Three species of the genus Swiftia are known for the NE Atlantic Ocean and Mediterranean Sea. Remotely-operated vehicle (ROV) surveys and sampling on board RV Maria S. Merian during cruise MSM 16/3 ‘PHAETON’ in 2010 provided footage and specimens of octocorals off Mauritania. Micro-computed tomography (micro-CT) reveals, for the first time in taxonomy of octocorals, the three-dimensional arrangement of the sclerites in a polyp. Swiftia phaeton sp. nov. is described for the continental slope off Mauritania. This azooxanthellate octocoral is distinctive from NE Atlantic and Mediterranean congenerics by the dark red colour of the colonies (including the polyps), the presence of a layer of rod sclerites on top of the polyp mounds, and different sizes of polyps and sclerites. Using micro-CT has allowed the observation and imaging of a layer of sclerites that is distinct from other species of the same genus. ROV images revealed live records of S. phaeton sp. nov. in submarine canyons and on cold-water coral mounds in the upper-bathyal off Mauritania (396–639 m depth), mainly attached to dead coral, coral rubble, or rocks. The new species represents an extension of the genus distribution to the tropical latitudes (17°07'N and 20°14'N) of the NE Atlantic Ocean.

Deep sea, gorgonian, micro-CT, NW Africa, Octocorallia, taxonomy

Only few deep-sea cruises have explored northwest Africa. During the 19^th and the beginning of the 20^th centuries, the Talisman, Michael Sars North Atlantic Deep-Sea expedition, and the Prince Albert I of Monaco, as well as expeditions of the vessels Thalassa and Discovery, made sporadic sampling (Bigelow 1911; Oñate 2017). Systematic exploration began later in the late 20^th century with the Dutch CANCAP (CANarian – CAPe Verdean Deep-Sea Basin) and Mauritania II expeditions and the Cooperative Investigation of the northern part of the Eastern Central Atlantic (CINECA) program (Den Hartog 1984; Van der Land 1988; Ramos et al. 2017a).

Knowledge of benthic deep-sea faunas of Mauritania is rare despite some scientific efforts passing by its coastline (Oñate 2017; Ramos et al. 2017b). Tydeman Madeira – Mauritania – CANCAP III in 1978 and Tyro Mauritania II in 1988 were the first expeditions to focus exclusively on Mauritania (Den Hartog 1984; Van der Land 1988) and, more recently, the Spanish MAURIT surveys during 2007–2010 (Ramos et al. 2017a). These exploration efforts revealed that the Mauritanian deep sea is home of the world’s largest coral mound barrier, with more than 580 km running parallel to the Mauritanian coastline (Ramos et al. 2017c; Wienberg et al. 2018). Live scleractinians were found scarcely distributed on mounds but forming vast frameworks in the submarine canyons off Mauritania (Colman et al. 2005; Westphal et al. 2012). Limited at the north by the Cap Timiris Canyon System and at the south by the Mauritanian Slide complex, the mounds are located on the upper bathyal between 450 and 550 m depth (Wien et al. 2007; Sanz et al. 2017). Mauritania has a large upwelling system increasing the productivity of its surface waters and creating a pronounced oxygen-minimum zone in deeper waters (Hagen 2001; Marañón and Holligan 1999).

Octocorals from Mauritania were mostly known from shallower depths, dominated by species of the genus Leptogorgia Milne Edwards, 1857 (Grasshoff 1986, 1988). The few octocorals recorded at deeper (> 200 m) Mauritanian waters were sea pens, acanthogorgiids, and plexaurids (Grasshoff 1981; Buschewski 2016; Ramos et al. 2017c; Sampaio et al. 2019). Worldwide bathymetrical records of the family Plexauridae Gray, 1859 are from 20 to 3000 m depth in tropical, temperate, and polar waters (Bayer and Weinheimer 1974; Grasshoff 1977, 1985, 1992; López-González 2006; Breedy et al. 2019; Sampaio et al. 2019). This speciose, widespread, and abundant family includes the genus Swiftia Duchassaing & Michelotti, 1864, whose position has been previously placed within the Gorgoniidae and Paramuriceidae and is still under discussion (Bayer 1961; Grasshoff 1977; Wirshing et al. 2005). At present, it forms part of the Plexauridae with the simplest sclerite forms: simple spindles, highly tuberculated sclerites, and bar-like rods but without thornstars in the coenenchyme (Grasshoff 1977; Bayer 1981). Three of 23 valid plexaurid species of the NE Atlantic Ocean and Mediterranean Sea belong to this genus: Swiftia dubia (Thomson, 1929), S. borealis (Kramp, 1930), and S. rosea (Grieg, 1887) (Grasshoff 1977). The genus is widespread throughout these ocean basins from 20 to 2400 m depth (Madsen 1970; Grasshoff 1977, 1981; Manuel 1981; Brito and Ocaña 2004; Sampaio et al. 2019). Swiftia borealis and S. rosea are distributed in northern latitudes (Greenland, Faroe Islands, Scandinavia, Ireland, Scotland) while S. dubia is widespread through the central and southern NE Atlantic and Mediterranean Sea: Gulf of Biscay, Galicia, mainland of Portugal, Mid-Atlantic Ridge, Josephine Bank, Macaronesia (Azores, Madeira and Canary Islands), Cape Verde Archipelago, the Mediterranean Sea, and NW Africa (Morocco, Western Sahara, Mauritania) (Madsen 1970; Grasshoff 1977, 1981; Manuel 1981; Brito and Ocaña 2004; Sampaio et al. 2019).

In 2010, RV Maria S. Merian cruise MSM 16/3 ‘PHAETON - Paleoceanographic and paleoclimatic record on the Mauritanian shelf off Mauritania’ visited the submarine canyons and coral-mound barrier off Mauritania with an ROV and an exploratory approach (Westphal et al. 2012). Herein we describe a new species of Swiftia collected during this cruise and highlight the potential of the micro-computed tomography (micro-CT) for octocoral taxonomy. The new species is the fourth of the genus reported for the NE Atlantic Ocean and Mediterranean Sea. Therefore, we increased alpha-taxonomic knowledge of a poorly explored area and have contributed to research on octocorals diversity, distribution, and conservation.

This is a pioneering taxonomic study on octocorals of the deep sea off Mauritania. Swiftia phaeton sp. nov. is the first octocoral species discovered at the upper bathyal off Mauritania, representing the southernmost record of the genus in the NE Atlantic Ocean. Modern image technology, micro-CT, was used for the first time in the taxonomy of octocorals, revealing potential for showing diagnostic characters not imaged so far.

Through the NE Atlantic, the genus Swiftia (including the species Swiftia dubia, S. borealis, and S. rosea), is distributed from Greenland to Morocco including the Medi­terranean Sea (Studer 1901; Tixier-Durivault and d´Hondt 1974; Grasshoff 1977, 1992; Manuel 1981). While S. borealis and S. rosea are inhabitants of the boreal Atlantic Ocean, S. dubia is widespread until the southern Macaronesian archipelagos and the Mediterranean Sea. Our record extends the known occurrence of the genus to the tropical latitudes of the NE Atlantic Ocean. Moreover, the recent observation of images of a living specimen of S. phaeton sp. nov. from the Fridtjof Nansen cruise to the Grand Tortue Ahmeyim oil and gas exploration area at the Senegalese-Mauritanian border is a potential distribution extension of this species (Andre Freiwald 2021, pers. comm.). Biogeographically, the species occurs in the Sahelian Upwelling ecoregion (Spalding et al. 2007), hinting at a geographical expansion for this species towards the south.

From a bathymetrical point of view, S. dubia is a eurybathic species, inhabiting sublittoral to abyssal depths (10–2400 m) (Grasshoff 1989; Sampaio et al. 2019). Nevertheless, the other species of this genus are more restricted in their depth ranges: S. rosea at the upper mesophotic and upper bathyal (20–400 m), S. borealis at the lower sublittoral down to the bathyal (83–1629 m), and S. phaeton sp. nov. at the upper bathyal (396–639 m) (Madsen 1970; Grasshoff 1977) (Table 1). However, the present geographical and bathymetrical range of S. phaeton sp. nov. is only based on data collected during the cruises Tyro Mauritania II, MSM 16/3 ‘PHAETON’, and one image from the Fridtjof Nansen cruise. Further exploration and research might uncover a more complete spatial and bathymetric distributions.

Seven species of Swiftia are known for the North Atlantic Ocean and the Mediterranean Sea (Deichmann 1936; Grasshoff 1977). Four species, Swiftia exserta (Ellis & Solander, 1786), Swiftia koreni (Wright & Studer, 1889), Swiftia casta (Verrill, 1883), and Swiftia pourtalesii Deichmann, 1936 are exclusive to the NW Atlantic Ocean and differ from the new species (see Deichmann 1936; Goldberg 2001). Swiftia exserta and S. koreni have fewer sparsely distributed polyps in thin branches, while in S. phaeton sp. nov. they are numerous and crowded. Moreover, S. exserta and S. casta differ in colony colour, exclusively white in both species. Another distinct characteristic of S. casta and S. pourtalesii is the absence of capstans in their sclerome.

In the NE Atlantic Ocean and Mediterranean Sea there are three species of Swiftia, S. dubia, S. borealis, and S. rosea. Swiftia phaeton sp. nov. differs from them. Swiftia borealis and S. dubia have a sparse distribution of polyps in their colonies (Grasshoff 1977). However, one might wrongly identify S. phaeton sp. nov. as S. rosea based on the dense distribution of polyps around the branches. Nonetheless, S. rosea is endemic to the boreal NE Atlantic Ocean and, after observation of museum specimens of this species, the morphological differences between both species are clear. Swiftia phaeton sp. nov. colonies and polyps (including anthocodiae) are of a darker red than S. rosea colonies, which are rose with white anthocodiae. Besides, S. phaeton sp. nov. has thicker branches and polyps and a clear division between polyp mound and anthocodiae (Fig. 2D–F). Long spindles from the inner layer of the coenenchyme of the new species are compacted and slightly thinner than the outer layer of capstans of the coenenchyme, while the coenenchyme inner layer of sclerites in S. rosea is thick compared to its outer layer (Fig. 3; Grasshoff 1977). At the sclerome level, S. phaeton sp. nov. differs from S. borealis, which has no capstans but flattened sclerites (Grasshoff 1977). Despite having similarly long, sharp spindles like S. dubia and S. rosea, S. phaeton sp. nov. has more rounded capstans in the outer layer of the coenenchyme. Its capstans resemble double-headed sclerites (see Bayer et al. 1983: fig. 159), but with more warts on their tips (Fig. 4C).

A unique morphological feature of S. phaeton sp. nov. is the presence of bar-like rods on top of the polyp mound, observed clearly with the micro-CT (Figs 4C, 5). “Finger-biscuit” rods were considered a definitive characteristic of the genus, based on the observation of specimens from the Pacific Ocean (Horvath 2019) yet, the species from the Atlantic Ocean do not have rods. The presence of rods in Swiftia was proposed to be related with northern latitudes of the eastern Pacific (Horvath 2019). Nonetheless, these sclerites occur neither in colonies from the eastern Atlantic temperate S. dubia collected from the Azores archipelago, nor in the boreal S. borealis and S. rosea that we studied. Also, S. phaeton sp. nov. rods are different from the common round-ended finger-biscuit rods, having instead pointed ends and prominent warts (Figs 4C, 5).

We applied micro-CT for the first time in the observation of morphological features of a new octocoral species. Similarly, Bayer (1973) used the scanning electron microscope (SEM) for the first time to photograph sclerites of octocorals that used to be drawn in previous taxonomic papers (e.g., Cairns 2009). The 3D image based on micro-CT of a polyp of S. phaeton sp. nov. with its sclerites (Fig. 5) is shown for the first time in a taxonomic work on octocorals (Fig. 5). Stereo pairs of polyps of Primnoidae species scanned by SEM were already used with the purpose of showing the 3D position of the sclerites (Cairns 2016). However, this technique is not as effective in octocorals with smaller and more complex sclerites, like those of the family Plexauridae, where the tissue in-between sclerites hinders the total visualisation of the calcareous structures. The micro-CT of S. phaeton sp. nov. has endorsed the observation of characters previously not shown but used to describe and differentiate species of plexaurid octocorals. This technique has the advantages of being non-destructive and of building a virtual 3D model that allows the observation of sclerites from distinct perspectives, including their relationships to each other and also the measurement of morphometric data. As already proven for bryozoans (Matsuyama et al. 2015), this technique has potential for future taxonomic studies of benthic marine invertebrates.

Taxonomic studies are essential for scientific and conservation endeavours. This is particularly true considering the unexplored deep-sea areas, such as off northwest Africa, and taxa as Octocorallia, both of which lack taxonomical expertise. In Mauritania, the natural and human impact has already occurred in the form of depleted oxygen levels, sedimentation, demersal fisheries, and local oil exploration (Colman et al. 2005; Ramos et al. 2017c; Wienberg et al. 2018). Therefore, it is crucial to describe its diversity before it becomes extinct.

The genus Swiftia now includes four species inhabiting the NE Atlantic Ocean. Swiftia phaeton sp. nov., an inhabitant of the upper bathyal off Mauritania, is the southernmost species. Micro-CT images of morphological details of gorgonians have the potential to show important diagnostic features, allowing us to visualise and compare them with other species. It is of fundamental importance in the description of new Octocorallia, which has only a handful of expert taxonomists around the globe. At the same time, in order to assess the consequences of the natural and anthropogenic impacts that are currently taking place on unexplored areas, such as NW Africa and the Mauritanian deep sea, it is very important to know, describe, and conserve the fauna of this geographical area. Discoveries of additional new octocoral species are foreseen for the coral framework off Mauritania during future deep-sea research.

The authors are thankful to the crew and scientific team of MSM 16/3 ‘PHAETON’ cruise onboard RV Maria S. Merian for collecting images, specimens, and data that made this work possible. Special thanks are givent to the ROV Pilot Thomas Lundälv from the Sven Lovén Center for Marine Infrastructure at Tjarnö, Sweden, for his skilful work. We also thank Prof. Dr. Bert Hoeksema, Mr. Koos van Egmond, and Ms. Karen van Dorp for facilitating access to specimens at Naturalis. We thank Dr. Alexander Kieneke for his guidance with the critical point dryer and Nicol Mahnken for her patience while taking sclerite and polyp SEM images. Thanks are due to the referees for their constructive comments on the manuscript. ÍS was funded by Fundação para a Ciência e a Tecnologia (FCT) Doctoral grant SFRH/BD/101113/2014 and has received a Marine and Environmental Sciences Centre (MARE) travel grant to visit Naturalis. This research received support from the SYNTHESYS Project http://www.synthesys.info/ which is financed by European Community Research Infrastructure Action under the FP7 “Capacities” Program. This publication is supported by the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) through the WASP (West African Biodiversity under Pressure) Project, Contract 81248171, to AF.