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Echinoderms are the most important taxa of megabenthic invertebrates on the Mauritanian deep-sea floor, both in density and biomass . The phylum is represented by approximately 82 species, Ophiuroidea and Asteroidea being the most diverse classes (25 and 23 species, respectively), followed by Holothuroidea and Echinoidea (19 and 12 species, respectively). Crinoidea is poorly represented by only three species. Holothuroidea (sea cucumbers) are, undoubtedly, the main component of deep Mauritanian echinoderms, accounting for 85.5% of numerical abundance and 96.0% of biomass. Species belonging to Echinothurioida are dominant among the sea urchins . The most common echinoderm species are Phormosoma placenta , Enypniastes eximia , Psilaster cassiope , Benthothuria funebris, Ophiernus alepidotus , Pseudarchaster gracilis gracilis and Ophiura flagellata . The Mauritanian echinoderm fauna are mostly composed of species with Atlantic distribution (80.5%) and a smaller group of wide distribution species (19.5%). Only three species, two sea stars and one ophiuroid, could be considered as endemic to Macaronesian African waters : Hymenaster roseus , Dyptaster mollis and Ophiothrix maculata . The bathymetric distribution of species and multivariate analysis clearly identify two main assemblages : the first groups the stations located on the deep shelf and upper slope (

During the four Maurit surveys carried out annually from 2007 to 2010, 342 trawl stations were sampled off the Mauritanian coast between 80 and 2000-m depth. Hydroids were captured at 174 stations, totalling 6169 colonies belonging to 63 species and 19 families . Most of the species were Leptothecata and only seven belonged to Anthoathecata . Thirty-two of the 63 species are new records for Mauritania. Plumulariidae showed the highest species richness (8 species), followed by Sertulariidae and Campanulariidae (7 species), Lafoeidae , Haleciidae and Aglopheniidae (6 species). The most abundant were Sertulariidae , followed by Aglaopheniidae, Plumulariidae, Campanulariidae, Halopterididae and Haleciidae . The ability of hydroids to colonize Mauritanian soft bottom s was driven by hydrorhizal modifications for anchoring the colony to the sediment and by epizoism on 11 invertebrate groups. Families whose species developed both strategies and those which were typically epibionts on hydroids were the most frequent and abundant. Most of the species were eurybathic (58 species), with four species recorded only in the coastal realm, 18 exclusively in the deep benthic realm and 41 in both realms. Therefore, overall hydroid diversity was highest in the deep benthic realm . Biogeographical components included two main groups: species with a wide distribution (59%) and species of Atlantic distribution (41%); our results also emphasize the similarity between the Mauritanian hydroid fauna and the Atlantic-Mediterranean region. Only one species, Hydractinia multitentaculata (Millard 1975), has West African distribution. No endemic species were reported.

The waters surrounding the Islamic Republic of Mauritania, located in the Northwest African region, harbour one of the four major upwelling systems and productive areas of the world’s oceans. Along 754 km of the desert coastline, the permanent upwelling phenomena and seasonal shifting of the oceanographic front strongly influence the water mass structures, climate and species distribution in the Mauritanian EEZ. Despite oceanographic and faunistic interest and the threats of demersal deep˗sea fisheries and oil exploration, for the past few decades, marine research in Mauritania was performed to obtain information on the Banc d’Arguin and other coastal ecosystems; only recently, research is being focused on the study of slope habitats. Between 2007 and 2010, the Spanish Oceanographic Institute launched a partnership program with the Mauritanian Institute for Oceanographic Research and Fisheries and the University of Vigo (Spain) to study the resources and ecosystems in Mauritanian deep waters. Four multidisciplinary expeditions were conducted using the Spanish R/V Vizconde de Eza , and geomorphological prospection and oceanographic and biological sampling were performed. Special efforts were made to characterize the Banc d’Arguin canyon systems, the giant coral carbonate mounds barrier and a newly discovered seamount. In this monograph, we tried to compile, in one volume, the main findings of the Maurit surveys and provide a detailed description of the bathymetry, geomorphology and oceanography, and biodiversity and structure of demersal fishes and benthic communities that inhabit the soft˗ and hard˗bottom habitats of the Mauritanian slope.

This chapter describes the main features of the giant mounds structure running parallel to the shelf break along the Mauritanian slope between the Senegalese border and Cape Timiris. At over 580 km long, it is the world’s largest identified cold-water coral mounds barrier and, in our opinion, conforms to a single province, the Mauritanian Province . The Maurit series of Spanish-Mauritanian oceanographic surveys collected multibeam echosounder data, performed 16 conductivity-temperature-depth (CTD) profiles and 13 rock dredge samplings , which led to the mapping of the reef complex , oceanographic characterization of water masses and faunistic studies. The reef framework is mostly composed of dead coral, mainly Lophelia pertusa , with a minor fraction of Madrepora oculata . Associated fauna mainly consists of Acesta excavata and other bivalve and prosobranch molluscs, soaked in abundant, fine compacted mud . At least 150 macrobenthic species of 27 high-range taxa inhabit this reef. The coral surface is either almost devoid of sessile fauna or poorly colonized by small encrusting epifaunal species. We only found fragments of living L. pertusa in the four southernmost stations. The multivariate analysis clearly groups some areas where corals and other suspension-feeders exhibit the highest diversity. Despite their small size, the 17 taxa of suspension-feeders represent 75% of abundance and biomass . Surprisingly, despite the unfavourable history of climatic change episodes, current environmental conditions, deterioration and overall faunistic poverty of the mounds, Lophelia specimens still survive in some areas , sheltering communities of apparently similar structure to those on well-developed cold-water coral reefs in Northern Atlantic latitudes.

The composition and structure of megabenthic communities of bathyal soft-bottoms off Mauritania were analysed, based on data and material collected at 291 trawling stations accomplished during the four Maurit surveys carried out in November–December of 2007–2010. A total of 551,281 specimens with a corresponding wet weight of 2.2 tons were recorded and more than 700 species of benthic invertebrates belonging to 37 high-range taxa were identified. The best-represented group in terms of specific richness was Decapoda with 118 species, followed by Polychaeta , Porifera and Hydrozoa (74–63 species). In terms of biomass , Holothuroidea clearly dominates the megabenthic communities representing the 83% of the total, while in numerical abundance the dominance was shared by Decapoda (39%) and Holothuroidea (34%). The distribution of megabenthos showed latitudinal patterns along the Mauritanian coast, with a decreasing trend southwards for specific richness , Shannon and Pielou indexes , but with a reverse figure for abundance and biomass that reached their maximum values in the southern area. The bathymetric distribution showed a decreasing trend with depth only for Shannon and Pielou but species richness, abundance and biomass reached their highest values in deep waters. Multivariate analysis identified four megabenthic assemblages structured by depth. The giant coral mounds barrier together with the minimum oxygen values obtained at the same depths seems to perform the main faunistic discontinuity separating the shelf and upper slope assemblages from those located at middle and lower slope .

One of the objectives of the Maurit˗1011 survey was the environmental and faunistic characterization of a small seamount of 200 m in height, discovered during the Maurit˗0911 expedition, on the upper continental slope south of Nouakchott, at 17°08′ 50″N and 16°46′ 38.1″W. It is an isolated conical structure with small ridges, almost parallel to the edge of the continental shelf and not connected to nearby coral mounds barrier. The geomorphology of this submarine elevation, as well as the debris from a hydrothermal vents or cold seeps discovered in the rock dredge samples suggests a diapiric association because no hydrothermal benthic fauna was found. The summit of the seamount was covered with sponges belonging mainly to the family Geodiidae, and ophiuroids belonging to the suspension-feeders family Ophiotrichidae. We also found accompanying fauna comprising about 50 species, mainly consisting of molluscs and decapods. Despite its small size but taking into account the richness of the suspension-feeders community living on it, this Wolof’s Seamount (so called to honour the artisanal fishermen who first discovered it) could be promoted as a marine protected area in the Mauritanian continental margin.

Quantitative data were collected with a 3.5 m beam-trawl gear at 25 stations along five latitudinal transects , covering five bathymetric strata from 100 to 2000 m depth. More than 500 species belonging to 35 epibenthic high-range taxa are present in Mauritanian deep-sea bottom s. Pisces , Decapoda and Polychaeta , represented by 111, 80 and 56 species, respectively, constituted the richest groups, being the first two collected at all stations. Decapoda were dominant in abundance (55.8% of the total), although in biomass they shared their dominance with Actinopterygii and Holothuroidea (29.4, and 26.0%, respectively). Although suspension feeders were represented by 131 species (25.7%), they only accounted for 9% of the abundance and 1% of biomass (small-sized species). Low diversity indices varied widely between localities but showed no latitudinal pattern . Richness and densities decreased with increasing depth, a strongly marked trend in the case of densities, which peaked on the deep shelf and upper slope (150–300 m), while the biomass remain standing showing only minimum values at 500 m depth. Depth seems to be the strongest factor determining the structure and composition of the four main epibenthic assemblages identified on the deep shelf (150 m), upper slope (300–550 m), middle slope (1000 m) and deep slope (1600 m). The two shallowest assemblages were clearly separated from the deeper ones and were typified by different decapods and fish species , while the two deepest assemblages were characterized by echinoderms and fishes.

The composition of megabenthic communities from the Northern Mauritanian canyons was studied using data and material obtained during the four Maurit surveys , accomplished in November–December of 2007–2010. Cartography of the canyons was carried out using a multibeam echosounder and a high resolution seismic system (TOPAS). Oceanographic data were obtained in 29 vertical CTD profiles located in four transects along the canyons. The megabenthos survey was limited to canyon edges and included 11 stations sampled with a rock dredge . Twenty nine canyons and other minor channels , grouped in four large complex systems, were located in the Arguin Province . The canyons, with very steep walls, can reach down to 700 m depth and extend over 8–70 km in length along the slope. The water mass influencing the canyons correspond to the South Atlantic Central Water (SACW). The megabenthos species richness was estimated to be over 206 species, distributed among 29 high˗range taxa . Hydroids , sponges , decapods, bivalves , polychaetes and prosobranchs showed the highest species richness, oscillating from 29 to 15 species. In terms of feeding strategies , suspension-feeders dominated the species richness (63%), abundance (75%) and also biomass (94%). While ophiuroids composed the largest proportion of total abundances (23%), demosponges dominated the biomass (80%). Multivariate analysis identified two megabenthic assemblages mainly segregated by its trophic strategy . The first cluster was defined by strong dominance of suspension˗feeders and higher values of diversity indexes, and included four stations where living corals were collected. The second cluster was dominated by scavengers , carnivorous and detritivorous species, with much lower diversity.