The CTD profiles obtained over and near Algarrobo Bank (Fig. 2) clearly indicate that the water mass covering the summit of the bank is the Levantine Intermediate Water. The decrease in salinity and increase in temperature triggered by mixing with the inflowing Atlantic water is not appreciable below 250 m and therefore one hundred metres above the summit of the bank.

The most abundant species was the crinoid Leptometra phalangium, represented in the sample by 815 individuals. Other important species of the benthic macrofauna sorted on board (Table 1) were the bivalve Limopsis aurita, the crab Ebalia nux, the shrimps Plesionika antigai and Lophogaster typicus, the poriferan Thenea muricata and the polychaete Hyalinoecia tubicola.

The sample below the 10 mm size fraction yielded 156 species of molluscs (Table 2), of which 83 were represented by at least one live specimen (total 1772 live-taken specimens and 1656 shells) in the fraction sorted down to 0.5 mm. Considering only the live-collected molluscs, the assemblages feature a high value of the Shannon-Wiener diversity index H’_(log2)=3.60 and Pielou’s evenness index J’=0.56.

Among molluscs, the dominant species of live specimens are Limopsis aurita (676; D=38.1%), Yoldiella philippiana (275; D=15.5%), Kelliella miliaris (148; D=8.3%), Bathyarca pectunculoides (132; D=7.4%), Pagodula echinata (70; D=3.9%) and Cardiomya costellata (66; D=3.7%), these species making up altogether 77% of the live molluscan specimens of the sediment fraction of the haul BT04 (Fig. 3). Gastropods slightly predominate in number of species (43 live-taken, 84 species in total) over bivalves (36 live-taken, 60 species in total) but in abundance the bivalves prevailed, with 73% of live-taken individuals, because of the overwhelming numbers contributed by Limopsis aurita.

There were 30 species represented by a single live specimen and 12 more represented by only two specimens. Some of the species found are considered rare or very rare (Fig. 4): the Solenogastres Neomenia carinata and Unciherpia hirsuta, the gastropods Adeuomphalus ammoniformis, Epitonium linctum, E. hispidulum, Eulima fuscozonata, Drilliola emendata, Spirolaxis clenchi, S. lamellifer, Tiberia minuscula and Philine monterosati, and the bivalves Crenella pellucida and Polycordia gemma. Two species are believed to be new to science and are described hereafter.

The gastropods Puncturella noachina, Anatoma tenuisculpta, Iothia fulva and Limacina retroversa, and the bivalve Bathyarca frielei were represented only by rather abundant shells, with a dull, opaque appearance and no live specimens of these species were collected (Fig. 5).

Family EULIMIDAE

Genus Melanella

(type species, Melanella dufresnii Bowdich, 1822, by monotypy)

Melanella scarifata Gofas and Rueda, n. sp.

Holotype: live-collected specimen from DEEPER 0409 cruise, BT04 (MNCN 15.05/60139).

Type locality: Algarrobo Bank, Alboran Sea (36°21.08’N, 03°58.58’E, 349 m to 36°21.06’N, 03°58.07’E, 365 m)

Description. Shell small (holotype 5.1×1.9 mm) with a tall, very regularly conical spire, comprising in total 11 whorls, quite transparent in the early whorls but becoming gradually more opaque on the two last whorls. Height of last whorl (measured along the axis on Fig. 6A) 2.3 mm (45% of total height), height of aperture 1.6 mm, spire angle 25.5°.

Larval shell distinct, the visible part 530 µm high and 350 µm broad, transparent and thin, smooth, comprising 3.5 whorls which are slightly convex with a distinct suture. Teleoconch comprising 7.5 whorls, the spire whorls almost flat, the last whorl having its maximum convexity near the periphery and very slightly convex in the abapical part. Suture of teleoconch whorls not very conspicuous but distinct; parallel to this just beneath the upper 1/4 of the whorls, there is a conspicuous internal suture (the “false suture”) seen by transparency.

Axis very slightly bent, resulting in that the profile opposite the aperture (right side of Fig. 6B) is imperceptibly convex. Surface of teleoconch whorls apparently smooth, with a few hardly distinct growth stages, with very finely incised, opisthocline lines visible only under strong magnification, regularly spaced about 50 µm.

Aperture oblong, rounded in its abapical part; the columella and the parietal edge are bordered by a callus and form a very obtuse angle. Outer lip simple, slightly flaring so, in apertural view (Fig. 6A) the edge would reach slightly outside the prolongation of the spire profile. In lateral view, the outer lip of the aperture forms a small sinus just below the suture, then forms a regular curve with the most protruding part situated at the middle of its height.

Remarks. Eulimids commonly have rather featureless, smooth shells which renders them difficult to characterize at the species level. This species is unusual in having a definite microsculpture consisting of regularly spaced scars parallel to the growth lines. A similar sculpture is found in Melanella costellata (Dautzenberg and Fischer, 1897) from the bathyal of the Azores, and M. turbonilloides Bouchet and Warén, 1986 from abyssal depths of the North Atlantic; however, in these species the incisions are more pronounced, the shape of the spire is much more slender, with convex whorls on the teleoconch also, and the larval shell is paucispiral. Other species of European Melanella with a shell microsculpture include Melanella alba (da Costa, 1778), a much larger species (up to 20 mm) and Melanella lubrica (Monterosato, 1890), only slightly larger (9 mm) than the present species. Both of these species live in the infralittoral zone and are holothurian parasites (Warén 1984: 55, Gofas et al. 2011: 227-228). In the former, the microsculpture is also parallel to the growth lines but consists of densely crowded, minute wrinkles; in the latter the microsculpture is spiral (see Gofas et al. 2011: 227-228). The larval shell of Melanella scarifata n. sp. is unusually large and denotes a planktotrophic larval development, so the species is probably not endemic to the Algarrobo Bank and may be found again in relatively remote places of the Atlantic and Mediterranean Sea.

Etymology. From Latin scarifare, to scratch, alluding to the microsculpture.

Family TOFANELLIDAE

Genus Graphis Jeffreys, 1867
(type species: Turbo unicus Montagu, 1803, by monotypy)

Graphis pruinosa Gofas and Rueda, n. sp.

Holotype: live-collected specimen from DEEPER 0409 cruise, BT04 (MNCN 15.05/60140H); 7 paratypes (1 live-taken specimen and 6 shells) from the type locality (MNCN 15.05/60140P).

Type locality: Algarrobo Bank, Alboran Sea (36°21.08’N, 03°58.58’E, 349 m to 36°21.06’N, 03°58.07’E, 365 m)

Description. Shell very small (holotype 2.1×0.65 mm) with a tall spire, comprising in total six whorls, somewhat translucent but having a surface with a frosty appearance, not transparent. Height of last whorl (measured along the axis on Fig. 7A) 0.8 mm (38% of total height), height of aperture 0.45 mm, spire angle 11°. Colour whitish.

Protoconch 450 µm high and 360 µm broad, smooth, comprising hardly 1.5 whorls, the first of which is globose and bears a couple of blunt keels, one of which persists on the upper third the following half-whorl.

Teleoconch comprising 4.5 convex whorls, increasing very slowly in diameter, with a deeply impressed suture. Sculpture of teleoconch whorls composed of weak, irregular wrinkles parallel to the growth lines, flexuose, and of a few growth scars; there are very faint and shallow spiral microstriae, spaced ca. 5 µm and detectable only under very high magnification. Aperture oval, with a continuous peristome, the outer lip simple, very slightly flaring, flexuous in lateral view (Fig. 7B); the columellar edge appressed.

Remarks. This species most resembles Graphis gracilis (Monterosato, 1874), from which it differs essentially by the absence of the strong axial ribs and the surface texture, which is shagreened rather than smooth, and under scanning electron microscope examination bears microstriae which are not present in the former. The protoconchs are essentially alike and this shared morphology indicates that these two species are probably related. Both species were found together on Algarrobo Bank, but G. gracilis was not collected alive.

Etymology. From Latin pruinosus, frosted, alluding to the aspect of the shell surface.

Anatoma tenuisculpta (Seguenza, 1880)

This species was reported, under the name Anatoma crispata (Fleming, 1828), from the Alboran platform as shells only by Peñas et al. (2006) but was then misidentified according to Geiger (2012). Numerous shells only were found in haul BT04, but living specimens were subsequently found on the Alboran platform (unpublished data from the INDEMARES project), so the species is still extant in the area.

Turbonilla cf. vaillanti Dautzenberg and Fischer, 1896

The presence of spiral sculpture and of a colour pattern on this species considerably limits the scope of possible comparisons among the nearly one hundred species of Turbonilla currently reported from European and western African coasts. Turbonilla vaillanti, originally described from the bathyal of the Azores and also reported from the Irving seamount (Peñas and Rolán 1999), is possibly the most similar species, with size and sculpture matching those of the present species. There are several differences: T. vaillanti is white, and the ribs are somewhat more prosocline on the early whorls. Nevertheless, we have observed several cases (e.g. the rissoid Alvania testae [Aradas and Maggiore, 1844], the columbellid Amphissa acutecostata [Philippi, 1844]) where the same species are white in deep-water and have a colour pattern in shallower environments. Therefore, although we cannot preclude that this is an undescribed species; we prefer to refer it as Turbonilla cf. vaillanti, in any case a new item for the Mediterranean fauna.

The Mediterranean deep sea is infamous for the poverty of its fauna (Fredj and Laubier 1985, Bouchet and Taviani 1992, Galil 2004), to the point that it once provided support towards Forbes’ (1844) “azoic theory”. However, ongoing exploration reveals that some areas may be not so poor (Koutsoubas et al. 2000, Sardá et al. 2004, Danovaro et al. 2010). The number of species reported here (156 spp.) is nevertheless unusual for the Mediterranean deep sea, usually reported as being less diverse than other adjacent basins such as the Atlantic Ocean. At similar depths, thanatocoenoses were reported as “rich” with 61 species off NW Sicily (Di Geronimo et al. 2005). In the area of S. Maria di Leuca, off SE Italy, Negri and Corselli (2011) reported as many as 112 species of molluscs, but this was in the context of a broader sampling including 24 different box cores, not at a single site as here.

The BT04 haul is outstanding at the local scale but this may be an artefact of a particularly successful operation. Of the other two beam trawl operations on Algarrobo Bank, one (BT03) was void and one (BT05) got stuck on the bottom shortly after landing although it managed to collect a small amount of Leptometra. The other four beam trawls on the nearby Ville de Djibouti Bank yielded a considerably lower number of species and did not bring up a noteworthy amount of sediment to be sorted. The other gears are less effective at collecting benthic molluscs: the otter trawl operations are essentially targeted to fish and usually bring up a limited sample of small invertebrates, and the rock dredge collects a considerably smaller sample and covers a much smaller area (150 m² versus 800 m²) of the bottom.

The solenogastre Unciherpia hirsuta (Pedrouzo et al. 2014) and the gastropods Spirotropis monterosatoi and Turbonilla cf. vaillanti are the first findings of these species in the Mediterranean fauna. The specimen of Adeuomphalus ammoniformis is the first unquestionable record of this species living in the Mediterranean, whereas previous records are suspected of being based on Pleistocene subfossil specimens (Kano et al. 2009). Likewise, the occurrence of several live-taken Trophonopsis barviciensis proves the presence of this species in the Recent Mediterranean fauna, which had been questioned by Bouchet and Warén (1985). Spirolaxis lamellifer and S. clenchi are the first specimens collected in Iberian waters.

The proportion of species of molluscs which would have been overlooked if the processing were limited to the shipboard sorting is considerable, since only seven species were recovered at that stage (Table 1). Admittedly, the dominant (Limopsis aurita) and characteristic (Antalis panorma) species were correctly represented, but over 90% of the species were found only after sorting the fine-fraction residue. Conversely, two of the megafaunal species (Heliacus fallaciosus and Xenophora crispa) were not found again in the residue. This bias is particularly acute for molluscs, which are mostly represented by small species (Fig. 8). Streftaris and Zenetos (2007) reached a similar conclusion after analysing the molluscan species composition in a suite of samples from the NE Aegean Sea separately in the size fractions >1 mm and 0.5-1 mm: among a total of 124 species, 84 (67.7 % ) were encountered in the >1mm fraction, and 107 (86.3 %) in the 0.5-1mm fraction. Groups such as fish and echinoderms are not in this case, but the bias may also be important for crustaceans, where the small peracarids would contribute many species in addition to the megafaunal decapods. This observation highlights the importance of laboratory sorting in order to obtain a complete assessment of species richness and diversity, most of which may be missed if the processing is limited to shipboard observations.

It is striking that one-third (54 out of 156) of the species found on Algarrobo Bank were not reported in the large species list (655 spp.) from the surroundings of Alboran Island (Peñas et al. 2006). The number of non-shared species was slightly reduced by the recent and intensive exploration of Alboran in the INDEMARES project, which added Neomenia carinata and Pitar mediterranea to the list of shared species, in a dataset of 191 live-collected mollusc species (4.331 specimens). The Alboran platform holds much more diverse habitats, with a much broader bathymetric range which starts from the supralittoral (Templado et al. 2006) and therefore it is straightforward that many of the species found in the shallower parts of this platform should not be found on Algarrobo Bank. Conversely the presence on the bank of a high number of species not shared with the Alboran platform may be related to a deeper summit (ca. 350 m) than the Alboran platform (ca. 50-200 m), but it can be also indicate genuine heterogeneity among different sectors of the Alboran Sea.

The bulk of the species found on Djibouti Banks have a more or less extensive range in the Atlantic and most of them were also previously known in the Mediterranean. An analysis of the molluscan fauna using presence/absence in the six regions listed in Table 2 and the Bray-Curtis similarity index (Fig. 9) shows that 103 of the species cluster together with a similarity above 75%, with various combinations of ranges involving western and/or NE. On the other hand, only three species (Curveulima devians, Aclis attenuans and Bogia labronica) are reported in the Mediterranean outside the Alboran Sea but not in the Atlantic. A similar biogeographical pattern of coastal molluscan assemblages was found by Rueda et al (2009) and Marina et al. (2012), indicating the importance of species with a broad Atlantic range in the bulk of fauna at different depth levels in the Alboran Sea.

A small group of species (Puncturella noachina, Iothia fulva, Limacina retroversa, Bathyarca frielei, Dacrydium ockelmanni) cluster together with a distribution currently restricted to northern and western Europe and not reported in the Ibero-Moroccan area. The three former are cited by Malatesta and Zarlenga (1986) as Mediterranean “cold hosts” during cold climatic epochs and are here interpreted as elements of a thanatocoenosis, now extinct in the Mediterranean Sea. Indeed, abundant cold-water coral remains occur at the summit of this and other banks, and related to high productivity during the Late Pleistocene and Holocene (Hebbeln et al. 2009, Fink et al. 2013).

Regarding potential for dispersal, the bulk of the species found in haul BT04 have a planktonic stage, albeit not always planktotrophic. Of the molluscan species taken alive, only Trophonopsis barvicensis, Pagodula echinata, Pseudobabylonella minima, Drilliola emendata, Spirotropis monterosatoi and Graphis pruinosa n. sp. are inferred to have direct development. Therefore, it can be hypothesized that the fauna benefits from a diverse influx of larvae carried in by the incoming Atlantic current. The Atlantic water is not directly in contact with the bottom but sufficiently close for sinking larvae to reach the bank and settle there. Obviously, the higher salinity of the Levantine Intermediate Water in contact with the summit of Algarrobo Bank, which Bouchet and Taviani (1992) put forward as the second “barrier” after the sill of Gibraltar, is not so severe as it could be overcome by a large number of species. García-Muñoz et al. (2012) indicated the presence of low hydrodynamic conditions and a high incidence of pelagic sedimentation at the summit of Algarrobo and Herradura banks based on sedimentological characteristics and this may benefit the vertical influx of larvae. Moreover, the dense population of Leptometra phalangium at the summit, as suspension feeders, benefit from a highly productive environment due to a high amount of particles from surficial levels (Colloca et al. 2004, Gil et al. 2010).

Regarding the species level, there is a considerable balance between the different trophic groups, with a large representation of specialized carnivores or ectoparasites such as the families Eulimidae or Epitoniidae. When the counts are made at the individual level, the balance is biased towards suspension feeders, because of the dominance of the single species Limopsis aurita (Fig. 10). Taking into account that the structuring species Leptometra phalangium is itself a suspension feeder, the abundance of this group suggest that there must be a good supply of particles at the summit of the bank. The other abundant trophic group is the detritus feeders, which together with the filter feeding form the base of the animal food web in the Algarrobo Bank, with its summit in the aphotic zone and therefore without the presence of primary producers. The latter, however, indirectly influence the food web in the bank because the detritus down from the photic zone constitutes the main source of energy in the bank. According to Moore et al (2004), the detritus-based food web results in a more diverse, stable and structured community, particularly for ectotherm organisms, because the detritus represents energy persistent in time.

The specialized predators are represented by many rare species, so they account for 18% of the species but only 2.5% of the individuals, suggesting a high faunal diversity but with low population density. On the other hand, a broad representation of trophic groups is considered as an indicator of a stable environment (Sanders 1968, 1979, Levinton 1972)

The structuring species Leptometra phalangium and the scaphopod Antalis panorma are reported as exclusive characteristic species of the offshore detritic bottom community (“Détritique du Large”, Pérès and Picard 1954, 1964: 94, Bellan-Santini et al. 2002). The bivalves Pseudamussium clavatum and Astarte sulcata, also reported as characteristic, were represented only by shells in our material. Unlike “Detritique Côtier”, where some facies may incorporate several species of algae including rhodolith-forming algae, benthic photosynthetic organisms are never involved in the community of the “Détritique du Large”. It is disputed whether this community should be encompassed in the circalittoral or in the bathyal stage. Formally speaking, it belongs to the aphytal zone, and therefore to the bathyal (the view defended by Emig 1997). Nevertheless, Pérès and Picard (1964) and Pérès (1982: 479) advocated grouping it with the circalittoral because so many animal species are shared with that stage. Our molluscan species list definitely supports the former view: it contains hardly any species that would normally occur shallower than 200 m (Arcopagia balaustina and Tellina distorta would be the only ones in this case and these are only represented by one live-collected specimen).

Because of its exceptional richness, both in numbers and in the quality of the species, this type of bathyal community with Leptometra and Limopsis should be considered a high priority for habitat conservation in the Mediterranean deep sea. Mediterranean communities of the offshore “detritic” bottoms have also attracted interest elsewhere and qualify as a “reservoir” of biodiversity (Colloca et al. 2004, 2006).

Circalittoral, shelf-edge and upper slope gravel bottoms are a primary target for trawling activities because (1) they are mostly situated below the legal limit of 50 m for this activity, (2) the sediment forms extensive stretches amenable to trawling and (3) many species of fishes and cephalopods, which predominate in trawling shelf landings, seem to be related to these beds. Trawling can be intuitively perceived as a threat to the benthic assemblages, particularly to the epifauna, and where quantitative data exist it has effectively been recognized as a pressure affecting this community (Mangano et al. 2013).

Particular attention is needed for the shelf break facies with abundant Leptometra phalangium, which is increasingly perceived as the habitat of an exceptionally diverse benthic fauna that is easily disturbed by trawling. The outstanding species richness may be favoured because the crinoid increases heterogeneity, but above all its presence is an indicator of favourable resources and it has been explicitly put forward as a species deserving a protection status (Colloca et al. 2004, De Ranieri et al. 2006, Angeletti et al. 2010). A strong case has been made for the recognition of Leptometra phalangium aggregates as Essential Fish Habitats by the working group (SGMED 06-01) of the scientific, technical and economic committee for fisheries (European Commission 2006), with several papers on the subject presented at the 2006 meeting (Abelló and Gil de Sola 2006, Colloca et al. 2006). This culminated in having Leptometra beds (along with Posidonia beds 1120, a priority habitat in Europe’s Habitat Directive, and along with maërl beds) as one of the 14 habitats selected by the Scientific Advisory Committee of the General Fisheries Commission for the Mediterranean as criteria for identifying Essential Marine Habitats (EMH) of relevance for the management of priority species (General Fisheries Commission for the Mediterranean 2009). We conclude that it is unfortunate that the area investigated herein was not included in the final proposal for deep-water Sites of Community Interest in Spanish waters.