Approximately 24 species of Plexauridae were found after studying 86 colonies, 27 fragments of gorgonians and ~24 colonies or colony fragments of gorgonians, which were sampled during the cruises of CANCAP and Tyro Mauritania II at the southern NE Atlantic Ocean (Tables 1, 3). The specimens were identified by the last author (M.G.) (13 species), by Dr. L.P. van Ofwegen (one species) and the first author (15 species) after the discovery of uncatalogued and unidentified specimens that were partly separated from or mixed with the catalogued species (Figures 2, 3; Table 3). Moreover, seven species were identified by two specialists (I.S. and M.G. or I.S. and L.P. van O.) (Tables 1, 3). Of the 24 plexaurid species, six lack certainty in their identification (listed as cf. or aff.) and six seem to represent species new to science (listed as sp., sp. I, and sp. II) (Tables 1, 3). The taxonomic description of these species will be presented in future works. The study produced 49 additional records of Plexauridae species that are mostly not encountered elsewhere in the NE Atlantic Ocean (Figures 2–5; Table 3).

Figure 3. 

Map of Plexauridae collected during CANCAP and Tyro II Mauritania cruises except uncertain Cape Verdean records (see Figure 4).

Geographical coordinates associated with the specimens were plotted in a map of the NE Atlantic Ocean. Specimens were from all Macaronesian archipelagos, as well as from off the Mauritanian coast (Table 1; Figure 3). The Cape Verde archipelago has appeared as the region with the highest species richness (11 species in four genera) followed by the Azores archipelago where eight species of six genera were recorded, the Canary Islands (seven species in five genera), the Madeira islands (three species in three genera) and Mauritania (three species in two genera), and lastly the Selvagens Islands and Morocco (one species each) (Figures 3, 4; Table 3). The generic diversity of Plexauridae is higher in the Azores than in other NE Atlantic regions. Moreover, species identified in the Azores have a more accurate identification, especially if compared with the Cape Verde plexaurid fauna, where 11 species represent four genera but, near half of them are putative new species to science (Figures 3, 4; Table 3). The uncertainty associated with the identification of another four species from the Cape Verde islands, namely Paramuricea aff. macrospina, Placogorgia cf. graciosa, Placogorgia aff. graciosa, Placogorgia aff. terceira, may also represent new fauna (Figure 4; Tables 1, 3).

Figure 4. 

Map of Plexauridae collected during Tydeman Cape Verde Islands – CANCAP VI and VII cruises.

Specimens ancillary data has also revealed new species records. Some plexaurids are known to occur in most of the NE Atlantic basin; however, within it, the CANCAP records have widened their distribution ranges on a regional scale. Here we report Muriceides lepida Carpine & Grasshoff, 1975 in Madeira, Canary and Cape Verde archipelagos (Figures 3, 4; Table 1, 3) for the first time. In the Cape Verde archipelago, Muriceides paucituberculata (Marion, 1882), Paramuricea aff. macrospina (Koch, 1882), Placogorgia aff. graciosa (Tixier-Durivault and d’ Hondt, 1974), Placogorgia cf. graciosa (Tixier-Durivault and d’Hondt, 1974), and Placogorgia aff. terceira Grasshoff, 1977 are newly reported (Figures 3, 4; Tables 1, 3). Placogorgia aff. terceira was also found at the Canary Islands along with P. terceira (Figure 3; Tables 1, 3). Finally, Paramuricea biscaya Grasshoff, 1977 has its geographic distribution extended to the Selvagens Islands and the Azores (Figure 3; Tables 1, 3).

In terms of bathymetric distribution, the depth range of various plexaurids is now also upgraded. Overall, most species were collected from their known bathymetrical range during CANCAP (Table 1). However, three species have increased their depth range in the NE Atlantic Ocean: Muriceides lepida Carpine & Grasshoff, 1975, Paramuricea candida Grasshoff, 1977 and Placogorgia intermedia (Thomson, 1927) (Tables 1, 3; Figure 5). Muriceides lepida, previously reported to live at 79–718 m depth (Carpine and Grasshoff 1975) is now reported at its deeper record from 1000–1350 m depth at the Cape Verde archipelago (Tables 1, 3; Figures 4, 5). Paramuricea candida Grasshoff, 1977 known to inhabit the range of 1069–1235 m depth (Tixier-Durivault and D´Hondt 1974; Grasshoff 1977) is now known to live down to 1350 m depth (Tables 1, 3; Figure 5). The same new high depth record of 1350 m depth is herein reported for P. intermedia, previously known to occur between 800–1235 m depth (Tixier-Durivault and D´Hondt 1974; Grasshoff 1977) (Tables 1, 3; Figure 5). Also, if Paramuricea aff. macrospina is in fact P. macrospina, this species widens its geographical distribution as it expands its greater depth limit to 350 m in the NE Atlantic Ocean (Cape Verde islands) (Tables 1, 3; Figures 4, 5). So far it is only known to occur at the Mediterranean, where it lives at 38–200 m depth (Carpine and Grasshoff 1975; Grasshoff 1977).

Figure 5. 

Bathymetric distribution of Plexauridae identified to species at different sampling stations of CANCAP and Tyro Mauritania II cruises on the NE Atlantic Ocean. Symbols represent precise records while bars represent distribution ranges. Colours represent distinct regions: Azores (blue), Madeira (yellow), Selvagens (white), Canary Islands (orange), Cape Verde (green) and Mauritania (rose).

While the overall depth range has increased for three species, the vertical distribution range has increased or has been specified at a regional level in eight of the species identified with certainty. The exceptions are Villogorgia bebrycoides (Koch, 1887), Paramuricea grayi (Johnson, 1861), Spinimuricea atlantica (Johnson, 1862) and Thesea talismani Grasshoff, 1986 (Table 1). Considering the available data and the new records (Carpine and Grasshoff 1975; Grasshoff 1977, 1981, 1985a), we now know M. lepida’s specific depth ranges at the Azores, Madeira, Canary and Cape Verde archipelagos (Tables 1, 3; Figures 3, 5). In the Azorean archipelago, three species have increased their greater depth limit to 1350 m: M. paucituberculata, P. candida and P. intermedia (Pax and Müller 1954; Tixier-Durivault and D´Hondt 1974; Grasshoff 1977) (Tables 1, 3; Figure 5). There is a new depth record of P. biscaya from 1650–2050 m (Table 1, Figure 5). In Madeira is presently known that M. lepida occurs at 300–400 m depth and that P. biscaya occurs at Selvagens between 2100 and 2500 m depth (Table 1, Figure 5). At Canary Islands, Bebryce mollis Philippi, 1842 has a greater depth limit at 330 m (Brito and Ocaña 2004) (Tables 1, 3; Figure 5). Moreover, P. coronata has increased its regional depth range from 1200 m to 550–1800 m depth (Brito and Ocaña 2004) (Tables 1, 3; Figure 5). Also, M. lepida is now known to occur between 180 and 320 m depth and P. terceira at 200 m depth (Table 1, Figure 5).

In the Cape Verde archipelago, M. lepida has a record between 1000 and 1350 m depth and M. paucituberculata at 515 m depth (Tables 1, 3; Figures 4, 5). Moreover, there are many depth records at the archipelago with taxonomic uncertainty (Table 1). Lastly, at Mauritania M. paucituberculata is present at 51 m depth, Swiftia aff. dubia (Thomson, 1929) at 85 m depth and a new species of Swiftia sp. at 500 m depth (Tables 1, 3; Figure 5).

The CANCAP and Tyro Mauritania II expeditions collected 15 species of the known Plexauridae through the southern NE Atlantic Ocean, representing 62.5 % of the 24 described species known to occur in this area (Grasshoff 1977, 1986, 1992; Table 3). Only the genus Dentomuricea was not represented in these samples. This taxon is known from the Great Meteor complex of seamounts and its known biotopes in the Azores that were not visited by the Dutch expeditions. Nonetheless, some species are still missing a definite taxonomic status, meaning that so far only 12 of the 15 identified species can be used as reference records for future studies. If Paramuricea aff. macrospina is indeed P. macrospina, the most expressive extension of the geographical species is here reported. This species was known as endemic to the Mediterranean Sea and therefore this may represent its first report in the NE Atlantic at the Cape Verde archipelago (Carpine and Grasshoff 1975; Table 2, Figure 4).

New regional records were found in distinct Macaronesian archipelagos. While the easier taxonomic assignments were found in Azorean specimens, the most difficult were found in Cape Verdean specimens (Figures 3, 4). Easier taxonomic assignments were found in Azorean specimens due to the higher deep-sea exploration and the effort in octocoral taxonomy within the region (Sampaio et al. 2019). Nonetheless, difficult taxonomic assignments were found for the Cape Verdean specimens due to the lack of knowledge on octocoral diversity of that archipelago (Figures 3, 4). The sampling and research effort of Azorean octocorals is much higher than those in other southern areas of the NE Atlantic Ocean, like Cape Verde, which explains why the octocoral fauna of those areas is mostly unknown (Pérez et al. 2016; Sampaio et al. 2019). As expected based on previous knowledge for the area, Azorean CANCAP plexaurids have a high species and genus richness and have revealed a new record for the region (Sampaio et al. 2019). Paramuricea biscaya has its type locality in the Gulf of Biscaya, and was further mentioned to occur in Tenerife, Canary Islands, the Mid-Atlantic Ridge and the Gulf of Mexico (Grasshoff 1977, 1985; Brito and Ocaña 2004; Molodtsova et al. 2008; Thoma 2013). Therefore, we know now that more than 50% of the Plexauridae species of the NE Atlantic inhabit the Azorean marine waters (Sampaio et al. 2019). Herein, we also report the first record of P. biscaya for Selvagens Islands (Table 3; Figures 2, 3). In the Canary Islands we raise the number of plexaurid octocorals from seven to ten with three new records for the archipelago according to previous data (Brito and Ocaña 2004): Muriceides lepida, Placogorgia coronata and P. terceira (Table 3; Figure 3).

Despite some sparse records found in the taxonomic literature (e.g., Acanella arbuscula (Johnson, 1862)) and a vast number of gorgonians mentioned for the area, no thorough revision of Octocorallia of Cape Verde was completed at this point (Molodtsova et al. 2008; Raddatz et al. 2011; Hansteen et al. 2014). In this study we report the highest number of species of Plexauridae collected in this archipelago, reflecting the two Tydeman expeditions that exclusively explored this area: Cape Verde islands CANCAP VI and CANCAP VII (Table 3; Figure 4). Moreover, two newly recorded species are reported for the archipelago with certainty, Muriceides lepida and M. paucituberculata, and three are still considered uncertain records Paramuricea aff. macrospina, Placogorgia aff. graciosa and Placogorgia cf. graciosa. Aditionally, in this archipelago, there are five species that seem to be new to science and are in need of further taxonomic investigation (Figures 2, 4; Table 3).

As undescribed marine species are commonly found in museum collections (Appeltans et al. 2012), also a sixth new species was discovered on the shelves of this NHMC (Sampaio et al., personal observation). After observing the samples and videos collected during the German expedition MSM 16/3 in 2010, the first author, in 2016, discovered that a new species of the genus Swiftia was preliminarily collected during the Tyro Mauritania II expedition in 1988 and identified until the genus level by the senior author (M.G.). Also from Mauritania, Thesea talismani, was reported as a new record for this location in 2016 based on the same German expedition and two Spanish expeditions Maurit-0911 and Maurit-1011 (Matos-Pita et al. unpublished data). However, it had been also sampled in 1988, identified and stored at NBC (Table 2; Figures 2a, 3).

Plexaurid species are commonly found forming coral ecosystems of high density (e.g., Dentomuricea meteor at the plateau of Great Meteor seamount) (Grasshoff 1977; Tempera et al. 2013). The existence of multiple colonies under a single museum collection number indicate that the colonies were collected during a single sampling event and may be present at the seafloor in high densities such as octocoral gardens. For example, 11 specimens of Bebryce mollis were collected at Punta de Jandia, South of Fuerteventura (Canary Islands) and 11 specimens of a new species of Swiftia sp. were sampled off Mauritania, potentially indicating coral garden communities dominated by these species in each area (Table 3).

Zoogeographical regions of the North Atlantic Ocean have clustered for deep-sea Scleractinia by Cairns and Chapman (2001) and were further adapted for deep-sea corals in general and octocorals in particular (Watling et al. 2011; Braga-Henriques et al. 2013). These studies cluster species known from the Mediterranean Sea with NE Atlantic species of the Lusitanian region which are denominated as Lusitanian-Mediterranean species. Therefore, the majority of the species of this study (twelve) would be considered exclusively Lusitanian-Mediterranean (IIIA) while three, Paramuricea biscaya, Paramuricea grayi and Spinimuricea atlantica, would inhabit also the New England and Corner seamounts, Bermuda (IIC) (Watling et al. 2011; Table 4). P. biscaya is also known from the Gulf of Mexico (Thoma 2013). This would be in accordance with the trend found for the Azorean Alcyonacea, where most species are Lusitanian-Mediterranean (Braga-Henriques et al. 2013).

Nonetheless, if we consider the regions defined by Grasshoff (1977) for the Paramuriceidae (now Plexauridae) and take into account the boundary he suggested between the gorgonians of the NE Atlantic Ocean and the Mediterranean Sea, then six species of the 12 known Plexauridae species identified here are exclusively inhabitants of the NE Atlantic Ocean (Table 4). Muriceides paucituberculata and Placogorgia graciosa are widespread and live from the temperate waters in Ireland to the tropical waters of the West coast of Africa. Placogorgia terceira occurs on the vicinity of the Tropic of Cancer, between temperate and tropical waters of the southern NE Atlantic Ocean, while two species are uniquely known from the Azores, namely Paramuricea candida and Placogorgia intermedia. Thesea talismani is exclusive to the west coast of Africa (Grasshoff 1977, 1986). All the new putative species are exclusively known to inhabit the southern NE Atlantic Ocean, with Swiftia sp. only found in the West of Africa.

Six plexaurid species from the CANCAP records live in the “natural whole” (Ekman, 1935), the NE Atlantic Ocean (Lusitanian, Moroccan, Mauritanian and Macaronesian territories) and Mediterranean region, representing all five species mentioned by Grasshoff (1977) plus Paramuricea aff. macrospina. Here this species is mentioned as being located in the NE Atlantic Ocean; however, this record needs to be considered carefully, as explained above. Moreover, Paramuricea biscaya, P. grayi, and Spinimuricea atlantica are amphi-Atlantic (Watling et al. 2011; Table 4).

The distribution of marine invertebrates is highly influenced by oceanographic conditions (Ansín-Agís et al. 2001). Several oceanic currents and different climates influence the Macaronesian archipelagos (Amorim et al. 2017). The Equatorial counter-current, for example, seems to influence the distribution of scleractinian corals in the deep sea. Scleractinian corals at the Canary Islands, a warm temperate area, cluster together with corals from Cape Verde archipelago at the tropics. Additionally, Scleractinia from temperate areas like the Azores and Madeira archipelagos make up the large cluster of the Lusitanian Province (Cairns and Chapman 2001). However, NE Atlantic plexaurid octocorals seem to be more influenced by the climate. Further taxonomy and biogeographical analyses need to be done in order to make conclusive comments. Notwithstanding, the plexaurid species of the Canary Islands are also present at the Madeira and Azores archipelagos, while the new putative plexaurid octocoral species of Cape Verde indicate a distinction between species in temperate cold deep-sea waters and those at cold deep-sea tropical climates.

The present study has altered the known bathymetrical distribution ranges of a few plexaurid octocorals in the NE Atlantic (Figure 5). One species, Paramuricea aff. macrospina, had its depth range increased by 150 m and is now known from the upper mesophotic to the upper bathyal (40–350 m depth) (Grasshoff 1977). Spinimuricea atlantica (20–875 m), Villogorgia bebrycoides (63–845 m), Bebryce mollis (71–1250 m) and Muriceides lepida (80–1350 m), the last with its depth range increased by 650 m, inhabit the lower sublittoral down to the bathyal (Stiasny 1940; Tixier-Durivault and D´Hondt 1974; Carpine and Grasshoff 1975; Grasshoff 1977, 1985, 1992; Brito and Ocaña 2004). Species exclusively inhabiting the bathyal zone are Thesea talismani (462–1090 m), Placogorgia cf. graciosa (769–1300 m), which increased its depth range by 360 m, Placogorgia intermedia (800–1400 m), increasing its depth by 285 m, and Paramuricea candida with an additional 165 m (1069–1400 m depth) (Tixier-Durivault and d’Hondt 1974; Grasshoff 1977, 1985; Alvarez-Claudio 1993; Matos-Pita et al. unpublished data). Moreover, five species are eurybath having ranges from sublittoral or mesophotic depths until upper abyssal depths: Muriceides paucituberculata (51–2100 m), Paramuricea grayi (20–2195 m), Placogorgia coronata (50–2080 m), Swiftia aff. dubia (10–2400 m), and Placogorgia terceira (170–3200 m) (Grasshoff 1977, 1981, 1985, 1989; Altuna et al. 2010). Finally, the deepest plexaurid of the NE Atlantic is Paramuricea biscaya (1200–4152 m), an inhabitant of bathyal and abyssal depths (Brito and Ocaña 2004; Molodtsova et al. 2008).

Natural history museum collections harbour long-term biodiversity collection data. Museum data collected over time are prone to being incomplete (e.g., by lacking geographic locality information) (Soberón and Peterson 2004; Ross et al. 2012). Therefore, NHMC data are often overlooked in marine biodiversity assessments. CANCAP geo-referenced records from the 1970s and 80s have a low position accuracy in comparison with precise records sampled with modern submersibles and remote operated vehicles (ROVs) (see for example Englebert et al. 2015; Hoeksema et al. 2017). Most samples from the Dutch expeditions were collected by dredges and trawls which operated through transects, while only a single point position was recorded for each sampling location, detailing coordinates exclusively in degrees and minutes (Table 3).

While digitisation is improving museum data quality and standardisation, it is still essential to visit NHMC in order to have an accurate source of information on specific taxa (Roy and Gagnon 2016). Yet, even when having access to all the data available in NHMC, there are some problems to sort out. For example, among the octocorals deposited in NBC collected during CANCAP and Tyro Mauritania II expeditions, some specimens were not yet catalogued, other specimens were mixed with specimens from a different family of octocorals having the same catalogue number for different taxa and, a specimen of Spinimuricea atlantica, present on Bioportal, could not be located in the museum at present time.

Similar concerns can be raised by mapping published species records without checking the original record and its auxiliary information (Ross et al. 2012). Unvouchered records can be easily misidentified without proper local taxonomic knowledge or observable taxonomical characters essential to identify the species (Henry and Roberts 2013). In consequence, poor quality data can be used in studies that model distribution of species and may lead to overestimation of their putative distributions (Davies and Guinotte 2011; Bullimore et al. 2013; Henry and Roberts 2013). Unvouchered records in deep-sea ecology studies need to be identified with care, particularly if plexaurid octocorals are present. Plexaurids are extremely diverse and difficult to identify in video transects and in situ images, despite high definition and highly magnified imagery. Colony morphology is not the main distinctive taxonomic feature of most octocorals, including the Plexauridae. Consequently, some plexaurid species identification based on imagery can be easily mistaken as species of different families like Acanthogorgiidae and Gorgoniidae when the sclerome of the specimens is not analysed.

Even when a specimen is available, plexaurids have a remarkable and little studied variability of their sclerites, which hampers an easy identification and description of new species (Grasshoff 1977). Moreover, genetic markers currently used in the DNA barcoding of octocorals are not as effective as it would be desirable to discriminate at species level. Notwithstanding, currently it is important to consider genetics when describing new octocoral species and their cryptic diversity (Breedy and Guzman 2011; Reijnen et al. 2014). Museum specimens, when suitable for genetics, may be used to sequence vouchered reference DNA barcodes (Morín et al. 2019). Antique octocoral reference samples can be compared with recently collected samples like some of the CANCAP and Tyro Mauritania II Plexauridae, which were already sequenced with this goal.

Deep-sea exploration is expensive and constrained to specific areas of the vast, unexplored and difficult to sample deep sea. Likewise, deep-sea sampling cruises are limited to specific sampling gears and determined depth strata. Therefore, locations where well-curated deep-sea specimens are well identified and stored through decades, or even centuries, represent inestimable access to baseline knowledge on deep-sea biodiversity. NHMC with type and reference octocoral deep-sea specimens are money savers because they decrease the need of much new expensive and time-consuming fieldwork (Suarez and Tsutsui 2004). Museum records already proved to be useful for cold-water scleractinian reef building species off the southeastern United States and for Canadian Arctic marine benthos distribution based on online databases, selected publications and visits to museums (Ross et al. 2012; Roy and Gagnon 2016). Besides, CANCAP deep-sea plexaurids represent high standard accounts for this family because their identification was made by leading experts on taxonomy of gorgonians. Additionally, they increase the knowledge on Plexauridae, a neglected but important octocoral family, with many structural species which form vulnerable marine habitats. Therefore, visits to NHMC should be encouraged to greatly increase known biodiversity, to gather reference samples with credible taxonomic status and auxiliary data associated to specimens. Visits to museums are also essential to correct errors that are hampering the accessibility to this knowledge.

More value needs to be given to NHMC like the Octocorallia collection stored at NBC. Reliability on taxonomic knowledge, which is fundamental for the quality of the following biological knowledge, is dependent on NHMC (Suarez and Tsutsui 2004). Contemporary biodiversity studies rely on the 300 years of historical research deposited in museums to have accurate knowledge on species distribution and their changes under anthropogenic pressures and climate change (Roy and Gagnon 2016). This is particularly true for deep-sea invertebrates for which there is little taxonomical knowledge, especially in unexplored geographical areas. In some cases, like the Plexauridae from Cape Verde islands herein mentioned, NHMC store the unique existing data (Graham et al. 2004; Funk 2018).

Still, many countries have no or very limited funding for taxonomy, their natural history museums have limited personal to curate and investigate collections, there are shifts in the scientific focus of NHC towards molecular studies and a trend in the scientific community in publishing biodiversity studies based on unvouchered records (Kemp 2015; Troudet et al. 2018). This raises concerns on the reliability of identifications that cannot be verified (Costello et al. 2013). Moreover, taxonomic papers are published in low-citation indexed journals and NHMC are in high risk of not being preserved in the long run (Andreone et al. 2014). The consequence of a lack of taxonomical knowledge is the loss of irreplaceable sources of high-quality biodiversity data, and the proliferation of unvouchered misidentified records with poor or no auxiliary data which, in turn, results in a doubtful source of knowledge for future generations (Yesson et al. 2007; Funk 2018; Troudet et al. 2018). This trend is alarming, particularly in the light of a biodiversity extinction crisis.

A taxonomist-ecologist partnership would benefit museums and ecological studies improving long-term storage of ecological specimens and the quality and reproducibility of ecological studies (Ward et al. 2015). Therefore, collections-based research would complement field surveys in all biodiversity disciplines to achieve a more comprehensive understanding of the taxa under study and to discover biodiversity hotspots that can be considered priority for future conservation (Minton and Perez 2010).

Threats to biodiversity emphasise the need to decrease the Linnean shortfall by gathering information on known species based in specimen collection and also in describing new species, as rapidly as possible, to understand their vulnerability and to conserve them (Costello et al. 2015; Hortal et al. 2015; Ceríaco et al. 2016; Troudet et al. 2018). Henceforth, the understanding of the current state and future effects on the NE Atlantic Plexauridae relies on a complete data gathering exercise. Considering the heterogeneous octocoral taxonomy effort in the Macaronesian archipelagos and West Africa, historical literature and specimen collection data have a great value to fill in gaps in areas where biodiversity is still unknown (e.g., at greater depths) (Sampaio et al. 2019). The present study has generated accurate baseline octocoral taxonomic status records reliable for species distribution, biodiversity and conservation studies. This Plexauridae museum database will be beneficial to decide future field surveys in geographic and taxonomic unexplored areas and for a better management of deep-sea areas where plexaurid species are rare or form vulnerable marine ecosystems (VMEs).