preys swimming among the colonies or climbing on them. This is the case for the predators observed swimming within the facies such as the sharks G. melastomus and E. spinax and the teleost fishes such as M. merluccius, M. poutassou, P. blennoides and L. boscii. IF also represents a trophic area for the opportunistic species which use I. elongata as a feeding podium, such as the decapod A. rissoana that climbs on the colony to capture its preys, or the crinoid A. mediterranea that arises from the muddy bottom. Anamathia rissoana, often associated to I. elongata (Dieuzeide 1960;Cartes et al. 2013), has recently been recorded on the black coral L. glaberrima (Bo et al. 2015). Despite the hydroid R. incrustans being considered a typical epibiont on the carapace of A. rissoana (Vervoort 1966), it was recently reported as a missed species in the Mediterranean Sea (Gravili et al. 2015). Therefore, its occurrence here is the first documented finding of the species since 1958 (Bouillon et al. 1995;Mastrototaro et al. 2016).
Moreover, IF represents an important recovery area for many fish and invertebrate species that use the gorgonian to camouflage or protect themselves. An interesting example is provided by the ray-finned fish B. robustus that camouflages itself among the branches of Isidella swimming vertically (Figure 8 (p) and (q)). This particular behaviour was also observed by Fabri et al. (2014) in the Bourcart and Cassidaigne canyons in the Gulf of Lion, but in that case B. robustus took refuge close to another alcyonacean species, Callogorgia verticillata, or near anthipatarians such as Leiopathes glaberrima and Antipathes cf. dichotoma. Benthocometes robustus was reported by Bo et al. (2015) to be characteristic of L. glaberrima gardens, while in the North-East Atlantic this fish was observed associated with deep-sea scleractianian corals, anthipatharians and gorgonians (Fabri et al. 2014). The present finding is the first that shows B. robustus associated with I. elongata, suggesting a nonspecific association of this fish to a single anthozoan species.
Stomias boa boa was also observed swimming vertically, with its chin barbel downward, probably waiting for preys (Figure 8(o)). The presence of this bathypelagic species within the IF could be accidental/coincidental, or could be related to the presence of the facies that enhances the chances for this species to catch food.
Other interesting findings not strictly related to the presence of the IF, but more likely related to the protection of the seabed, are those of the sea pen P. carpenteri. This species has recently been reported for the Mediterranean Sea by Mastrototaro et al. (2015) in the Ionian Sea, close to another area protected from trawling and characterised by cold-water coral reefs, Santa Maria di Leuca coral province (Italy, Ionian Sea). The occurrence of the Foraminifera Pelosina sp. represents the first finding of this genus in the Mediterranean Sea. It proved very common in both areas surveyed and probably it passed undetected until now only due to its small size.
Peniplidia ludwigi represents the only Elasipoda species found so far in the Mediterranean Sea (Fiege & Liao 1996;Mecho et al. 2014). The present observations clarify its swimming behaviour (Figure 7(h-n)), suggested by Pagès et al. (2007) but never documented before (Gebruk et al. 2013). The finding of the arborescent bryozoan Kinetoskias sp. also represents one of the rare occurrences of this genus in the Mediterranean Sea, considering that this genus was first reported in 1993 by Harmelin and d'Hondt (1993) close to the Gibraltar Strait. In particular, these authors reported K. smitti Danielssen, 1868 in the Alboran Sea; thus, the specimens here observed could belong to this species (Aguilar et al. 2013).
The felled colonies and especially the root-shaped bases of I. elongata act as a refuge for many reptant species such as the crustacean decapods belonging to the genera Munida and Plesionika, the lobsters N. norvegicus and P. mauritanicus and also some cephalopods such as O. salutii.
The small crustaceans swimming around the colonies probably belonged to Euphasiacea and Copepoda species. These crustaceans are likely to provide food for many of the species associated to the facies, and for I. elongata itself.
The red shrimps A. antennatus and A. foliacea, often considered characteristic of the I. elongata facies (Nouar & Maurin 2001;Maynou & Cartes 2012), were not observed. However, since these Aristeidae species have been observed down to 3000 m deep off the Balearic Islands (Sardà et al. 2004;Cartes et al. 2009), it is possible that they live in deeper waters than the bathymetry explored by the present study in the Balearic Sea.
Considering the number of taxa observed, a preliminary qualitative comparison showed relevant differences between the untrawled and trawled areas. Despite many of the sessile species being still present in both the areas (e.g. sponges and cnidarians), the number of taxa observed in the IF (50 taxa) is considerably higher than in the trawled area (19 taxa). This difference in species number could likely be partially influenced by the different sampling effort; however, it can also be explained with the contraction of the habitat (reduction of I. elongata density) and the fishing impact itself occurring in the trawled area.
Despite the occurrence of some traces of illegal fishing (Figure 4(a) and (b)), the presence of a large IF suggests a certain stability of the environment and low human impact affecting it, since I. elongata is characterised by slow growth rates (Andrews et al. 2009) and a long life span (up to 400 years for the Isididae family) (Sherwood & Edinger 2009). Its actual persistence in the Mallorca Channel, not due to any protection strategy notwithstanding the unquestioned worth of protection (FAO 2009(FAO , 2011Aguiliar & Marín 2013) urges reflections about the role of its destruction and paradoxical protection by some anthropic impacts such as submarine cables.