The deep Mediterranean Sea has a wide variety of geological and ecological settings. Their faunal composition and local biodiversity are largely unknown (Danovaro et al. 2010). The western Mediterranean deep basin is no exception. It has a complex assemblage of markedly different habitats (Sardà et al. 2004), including sedimentary slopes, submarine canyons and seamounts (Company et al. 2012). The specific geomorphological characteristics of these habitats (e.g. the elevation of seamounts, the walls and axes of the submarine canyons and the inclination of the continental slopes) and associated abiotic processes (e.g. variation in oceanographic currents, hard vs. soft substratum and food availability) result in large-scale heterogeneity of the continental margin seafloor (Carpine 1970, Emig 1997, D’Onghia et al. 2003). This high habitat heterogeneity plays a major role in the establishment and maintenance of diverse faunal communities (Levin et al. 2010), which, to date, are still largely unexplored in the deep Mediterranean Sea (Bienhold et al. 2013, Mecho et al. 2014).

The shallow Mediterranean marine fauna inhabiting the shelf and upper slope areas have been studied since ancient times. Consequently, they are relatively well known at many levels (taxonomic, ecological, and biological) (Riedl 1986, Bolam et al. 2002, Danovaro and Pusceddu 2007, Coll et al. 2010). Nevertheless, because of the difficulties in sampling the deep sea, the bathyal and abyssal fauna of the Mediterranean Sea remains poorly studied (Pérès and Picard 1956a, Fredj 1974, Galil and Goren 1995, Danovaro et al. 2010, Tecchio et al. 2011a,b).

The description of the benthic fauna occurring deeper than 800 m in the Mediterranean started in the 19th century. Cruises carried out by the RN Washington (1881-1882) and SMS Pola (1890-1898) provided the first extensive descriptions of bathyal and abyssal Mediterranean fauna (Marenzeller 1893, Bartolini Baldelli 1914), including many new species of non-crustacean invertebrates. From the late 1920s to the 1960s the number of deep-sea Mediterranean research cruises decreased, resulting in limited new information (Pérès and Picard 1956a,b, Pérès 1958). Since the late 1970s, improvements in sampling methods and equipment have allowed a second period of deep-sea scientific exploration and investigation below 1000 m depth, conducted by ships such as the Bambu, Mango, and Ruth Ann in Italian waters, the RV Jean Charcot in the Alboran Sea, and the RV Garcia del Cid in the Balearic Sea.

Specimens collected by these expeditions have stimulated a number of publications and new records of species (Carpine 1970, Parenzan 1970, Reyss 1971, Fredj 1974). However, most of this deep-sea literature focuses on the dominant groups such as fishes and crustaceans, the commercial use of Mediterranean marine resources, and the management of these resources (Sardà et al. 1994, 2004, Moranta et al. 1998, Company et al. 2004, Aguzzi et al. 2009, Bahamon et al. 2009). Thus, both fish and crustaceans are well known taxonomically in comparison with other megafaunal groups, such as ascidians, sponges, echinoderms, sipunculans and echiurans (Monniot and Monniot 1975, Alvà 1987a, Uriz and Rosell 1990, Villanueva 1992, Pancucci-Papadopoulou et al. 1999, Quetglas et al. 2000).

In this context, Mediterranean Echinodermata from middle and lower slopes have been poorly studied, particularly in comparison with the Atlantic Ocean, where echinoderms are important in terms of abundance, biomass and ecosystem function (Billett 1991). The large number of investigations conducted in the Atlantic Ocean have resulted in a good taxonomic knowledge of the echinoderms (Mortensen 1903, 1927, 1943, Koehler 1921, 1927, Hérouard 1923, Hyman 1955, Sibuet 1979, Borrero Perez et al. 2003, among others). In contrast, there have only been a few studies on the taxonomy of Mediterranean deep-sea echinoderms (Marenzeller 1893, Bartolini Baldelli 1914, Tortonese 1954, 1965, Sibuet 1974, Alvà 1987b). Most reports provide only species lists; morphological descriptions are of secondary importance (Cherbonnier and Guille 1967, Alvà 1987a, 1991, Koukouras et al. 2007) or totally absent (Tortonese 1958, 1972, 1979, Pérez-Ruzafa and López-Ibor 1988, Rinelli 1998, Coll et al. 2010).

It is in this context of dispersed and relatively scarce information that we have undertaken a study of all bathyal echinoderms, including samples collected in the last five years in the northwestern Mediterranean in the framework of four different projects. New records of species and their bathymetric distributions have been added to provide a thorough review of existing data and an updated account of the taxonomy, geographical and bathymetrical distribution of bathyal echinoderms in the Mediterranean Sea.

Class ASTERIODEA de Blainville, 1830

Two species of Asteroidea were collected in our study: Ceramaster grenadensis (Perrier, 1881) (n=149) and Hymenodiscus coronata (G.O. Sars, 1872) (n=47). Both are bathyal species. Ceramaster grenadensis sampled in the present study has a wide bathymetric range (850 to 2845 m, Fig. 2). The second species, Hymenodiscus coronata shows a narrower bathymetric range (1500 to 2250 m; Fig. 2).

Order VALVATIDA Perrier, 1884
Family GONIASTERIDAE Forbes, 1841
Genus Ceramaster Verrill, 1899
Ceramaster grenadensis (Perrier, 1881)
(Fig. 3)

Pentagonaster deplasi Perrier, 1885: 34.
Pentagonaster gosselini Perrier, 1885: 35.
Pentagonaster haesitans Perrier, 1885: 36.
Ceramaster grenadensis Halpern, 1970: 213-218, Figs. 8-9.

Material: 149 specimens collected during the PROMETEO 01-02-03-04-05, BIOFUN, PROMARES and DOSMARES 01-02-03-04 cruises. Depth of occurrence: from 850 to 2845 m. Zones: western Mediterranean Sea open slope, Blanes Canyon, Cap de Creus Canyon (Table 2).

Description: Shape pentagonal to stellate, very variable (Fig. 3A, B). Body flattened dorso-ventrally. Oral and aboral surface composed by more or less tabulate hexagonal plates covered by little granules. Marginal plates thick and massive, from 18 to 22; sampling methods could remove them. R=6 to 45 mm. r=3 to 25 mm. R/r=1.54 to 2.53. Colour variable, from cream, pale-yellow to pale pink. Polygonal madreporite, well defined, larger than surrounding plates. Adambulacral plate with 4 to 6 furrow spines, outside these a series of usually four club-shaped spines and outer spines similar to internal ones. Pedicellariae valvate, scarce on aboral side, larger and more numerous on oral side near ambulacral furrow. One of the specimens collected in the present study had six arms (Fig. 3C)

Distribution: Atlantic Ocean and Mediterranean Sea (Clark and Downey 1992).

New depth range: 200-2845 m (present study). The previous reported maximum depth of distribution for this species was 2500 m in the Atlantic Ocean (Clark and Downey 1992). The previous Mediterranean Sea bathymetric range was 600-2400 m (Tortonese 1972).

Remarks: Similarities were observed between the genus Litonotaster described by Halpern (1969, 1970). However, owing to 1) the absence of the characteristic flat and thin abactinial plates of the genus Litonotaster, and 2) the presence of tabulate abactinial plates covered by granules, the marginal plate disposition, and in agreement with available literature, we consider our specimens to be Ceramaster grenadensis. Litonotaster has not been reported in the Mediterranean Sea. Great intraspecific morphological variations have been signalled for Ceramaster grenadensis in the Mediterranean (Halpern 1970, Tortonese 1972, Sibuet 1974, Alvà 1987a). It is likely that a revision of the genus Ceramaster is needed.

Order BRISINGIDA Fisher, 1928
Family BRISINGIDAE G.O. Sars, 1875
Genus Hymenodiscus Perrier, 1884
Hymenodiscus coronata (G.O. Sars, 1872)
(Fig. 4)

Brisinga coronata Sars, 1873: 102
Brissingella coronata Tortonese, 1965: 194-196, Fig. 93.

Material: 47 specimens collected during cruises PROMETEO 05, BIOFUN and DOSMARES 01-02. Depth of occurrence: from 1500 to 2250 m. Zones: western Mediterranean Sea open slope and Blanes Canyon (Table 2).

Description: Diameter of disc 11 mm; from 9 to 13 long and slender arms. Colour orange to reddish. Very difficult to collect intact, usually the disc and the arms are broken and separate (Fig. 4). Madreporite large, channelled. Gonadal region slightly to highly inflated. Abactinal arm plates rod-like. Two to four tiny, acicular furrow spines and one to two moderately long subambulacral ones.

Distribution: North Atlantic and Mediterranean Sea (Alvà 1987a).

Depth range: 100-2904 m (Bartolini Baldelli 1914).

Remarks: Description taken from (Clark and Downey 1992).

Class ECHINOIDEA Leske, 1778

Only two sea urchin species were sampled: the regular echinoid Gracilechinus elegans (Düben and Koren, 1844) (n=7) and the irregular echinoid Brissopsis lyrifera (Forbes, 1841) (n=487). Gracilechinus elegans, known in the Atlantic, has been reported for the first time in the Mediterranean Sea in the present study. It was sampled in the Blanes Canyon at 1500 m depth (Fig. 2). Other specimens were observed and collected with the ROV during the PROMARES cruise (Mecho, pers. obs.) in the lower Palamós Canyon and Blanes Canyon areas (1500 m). Brissopsis lyrifera was found over a wide bathymetric range in the present study (from 900 to 2250 m; Fig. 2). It was abundant in some canyons between 900 and 1500 m (Table 2). In contrast, only five small specimens of B. lyrifera were collected on the open slope at depths between 1750 and 2250 m (Table 2).

Order CAMARODONTA Jackson, 1912
Family ECHINIDAE Gray, 1825
Genus Gracilechinus Fell and Pawson, in Moore, 1966
Gracilechinus elegans (Düben and Koren, 1844)
(Fig. 5)

Echinus elegans Düben and Koren, 1844: 272. Koehler, 1927: 51-53, pl. XII, Fig. 12 a-g; pl. XVII, Fig. 5

Material: 7 specimens from cruises PROMETEO 04, PROMARES and DOSMARES 04. Depth of occurrence: 1500 m. Zones: Blanes Canyon and Palamós Canyon (Table 2).

Description: Diameter test 38.5 to 48.3 mm; h=25.6 to 34.7mm. Test low, from conical and flattened above to slightly flattened on both sides, usually the height of the test is more than half the diameter (Fig. 5A). Colour whitish pink to pink, sometimes a few green (Fig. 5B, C). Long primary spines usually flat at the end. One primary tubercle present on each plate, forming a very regular series from oral to aboral side; usually secondary ones form a short longitudinal series from the middle to the oral side. A small tubercle is present between the pores and the primary tubercle, but not between the pores and the end of the plate. Some miliary tubercles are present, giving a rough appearance to the test. Three pairs of pores very clear and disposed in a sharp angle. The boundary between the areas was more straight than sinuous. Periproct (Fig. 5D) covered by large irregular plates, one of them with a spine. The plates surrounding the anus are irregularly club-shaped and smaller than the other plates. Ocular plates not in contact with the periproct. No spines on the buccal plates, where pedicellariae were present and abundant. Tridentate pedicellariae have the valves flat, narrow and mesh-worked, with the edge sinuate (500 to 650 µm long). In some cases small individuals had flatter valves than larger individuals (Fig. 5E). These valves have a narrow area near the base (Fig. 5F). Globiferous pedicellariae (500 to 550 µm) usually have 1 or 2 lateral teeth on either side of the blade and a more or less round to rectangular shape (Fig. 5G-I). Ophicephalus pedicellariae, broad, sinuate and with small teeth in the edge, and an intricate mesh-work.

Distribution: North Atlantic (OBIS). First record in the Mediterranean Sea.

Depth range: 50-1710 m. (Mortensen 1943, Minin 2012). Only reported at 1500 m depth in the Mediterranean Sea (present study).

Remarks: Mortensen (1903) reported this species from the Mediterranean, but he later discarded this identification (Mortensen 1943). Alvà (1987b) described another species, Gracilechinus alexandri, in the Mediterranean Sea. Both G. elegans and G. alexandri have many similar characteristics, making their true identification difficult (Mortensen 1903, Ramírez-Llodra and Tyler 2006, Minin 2012). Furthermore, juvenile G. alexandri have characteristics that might be confused with G. elegans. It is possible that the specimen of G. alexandri reported by Alvà (1987b) was a juvenile and was a misidentification of G. elegans. The specimen is no longer available for comparison. In our specimens, the presence of one or two teeth on the globiferous pedicellariae, their narrow base and their mesh-work are similar to those described in the literature (Mortensen 1903, Minin 2012). The tubercular pattern, the periproct, the shape of the ocular and genital plates and their disposition allowed us to classify these specimens as G. elegans. Mortensen (in 1903, p. 144, pl. XX, Fig. 9) found a small form for G. elegans with tridentate pedicellariae that had more flattened and truncate blades without mesh-work. This characteristic and the overlapping range in the number of teeth in the globiferous pedicellariae (1 to 4 in G. elegans and 2 to 5 in G. alexandri) could lead to a misidentification if only one individual was available, as appears to be the case in Alvà (1987b).

Order SPATANGOIDA Agassiz, 1840a
Family BRISSIDAE Gray, 1855
Genus Brissopsis Agassiz, 1847
Brissopsis lyrifera (Forbes, 1841)
(Fig. 6)

Brissus lyrifeer Forbes, 1841: 187
Brissopsis lyrifera Tortonese 1965: 372-374

Material: 487 specimens from cruises PROMETEO 02-04-05, PROMARES and DOSMARES 01-03. Depth of occurrence: 900 to 2250 m. Zones: western Mediterranean Sea open slope, Blanes Canyon, Cap de Creus Canyon and Palamós Canyon (Table 2).

Description: Body oval, arched, sloping anteriorly. Colour from yellow to red-brown with a narrow band of ciliated dark spines which rings all five ambulacra petals on the upper surface (Fig. 6A, B). Anterior ambulacral zone slightly depressed. Periproct terminal, near aboral zone. Posterior petals shorter than the anterior ones, diverging and well separated. Globiferous pedicellariae short, ending in two long teeth. Tridentate pedicellariae of various forms, with three more or less leaf-shaped blades. Rostrate pedicellariae blade slender.

Distribution: Atlantic and Mediterranean Sea (OBIS).

New depth range: 200-2845 m (present study). The previous reported maximum depth of distribution for this species was 1650 m in the Atlantic Ocean (OBIS). Previous Mediterranean maximum depth was 1500 m (Tortonese 1965).

Remarks: Differences from Brissopsis atlantica mediterranea (Mortensen, 1913) are evident in the posterior petals: diverging and well separated in B. lyrifera and confluent on the base, as opposed to nearly parallel in B. atlantica mediterranea (Lacour and Néraudeau 2000).

Class HOLOTHUROIDEA de Blainville, 1834

The Holothuroidea was the most abundant echinoderm class sampled in this study, with a total of 813 specimens and 7 species (Table 2). Three species belonging to the order Aspidochirotida were collected: Mesothuria (Allantis) intestinalis, (Ascanius, 1805) Östergren, 1896 (n=56), Pseudostichopus occultatus, Marenzeller 1893 (n=474) and Holothuria (Panningothuria) forskali, Delle Chiaje, 1823 (n=1).

In the present study, Mesothuria intestinalis had a bathymetric range between 900 and 1750 m depth (Fig. 2). In contrast Pseudostichopus occultatus had a very narrow depth range (2000 to 2250 m; Fig. 2). This species was sampled only in open slope areas. Although one individual was collected at 2250 m depth in the Blanes Canyon, we consider this as a residual sample, based on the high number of specimens collected in the previous catch, the bad condition of the specimen and the absence of this species in other trawls conducted at this depth in the canyon. This species was sampled in great numbers at 2250 m (maximum of 145 individuals) (Table 2). Only one individual of H. forskali was sampled (850 m in the Blanes Canyon).

The order Molpadiida was represented by two species: Molpadia musculus, Risso, 1826 (n=25) and Hedingia mediterranea (Bartolini Baldelli, 1914) Tortonese, 1965 (n=11). Molpadia musculus had a bathymetric range between 900 and 1050 m depth (Fig. 2) and was sampled only on the open slope. Hedingia mediterranea had a bathymetric range between 900 and 1500 m (Fig. 2) and was sampled mainly in canyon areas.

The order Elasipodida was represented by one species Penilpidia ludwigi (von Marenzeller, 1893) (n=219). The bathymetric distribution of this species ranged from 900 to 1500 m. Most of the individuals (n=200; Table 2) were sampled by the epibenthic sledge at a single open slope site in the western Mediterranean Sea at 900 m depth. A few individuals (n=19) were reported from sediment trap samples located in the Blanes Canyon at 1200 and 1500 m depth (Fig. 2).

The order Dactylochirotida was represented by a single species: Ypsilothuria bitentaculata (Ludwig, 1893) (n=27). This species was distributed in the present study between 900 and 1350 m depth (Fig. 2) and was sampled only in an open slope area (Table 2).

Order ASPIDOCHIROTIDA
Family Synallactidae Ludwig, 1894
Genus Mesothuria Ludwig, 1894
Subgenus Allantis Heding, 1942
Mesothuria (Allantis) intestinalis (Ascanius, 1805) Östergren, 1896
(Fig. 7)

Holothuria intestinalis Ascanius 1805: 5, pl. 45
Mesothuria intestinalis Gebruk 2012: 291-391, Fig.1-9C, D

Material: 56 specimens were collected during cruises PROMETEO 02-03-04-05, BIOFUN and PROMARES. Depth of occurrence: 900 to 1750 m. Zones: western Mediterranean Sea open slope, Blanes Canyon and Cap de Creus Canyon (Table 2).

Description: Large species, up to 30 cm long (Koehler 1927). Body nearly cylindrical with both ends flattened (Fig. 7A). Mouth subventral surrounded by 20 peltate tentacles. Scattered small tube feet all over the body, more abundant near the anterior and posterior ends. Dermis usually covered by shells, skin very fragile and thin in fresh specimens. On preservation, the dermis becomes thicker and more wrinkled. Characteristic ossicles are round tables (±100 µm), more or less regular with small peripheral holes around a central hole, and with central spire built by four rods, ending in a crown of several thorns (Perrier 1898) (Fig. 7B, C). Hermaphroditic species (Hyman 1955), gonads constituted by one branched tuft attached to left side of the dorsal mesentery, with some tubules male and some female, not found ripe at the same time (Mortensen 1927). Two respiratory trees, gelatinous and transparent. The species produces a substance which gels in formaldehyde and alcohol when preserved. Specimens usually eviscerate during capture.

Distribution: Mediterranean Sea, North Atlantic and West Indian seas (Gebruk et al. 2012).

Depth range: 18-2000 m (Gebruk et al. 2012). Mediterranean depth range 20 to 1927 m (Cartes et al. 2009).

Remarks: The presence of a second Mesothuria species of the genus in the Mediterranean Sea, Mesothuria verrilli (Théel, 1886), was discarded by Gebruk et al. (2012).

Genus Pseudostichopus Ludwig, 1894
Pseudostichopus occultatus Marenzeller 1893
(Fig. 8)

Pseudostichopus occultatus Marenzeller, 1893a: 15-17, pl. 4, Fig. 9. O’Loughlin, 2005: 173-174.

Material: 474 specimens collected from cruises DOSMARES 01-02-04. Depth of occurrence: 2000 and 2250 m. Zone: western Mediterranean Sea open slope (Table 2).

Description: Specimens smaller than 40 mm long; usually with pteropods and sand encrusted in the skin giving an external vitreous structure, colour dusty brown (Fig. 8A). Body dorsally convex, flat ventrally. The specimens sampled in this study do not have the pygal furrow which is generally characteristic of the group; some authors also note the absence of a pygal furrow in some specimens. Mouth subventral surrounded by 16-20 orange peltate tentacles, anus terminal. When the encrusted material is discarded the dermis is thin. The dorsolateral tube feet are sometimes difficult to see (Fig. 8B). Muscular bands cylindrical and subdivided, visible by transparency. Calcareous ring solid, radial plates with two central and lateral projections providing a ribbon-like shape to each plate (Fig. 8C). Two respiratory trees long and slim clustering along a central strap. Usually dredged in great numbers. Ossicles present in tentacles, tube feet, respiratory trees and near anus; absent in skin and gonads. Spiny rods (150 to 350 µm) (Fig. 8D) and scarce irregular, mesh-like perforate plates. Gonads in one tuft, with long unbranched tubules arising separately along gonoduct; one dissected specimen had little tufts full of eggs free in the coelom.

Distribution: Mediterranean Sea, North Atlantic (O’Loughlin and Ahearn 2005).

Depth range: 360-4400 m (Koehler 1927). Mediterranean depth range 415 to 3624 m (Bartolini Baldelli 1914).

Remarks: O’Loughlin (2002) reconsidered the genus Pseudostichopus and classified P. occultatus as Meseres occultatus. Later, (O’Loughlin and Ahearn 2005) returned this species to the genus Pseudostichopus. The colour of tentacles and internal structures shows great variability between individuals and is not suitable as a diagnostic character.

Family HOLOTHURIIDAE Ludwig, 1894
Genus Holothuria Linnaeus, 1767
Subgenus Panningothuria Rowe, 1969
Holothuria (Panningothuria) forskali Delle Chiaje, 1823
(Fig. 9)

Holothuria forskahli Delle Chiaje, 1824: 77-116, pl. 6-8. Tortonese, 1965: 64, Fig. 23

Material: 1 specimen from cruise PROMETEO 05. Depth of occurrence: 850 m. Zone: Blanes Canyon (Table 2).

Description: 60 mm long. Cylindrical body flattened ventrally (Fig. 9A). Numerous tube feet in three or four rows. Conical papillae on its dorsal surface. Subventral mouth with about 20 stumpy, branched tentacles. Calcareous deposits scarce, as small discs in skin (Fig. 9B) and branched and curved rods in tube feet and tentacles. Colour, usually black with white spots, sometimes brown with a yellow ventral side. Cuverian tubules are present.

Distribution: Mediterranean Sea and northeast Atlantic Ocean (Pérez Ruzafa et al. 1987).

New depth range: 20-850 m depth (present study). The previous maximum depth reported for this species in the Atlantic Ocean was 348 m (Pérez Ruzafa et al. 1987). The previous Mediterranean Sea maximum depth was 193 m (Pérez Ruzafa et al. 1987).

Remarks: The one small specimen collected had a pale grey-pinkish colour. Some authors (Koehler 1921, 1927, Tortonese 1965) described deeper specimens of H. forskali as pale in colour and smaller in body length compared with shallower individuals. O’Loughlin and Paulay (2007) describe a related species to H. forskali, living at greater depths (800 m) in Australian waters.

Order MOLPADIIDA
Family MOLPADIIDAE Müller, 1850
Genus Molpadia (Cuvier, 1817) Risso, 1826
Molpadia musculus Risso, 1826
(Fig. 10)

Molpadia musculus Risso, 1826: 293. Pawson, 2001: 317-318, Fig. 2A-B.

Material: 25 specimens collected during cruises PROMETEO 01-02-03-04-05, PROMARES and DOSMARES 04. Depth of occurrence: 900 and 1050 m. Zones: only present on western Mediterranean Sea open slope (Table 2).

Description: Up to 50 mm long. Sausage-shaped, with a small tail (Fig. 10A). Terminal, mouth surrounded by 15 pink digitate tentacles with two small prolongations (Fig. 10B). Skin rough and thick, coloured from grey to dark purple due to phosphatic deposits (Fig. 10A, B). Ossicle tables have few perforations and a small solid spine (500 to 700 µm). Rosette and racquet-shape plates and anchors present (Fig. 10C). Fusiform rods (±1000 µm) always present in tail, usually also on body wall (Fig. 10D). Calcareous ring with posterior bifurcate projections on radial plates. Two long and slender respiratory trees. Ossicles and body shape could vary, but fusiform rods of the tail are diagnostic. Colour varies with the age and growth of the animal. In the early stages they are grey-white and, when grown to the adult size, the colour turns darker from the accumulation of phosphatic deposits.

Distribution: Cosmopolitan (Pawson et al. 2001).

Depth range: 35-5205 m (Pawson et al. 2001). Mediterranean Sea depth range 50 to 2500 m (Parenzan 1970).

Remarks: In the Mediterranean Sea, the maximum depth of distribution for this species was 1050 m (Tortonese 1965, Sibuet 1974, Cartes et al. 2009, Ramírez-Llodra et al. 2010, present study). However, Parezan (1970, pp. 10 and 33) sampled ten M. musculus between 2300 and 2500 m, with the RV Ruth Ann in 1969 while dredging the Ionian Sea (central Mediterranean Sea).

Family CAUDINIDAE Heding, 1931
Genus Hedingia Deichmann, 1938
Hedingia mediterranea (Bartolini Baldelli, 1914) Tortonese, 1965
(Fig. 11)

Trochostoma mediterraneum Bartolini Baldelli, 1914: 105-107, pl. 6 Figs. 9-10.

Hedingia mediterranea Tortonese 1965: 100-101, Fig. 43.

Material: 11 specimens collected from cruises PROMETEO 02-05 and PROMARES. Depth of occurrence: 900 to 1500 m. Zones: western Mediterranean Sea open slope and Blanes Canyon (Table 2).

Description: Fresh specimens pale violet or white, acquiring a yellowish white colouring when conserved (Fig. 11A, B). Body divided into two regions, an elongated body and a long caudal appendage (more than half the length of the body). Body oval, without podia. Rough skin due to calcareous plates. Anterior region wrinkled and cylindrical, with a terminal mouth. Skin without phosphatic deposits. Fifteen tentacles without digitations. Anus situated at the end of the caudal appendage. Five subdivided muscular bands visible by transparency. Ossicles very similar to H. albicans; tables (from 150 to nearly 250 µm) present all over the skin with very irregular holes and a central spine with three spiny columns (Fig. 11C-E). Smooth plates on anal papillae (Fig. 11F, G). Two respiratory trees (right and left), low-ramified and attached along the mesentery. Gonads long and unbranched tubules extending to the posterior end of the body, disposed in two tuffs attached to the mesentery on the upper part and free for the rest of their length in the coelom (Fig. 11H). Calcareous ring with five radial pieces, each with two posterior bifurcated projections and five interradial pieces (Fig. 11I, J). Tentacular ampullae long and digitate.

Distribution: Endemic from Mediterranean Sea, reported once on Tyrrhenian Sea. First citation in the western Mediterranean Sea.

Depth range: 800-1500 m (present study). The previous Mediterranean Sea depth range was 800 to 1000 m (Bartolini Baldelli 1914).

Remarks: Only one specimen has been reported previously in the Mediterranean Sea, dredged by RN Washington (1881-1882) in the Tyrrhenian Sea at 800-1000 m depth and described as Trochostoma mediterraneum by Bartolini Baldelli (1914). Later, Koehler (1927) classified the specimen as Trochostoma articum. Tortonese (1965) classified it definitively as Hedingia mediterranea. Pawson (2001) considered the specimen to be Hedingia albicans (Théel, 1886) Deichmann, 1938, and cited it in the Mediterranean. Molecular data are required for Hedingia species in order to resolve their taxonomic status.

Order ELASIPODIDA Théel, 1882
Family ELPIDIIDAE Théel, 1879
Genus Penilpidia Gebruk, 1988
Penilpidia ludwigi (von Marenzeller, 1893)
(Fig. 12)

Kolga ludwigi Marenzeller, 1893: 20-23, pl. III Fig. 7, pl. IV Fig. 8.
Penilpidia ludwigi Gebruk, 2013: 1030-1032, Fig. 1.

Material: 219 specimens from cruise PROMETEO 01 and sediment traps of PROMETEO project. Depth of occurrence: from 900 to 1500 m. Zone: western Mediterranean Sea open slope and Blanes Canyon (Table 2).

Description: Small species of 5-20 mm in length. Fragile animals with skin usually broken. Digestive tract visible by transparency (Fig. 12A). Body elongated ovoid, with ventral side flattened. Six pairs of tube feet on the posterior half of the flattened ventral sole. Three pairs of papillae are present on the dorsal side, two pairs on the anterior part of the body and one pair on the posterior part. Ten tentacles surrounding the mouth (Fig. 12B), each divided into six to eight marginal lobes. Tentacles spicules curved rods with spines (130-300 µm) at their ends and in the middle on the external side of the curve (Fig. 12C). Calcareous ring with five interlinked pieces, usually visible by transparency. Each piece has four pair of arms radiating from the centre (Fig. 12D). Arched rods with one or two spines and four spiny leg ossicles (Fig. 12E, F). Papillae spicules smooth rods (Fig. 12G). Marenzeller (1893) reports males and females, describing gonads as one tuft slender and ramified for males and short and less ramified for females.

Distribution: Endemic to the Mediterranean Sea (Pagés et al. 2007, Gebruk et al. 2013).

Depth range: 755-4766 m (Fiege and Liao 1996).

Remarks: Penilpidia ludwigi has been reported twice in the eastern Mediterranean Sea basin (Marenzeller 1893, Fiege and Liao 1996) at depths of 755 to 4766 m. Its presence was reported in the northwestern Mediterranean Sea from sediment traps at 22 m above the bottom at depths between 1200 and 1700 m in the Palamós Canyon (Pagés et al. 2007). Although a specimen has been reported from a depth of only 48 m on the southwestern coast of Portugal (Cunha de Jesus and Cancela da Fonseca 1999), there is some doubt about this identification owing to depth (very shallow) and substrate (i.e. rocky area), as well as the poor condition of the specimen. Gebruk et al. (2008, 2013) described a related species in the North Atlantic and included a re-description of the genus and its species.

Order DACTYLOCHIROTIDA Pawson and Fell (1965)
Family YPSILOTHURIIDAE Heding, 1942
Genus Ypsilothuria E. Perrier, 1886
Ypsilothuria bitentaculata (Ludwig, 1893)
(Fig. 13)

Sphaerothuria bitentaculata Ludwig, 1893:184. 1894: 141 pl. 12-14.
Ypsilothuria bitentaculata attenuata Alvà, 1991: 459-460.

Material: 27 specimens collected during cruises PROMETEO 01 to 05, PROMARES and DOSMARES 01. Depth of occurrence: 900 m to 1350 m. Zone: western Mediterranean Sea open slope (Table 2).

Description: Typically U-shaped (Fig. 13A). Two opposite siphons, oral and anal. Body wall thorny due to the presence of intricate scales, also visible with naked eye. Eight digitiform tentacles, of very unequal size, one on each side, being larger than the others. Calcareous plates visible with naked eye (Fig. 13B). Plates subcircular. Strong short spire placed near the edge of the plate (Fig. 13C). The plates are perforated by many small holes giving an irregular shape. Calcareous deposits in tentacles. Calcareous ring with eight plates. Lateral interradial plates with anterior bifurcated projections (Fig. 13D, E). The projections are often asymmetric.

Distribution: Cosmopolitan (Cherbonnier and Féral 1978).

Depth range: 225-4440 m (Cherbonnier and Féral 1978). Mediterranean Sea depth range 900 to 1560 m (Alvà 1991).

Remarks: Differs from Y. talismani in the bifurcated projections of the calcareous ring and the size of the plates (Gage et al. 1985, Alvà 1991).