Description of a new and widely distributed species of Bathypathes (Cnidaria: Anthozoa: Antipatharia: Schizopathidae) previously misidentified as Bathypathes alternata Brook, 1889

For many years an undescribed species of the genus Bathypathes has been misidentified as Bathypathes alternata Brook, 1889 (a species currently re-assigned to the genus Alternatipathes). This new species is rather common at mid- and lower bathyal depths of the Pacific, Atlantic and Indian oceans, often in areas with high concentrations of commercially valuable cobalt-rich ferromanganese crusts, where it was observed in underwater photo and video transects to occur in high densities. Under the name B. alternata this species is recorded in several inventories and databases. There is an urgent need for a formal description of this misidentified and widely distributed species to avoid further confusion. The new species is superficially similar to A. alternata in having a monopodial corallum and simple, bilateral and alternately arranged pinnules. However, it differs from the former in that it has an upright corallum with a straight pinnulated part (vs. a horizontally bent pinnulated part), pinnules of uniform length and density (vs. decreasing regularly distally), and a constant distal angle formed by the pinnules and the stem along different parts of the corallum (vs. a decreasing distal angle near the top). The new species can therefore be easily distinguished from A. alternata in underwater imagery. We formally describe this new species in the genus Bathypathes and assign it the new name B. pseudoalternata. An extensive synonymy list with previous misidentified records is provided. To evaluate the distributional patterns of the new species we review the geographic distribution of antipatharians reported below 800 m. The majority of the hitherto described lower bathyal and abyssal species have been recorded from one biogeographic province; however, 20 species are known from more than two provinces, and only three species are widely distributed (>5 provinces), including the newly described Bathypathes pseudoalternata. Members of the family Schizopathidae, to which the new species belongs, represent the majority of the lower bathyal (50.54%) and abyssal (82.35%) species.


INTRODUCTION
Corals, including black corals, are key components of hard-substrate ecosystems in the deep sea. Black corals are known to be an important habitat for a diverse range of organisms, providing food, shelter, nurseries and breeding grounds for number of organisms, thus enhancing biodiversity in the deep sea (Braga-Henriques et al., 2013;Wagner, Luck & Toonen, 2012;Yesson et al., 2017;De Clippele et al., 2019). Being slow-growing and expected to have slow rates of repopulation, black corals are primary indicator species for deep-sea vulnerable marine ecosystems including those associated with deep-sea mineral resources such as cobalt-rich crusts and polymetallic nodules (Molodtsova & Opresko, 2017). As a result, they are increasingly becoming the focus of conservation efforts (Braga-Henriques et al., 2013;Yesson et al., 2017). Therefore, it is important to be able to accurately identify the species at risk and to document the overall diversity of black corals in those areas. In addition, because information regarding the distribution and diversity of deep-sea organisms (e.g. cold-water corals) is increasingly being derived from imagery alone (Howell et al., 2019), there is a need to accurately identify species using gross morphological features whenever possible.
For a number of years two species of antipatharian corals of the family Schizopathidae have been reported in the literature under the name Bathypathes alternata Brook, 1889, due to the superficially similar appearance of their corallum (Molodtsova & Opresko, 2017). Each of these species forms a monopodial corallum with simple, bilateral and alternately arranged pinnules. However, the holotype of B. alternata and conspecific specimens from the lower continental slope and abyssal plain of the North Pacific (2,670-5,089 m; Molodtsova & Opresko, 2017), demonstrate a very distinctive pinnulation pattern in which the pinnules show a regular decrease in length from the lower parts of the pinnulated section of the corallum to the apex. In addition, the distal angle that the pinnules form with the stem decreases from around 60 on the lower section of the stem, to 30 or less towards the apex of the corallum. Thus, the overall shape of the pinnulated section of the corallum is triangular. In this morphotype the stem of the corallum is often bent over such that it extends out almost horizontally to the substrate with the pinnules curved down towards the substrate giving the corallum a windsock-like appearance (Molodtsova & Opresko, 2017). In contrast, a number of specimens that have also been assigned to "Bathypathes alternata", but reported from shallower depths (see e.g., Opresko, 1974;Wagner, Waller & Toonen, 2011;Barnich, Beuck & Freiwald, 2013;Brugler, Opresko & France, 2013;MacIsaac et al., 2013) have a very different colony shape and pinnulation pattern. These colonies are typically upright; the pinnules along most of the stem are similar in length, except those lowest on the stem and those near the apex where they are shorter and more or less straight (Fig. 1). Samples from the latter morphotype from Hawaii and the Northwest Atlantic have been sequenced using several different mitochondrial gene regions and the sequences have been deposited in GenBank under the name Bathypathes alternata (Brugler, Opresko & France, 2013;MacIsaac et al., 2013). These sequenced specimens were shown to have a close affiliation to species currently assigned to the genus Bathypathes Brook,1889, whereas a specimen having a similar  . (E) NOAA Ship Okeanos Explorer EX1703 Dive 7. Titov Seamount (Central Pacific) (Kennedy, Demopoulos & Auscavitch, 2017). pinnulation pattern as the typical form of Bathypathes alternata grouped with Umbellapathes bipinnata Opresko, 2005(see Brugler, Opresko & France, 2013Chery et al., 2018;Horowitz et al., 2020). Bathypathes alternata sensu stricto and U. bipinnata have since been assigned to the new genus Alternatipathes Molodtsova & Opresko, 2017. DNA sequencing studies indicate that specimens of the second morphotype, which have also been identified as "Bathypathes alternata" are not related to Alternatipathes alternata (Chery et al., 2018;Bilewitch & Tracey, 2020). Therefore, there is an urgent need for a formal description of this misidentified and widely distributed species to avoid further confusion, which is the purpose of this study.

MATERIALS AND METHODS
All examined material of the new species is listed in Specimens were studied at IORAS and NMNH. At IORAS, microscopic skeletal features were examined using a TESCAN VEGA 3 LMU and Camscan S2 scanning electron microscope (SEM). At the NMNH, specimens were examined using an AMRAY 1810 or a Zeiss EVO MA 15 SEM. Fragments of pinnules up to 10 mm long were cleared of tissue, air-dried and coated with a 30-40 nm thick layer of 60% gold and 40% palladium prior to scanning. Measurements of the microscopic skeletal features were made using an optical dissecting microscope or light microscope equipped with an ocular micrometer or from the photographs taken under SEM. SEM stub numbers are from a specimen series deposited at the NMNH.
In situ images of colonies used for distributional records (Supplemental Material S1) were retrieved from the NOAA National Database for Deep-Sea Coral and Sponges Version 20210414-0 (https://deepseacoraldata.noaa.gov/) (Hourigan et al., 2015), the NOAA Benthic Deepwater Animal Identification Guide V3 (https://oceanexplorer.noaa. gov/okeanos/animal_guide/animal_guide.html), and Ocean Networks Canada SeaTube V3 (https://data.oceannetworks.ca/ExpeditionManagement). Only images of colonies that matched the external diagnostic features in overall colony morphology and branching pattern were included in the analysis. It is important to point out that diagnostic characters for this species in terms of polyp and spine morphology are not visible in in situ photos. Therefore, the species-level assignments of the photo records should be considered tentative.
The terminology used in the species descriptions generally follows that outlined in Opresko, Tracey & Mackay (2014). The size of the polyps, referred to as the transverse diameter, is the distance between the distal edge of distal lateral tentacles and the proximal edge of the proximal lateral tentacles of the same polyp. The polyp density is the number of polyps along a given segment of pinnule. The distance between spines is the distance between centers of the bases of adjacent spines in the same axial row. The height of a spine is the distance between the apex and the center of the base of a spine. The number of axial rows of spines is determined as the number of complete rows (those in which the base of the spines is visible) that can be counted in one lateral view (also referred to as one aspect). Counts and measurements for selected specimens of Bathypathes pseudoalternata sp. nov. are provided in Table 2. We analyzed literature distribution records of deep-sea black corals (Supplemental Material S2), which were supplemented with unpublished distribution records validated by the authors (Supplemental Material S3). As fauna of the black corals in the mid-and upper bathyal zones is understudied, we restricted our analysis to black corals known from depths below 800 m, and included those in the lower bathyal zone (801-3,500 m) and abyssal zone (3,501-6,500 m; Watling et al., 2013). Currently accepted biogeographical  provinces for these two depth zones (Watling et al., 2013) were used. To avoid ambiguities in the analysis of geographical distribution patterns, only species determined to species level or provisionally identified as new species were considered (Supplemental Material S2). The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: [urn:lsid: zoobank.org:pub:58FF4531-1478-4462-84ED-A7E96920710E]. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS. The holotype of the new species is deposited in the collections of the NMNH and the paratype is deposited in the collections of the BPBM. Diagnosis. Corallum monopodial, unbranched or rarely branched, and pinnulate. Pinnules simple, arranged alternately or suboppositely in two anterolateral or lateral rows. Length of pinnules on stem and branches usually longest near the middle of the pinnulated section of the corallum. Striatum present or absent. Spines conical, smooth, usually simple, but in some cases forked or multiply knobbed at apex, with acute to slightly rounded apex. Spines often larger on polypar side of axis than on abpolypar side. Polyps elongated in the direction of the skeletal axis from 2 mm to as much as 17 mm in transverse diameter.      present it begins near the middle of the unpinnulated portion of the stem and generally extends past the second pinnule from the bottom. The striatum is generally more distinct on the abpolypar side of the stem. Pinnules simple, subequal in length over most of corallum, slightly shorter at the bottom of the pinnulated section and also decreasing in length near the apex; length usually about 10 cm in colonies approximately 20 cm tall and up to 22 cm in colonies 40 cm or taller; arranged alternately in two lateral to anterolateral rows along the stem. Spacing of pinnules slightly variable within colonies (increasing or decreasing distally along stem), but very variable between colonies (5-12 mm apart in each row); resulting in pinnular densities ranging from 6-8 to 10-12 (total per 3 cm) between colonies (or 8-18 pinnules total per 5 cm). Spines small, smooth, conical, with rounded apex; usually 0.03-0.05 mm tall (maximum about 0.08 mm on Description of holotype. The corallum of the holotype (USNM 1071044) is about 24 cm tall and 15 cm wide ( Fig. 2A). The entire unpinnulated lower section of the stem and holdfast are missing. Comparison of the in situ photo (Fig. 1A) with that of the collected specimen suggests that the lower unpinnulated section of the stem was about 6 cm long prior to collection. The diameter of the stem at the broken basal end is 1.9 mm. The pinnules are simple, bilateral to slightly anterolateral, strictly alternating, and similar in length along most of the stem. The longest pinnules are about 10 cm long and 0.6 mm in diameter near the base; they occur slightly above the middle of the pinnulated section of the corallum. The two rows of pinnules form a wide interior angle, and the individual pinnules are directed distally. The distal angle of the pinnules with the stem ranges from about 60 to 70 . The pinnules in each row are about 8 mm apart proximally, decreasing to 6 mm distally along the stem, resulting in a pinnular density of 8 (total for both rows) per 3 cm on the lower part of the stem, but increasing to 10 per 3 cm distally (13 per 5 cm increasing to 16 or more per 5 cm distally). Along 10 cm of stem there are a total of 28 pinnules.

Systematic description
The pinnular spines (Fig. 2C) are small, conical, slightly compressed laterally, with a rounded apex, and flared out at the base in the proximal and distal direction. They stand out at a right angle to the axis. On a section of pinnule about 0.5 mm in diameter, the  (Table 1), as well as photo records of specimens that show diagnostic characters for this species (Supplemental Material S1).
Full-size  DOI: 10.7717/peerj.12638/ fig-7 polypar spines are 0.04-0.045 mm tall and the abpolypar spines 0.02-0.03 mm tall. Five to six rows of spines are visible in lateral view. The spines are 0.15-0.38 mm apart in each row, with about 5-6 spines per mm. The polyps (Fig. 2B) on the pinnules are mostly 4.0-4.5 mm in transverse diameter. The smallest polyps (~3.5 mm) are often found near the base of the pinnules. The interpolypar space is small, and the polyp density over most of the pinnules is typically about 2.5 per cm.
Description of paratype. The paratype (BPBM D1775, Fig. 3A) is about 42 cm high and 30 cm wide, with pinnules up to about 15 cm long. The longest pinnules occur about 25 cm below the top of the corallum and about 10 cm above the lowermost pinnules which are 10-11 cm long. The pinnulated section is 37 cm long; the unpinnulated stalk is 6 cm (the basal plate is missing); and the diameter of the stem at its basal end is about 4 mm. The alternately arranged pinnules are up to 12 mm apart proximally, decreasing to 7 mm distally resulting in a pinnular density of 5-6 per 3 cm on the lower part of the stem and increasing to 8 per 3 cm distally.
In the in situ photo of the colony (Fig. 1B), the interior angle formed by the two rows is about 120 on the lower part of the corallum, decreasing to about 90 higher up. In the preserved specimen, however, the interior angle is closer to 180 over most of the corallum. The distal angle of the pinnules with the stem ranges from about 75 to almost 90 . Pinnules on the lower parts of the corallum tend to be less inclined distally than those in the upper part of the corallum.
On the pinnules the polypar spines are up to 0.064 mm tall, and the abpolypar spines are up to 0.03 mm tall. There are 6-7 rows of spines. The spine density is 5 per mm on the pinnules and 8 per mm on the stem.
In the preserved specimen the polyps over most of the proximal section of the pinnules are 3.5 to 4.2 mm in transverse diameter; in the midsection and distally they are up to 5 mm in transverse diameter (Fig. 3E). The polyp density ranges from about 3 per cm lower on the pinnules to 1.5-2 per cm near the tip of the pinnules.
Additional material. We include here detailed descriptions of specimens from the western Atlantic and New Zealand, two localities very distant from the type locality, to illustrate the fact that based on our present information it appears that the species is cosmopolitan in distribution. The preserved specimen from the western Atlantic (USNM 1093063, Fig. 4A) has a 45.5 cm long curved stem which at its basal end is 2.7 mm in diameter. A striatum is present and starts at 1 cm above the basal end of the stem and extends for 5 cm beyond the point where the pinnules first appear. The lower 5 cm of the stem lacks pinnules. The pinnules in each row are spaced 7 mm apart on the lower part of the stem, increasing to 10 mm on the upper part. The pinnular density is 9 (total) per 3 cm on the lower parts of the corallum and 7 per 3 cm on the upper parts. The longest pinnules occur near the middle of the corallum and are 15.5 cm and have a basal diameter of about 0.6 mm. The interior angle formed by the two rows varies over different parts of the stem, from greater than 90 on the lower parts of the stem to less than 90 in the upper parts and greater than 90 near the apex. The pinnules are directed distally, with the distal angle being about 60 . The polypar spines (Fig. 4B) are 0.05-0.058 mm tall and the abpolypar spines 0.038 to about 0.05 mm. Seven or eight rows of spines are visible in lateral view and the spine density is 5 per mm. The polyps are close to 5 mm in transverse diameter, and there are 2 polyps per cm.
The specimen from New Zealand (NIWA 24195, Fig. 5A) is a 71 cm tall, upright colony about 40 cm wide, and with stem diameter 1 cm below the pinnulated section of 5 mm by 7 mm. The pinnules are up to 22 cm in length in the middle section of corallum and about 2 mm in diameter near the base. The pinnular density is 8-10 (total for both rows) per 3 cm. The polypar spines on a pinnule 0.7 mm in diameter are 0.036 mm tall and the abpolypar spines 0.025 mm tall (Fig. 5B). The small conical spines have a very rounded apex, and are arranged in axial rows with six to eight rows visible in one view on a section of pinnule 0.7 mm in diameter. Within each row the spines are spaced 0.16-0.26 mm apart, resulting in 5-6 per mm in each row. The polyps are about 4 mm in transverse diameter, with about 2.5 polyps per cm.
Intraspecific variation. The taxonomic characters of the specimens assigned to this species are shown in Table 2. They are divided into groups by geographic region. The data presented leads to the following conclusions: (1) colonies can reach 70 cm or more in height and are usually very upright; (2) the lower unpinnulated section of the stem is very short, less than 7 cm; (3) a striatum can be present or absent (if visible, it begins in the middle of the unpinnulated section, normally 1.5-2.0 cm below the first pinnule and continues up to the base of fourth to eleventh pinnule); (4) the maximum length of the pinnules in the largest colonies is about 20 cm; (5) the pinnules are subequal in length over most of a colony with the exception of those lowermost on the stem and the newly developing ones at the apex; (6) the length of the pinnules is not necessarily correlated with the size of the colony; (7) the spacing of the pinnules within a row ranges from 4-12 mm, but is most often 6-10 mm; (8) pinnular density (total for both rows) ranges from 8 to 18 per 5 cm between colonies, but varies little within colonies or increases or decreases slightly distally; (9) the interior angle formed by the two rows of pinnules is very variable and ranges from less than 90 to 180 ; (10) the distal angle of the pinnules is usually 60-80 ; (11) polypar spines are typically 0.03-0.04 mm tall, but can be up to 0.07 mm in some colonies and on the thicker pinnules; (12) the spines can split to form double spines and additional rows; (13) the transverse diameter of the polyps is generally 4.0-4.5 mm, but can range from about 3 to 5 mm, with the smaller polyps often found at the proximal end of the pinnules; and (14) the polyp density is most commonly about 2.5 per cm, but can range from of 2 to 3 per cm. In the largest specimen examined (USNM 96356, >67+ cm) a striatum is not visible.
There are slight differences in the morphology of the spines in the examined specimens. In the specimens from Hawaii, the spines are generally triangular, with a wide base that extends out along the axis in both the proximal and distal direction (Figs 2B and 3B-3D), whereas some colonies from the Atlantic have spines that are narrower, more upright and a bit more conical (Fig. 4B). Some specimens from New Zealand have spines with a strongly rounded apex (Fig. 5B).
One of the Hawaiian specimens assigned to this species (USNM 1163569, Fig. 6A), differs from the typical form in that the spines tend to lie along very distinct ridges (Fig. 6B), a characteristic that also occurs to varying degrees, in other Hawaiian colonies. This specimen also has relatively short pinnules given the size of the colony. The pinnules are not more than about 10 cm long, although the entire colony is 44 cm tall, more than twice the size of the holotype which has pinnules of about the same size.
Associated fauna. A specimen from the eastern Gulf of Mexico was reported to harbor the polychaete Eunoe purpurea Treadwell, 1936 (Polynoidae) along the main stem (Barnich, Beuck & Freiwald, 2013: fig. 4a). In a similar way many colonies from the North Pacific, including smaller colonies 16 cm high were observed to harbor similar unidentified polynoid polychaetes (identification T. Britayev and D. Martin;see Figs. 1B and 1F,Supplemental Material S4). The polychaete was always observed nestling along the polyps on the main stem of the coral, with the tentacles forming a soft tunnel around the polychaete worm. Neither worm-runs nor apparent changes in pinnule arrangement, as often described in other associations of scale-worms with antipatharians (Molodtsova, Britayev & Martin, 2016), were reported. Several species of squat lobsters of the family Chirostylidae were also reported in association with B. pseudoalternata sp. nov. Comparisons. Most of the currently known species in the genus Bathypathes have pinnules arranged in subopposite pairs. The only other species of Bathypathes with alternately arranged pinnules is B. platycaulus Totton, 1923 (Fig. 8). It also forms an upright bilaterally pinnulate colony; however, it differs from B. pseudoalternata sp. nov. in having much more densely arranged pinnules (25-30 per 5 cm vs. 8 to 18 per 5 cm), thinner pinnules and smaller polyps (mostly 2-3 mm in transverse diameter vs. 3 to 5 mm).
Stichopathes robusta is a problematic nominal species with an unknown colony shape and pinnulation pattern which was originally described from a fragment (Fig. 9) is proposed here above as Schizopathidae incertae sedis. It differs from the new species by having more rows of spines on the pinnules (9-13 vs. 6-8 visible from side) and very characteristically multilobed and furcated spines (Figs. 9B, 9C and 9F).
Determinations from underwater imagery. Bathypathes pseudoalternata sp. nov. forms a characteristically Bathypathes-type monopodial colony with two rows of long pinnules, longest near the middle of the pinnulated section. Upright colonies of Bathypathes pseudoalternata sp. nov. with pinnules of a uniform length and forming an inner angle not exceeding 180 can be easily distinguished from the characteristically windsock-like colonies of representatives of the genus Alternatipathes (Supplemental Material S4). Compared to Alternatipathes, B. pseudoalternata sp. nov. has an upright corallum with a straight pinnulated part (vs. bent pinnulated part in Alternatipathes spp.), pinnules of uniform length and density (vs. decreasing regularly distally), and a constant distal angle formed by the pinnules and the stem along different parts of the corallum (vs. decreasing of distal angle near the top) (Supplemental Material S4). The new species can be distinguished from most of the hitherto known species of the genus Bathypathes by its alternating pinnules and from Bathypathes platycaulus (normally reported from shallower depths of less than 150 m) by less densely arranged pinnules (16-30 vs. > 50 per 10 cm).
Etymology. Species name "pseudoalternata" derived from species name "alternata" with Greek prefix "pseudo", meaning "false", is chosen to denote that for many years this species was misidentified as Bathypathes alternata.
Distribution. Currently recorded from the North Pacific, the North Atlantic, New Guinea, the Great Australian Bight, New Zealand; Tasmanian seamounts, at depths ranging from 331 to 4,152 m (Fig. 7).

Distribution of black corals in the lower bathyal and abyssal zones
Based on published data and additional distributional records checked by the authors, of the 285 currently described black coral species, many occur in the deep sea (Table 3). We analyzed lower bathyal and abyssal distributions separately because several species were reported from the transition zone and thus a single record belongs to both abyssal and lower bathyal zones. We did not use GBIF (www.gbif.org) distribution records that were not checked by us due to the high risk of possible misidentifications introduced by machine observations. Thus, several GBIF distributional records for Alternatipathes alternata based on machine observation of underwater photographs appeared in fact to be holothurian feces. Also, museum collections data determined earlier, especially before 2001, have to be checked for consistency as after 2001 several taxonomic revisions were published and many new species and genera were described (see e.g., Opresko, 2002Opresko, , 2005Opresko, , 2019Molodtsova & Opresko, 2017;Opresko & Molodtsova, 2021 for Schizopathidae). We caution others not to use such data without first checking the identifications. Based on our compilation (Supplemental Material S2, Table 3) 31.57% of the total number of species described (90 nominal species in 27 genera) occur below 800 m, and of this number 14 species in 6 genera occur below 3,500 m. The majority of the lower bathyal and abyssal black corals belong to the family Schizopathidae, representing 51.58% (42 nominal and seven potential new species) of the total number black coral species (95) reported from depths of 800-3,500 m and 85.71% (12 of 14 nominal species) from depths below 3,500 m (Figs. 10A and 10B). Only two schizopathid genera (Dendropathes Opresko, 2005 andSaropathes Opresko, 2002) are known exclusively at depths less than 800 m.   Genera Dendrobathypathes Opresko, 2002, Parantipathes Brook, 1889and Stauropathes Opresko, 2002 are predominately bathyal with the majority of species known from lower bathyal depths. Genus Abyssopathes Opresko, 2002 is known mainly from abyssal depths (Molodtsova & Opresko, 2017), whereas genera Alternatipathes, Bathypathes, and Schizopathes are equally present in the lower bathyal and abyssal zones. Six of 14 abyssal species are Bathypathes species (Table 3). Based on existing records (Supplemental Material S2), the most diverse lower bathyal province, in terms of number of species reported, is the North Atlantic (BY4) with 43 species reported, followed by New Zealand-Kermadecs (BY6:18 species), Indian Ocean (BY11:16 species), Subantarctic (BY10) and South Atlantic (BY13), each with 15 species reported. Not a single species of black coral has been recorded from the High Arctic (BY1) or from the Southeast Pacific Ridges (BY5) lower bathyal provinces. In the abyssal zone, the most diverse province is the North Pacific (AB13) with 8 species reported below 3,500 m, followed by the Indian (AB8:5 species); and North Atlantic (AB2), Antarctica East (AB6) and Equatorial Pacific (AB11), each with four reported species. The great majority (59 species of 95 determined to the species level including potential new species for the lower bathyal and 9 species of 14 determined to the species level for the abyssal

DISCUSSION
Although many species inhabiting the slopes of oceanic ridges and seamounts have been reported to show a high degree of endemism (see e.g. Parin, Mironov & Nesis, 1997;Richer de Forges, Koslow & Poore, 2000); seamount-scale endemism has not yet been shown to occur in antipatharians, although it should be noted that only one study using a limited number of mitochondrial markers has evaluated this possibility (Thoma et al., 2009). As it is possible to see from existing distribution data (Supplemental Material S2) most of the hitherto known species of black corals are limited to a single biogeographical province or two adjacent provinces. Only a few species demonstrate a wider geographic distribution pattern, including the new species, Bathypathes pseudoalternata, described here. On the other hand, it is possible to see that data on the distribution of the deep-sea black corals are critically scarce and uneven. Many species are known from a single locality. Consequently, some identified trends reflect sampling effort rather than actual distribution patterns. Many new species are currently being described, but there is a crucial need for local faunal reviews and faunal lists, to document the vertical and geographical distribution patterns. Given the high sampling effort in the High Arctic province (BY1), the absence of antipatharian records from this region likely is due to black corals being rare in this region. In contrast, the fact that only a single species is recorded from the Southeast Pacific Ridges lower bathyal province (BY5) is likely due to limited sampling efforts in this region. Abyssal provinces are even less studied in term of black corals (see Supplemental Material S2). Most local faunal reviews consider shallow-water species only, with a few exceptions, including the: North Atlantic (Molodtsova, 2006;Braga-Henriques et al., 2013;Molodtsova, 2014), South Atlantic (Lima, Cordeiro & Perez, 2019), Great Australian Bay (MacIntosh et al., 2018), South Pacific (Horowitz, Opresko & Bridge, 2018) and Central Pacific (Molodtsova & Opresko, 2017) with a few additional reviews forthcoming in the near future. The material of Bathypathes pseudoalternata sp. nov. examined as part of this study came from six lower bathyal provinces (North Atlantic BY4, New Zealand-Kermadecs BY6, Subantarctic BY10, Indian Ocean BY11, West Pacific B12, and North Pacific BY14), one abyssal province (North Atlantic AB2), and also from mid-bathyal depths of the Gulf of Mexico, North-West and North-East Atlantic, slopes of Hawaii, New Zealand and Madagascar. Consequently, this species has a cosmopolitan distribution. While our study focused on morphological analyses, there also exists genetic support for such a wide distribution pattern of Bathypathes pseudoalternata. Brugler, Opresko & France (2013) sequenced ten specimens of B. pseudoalternata (identified at the time as B. alternata). Nine of these specimens were collected in the North Atlantic and one was collected in the North Pacific near Hawaii (paratype, BPBM D1775). Brugler, Opresko & France (2013: Table S1) found that all 10 specimens share identical haplotypes for three mitochondrial DNA regions (igrW between trnW-nad2; igrN between nad5-nad1; and cox3 to cox1, excluding the Igr) which were different from all hitherto studied species in the genus Bathypathes. Subsequent morphological study of some of the same specimens sequenced by Brugler, Opresko & France (2013) has shown that a unique combination of haplotypes across all 3 mt gene regions can be indicative of a genus level taxon (see description of Telopathes magna MacIsaac & Best, 2013in MacIsaac et al., 2013 and a unique combination of haplotypes across 2 or 3 mt gene regions can be indicative of species-level taxa (see descriptions of Bathypathes alaskensis and B. ptiloides in Opresko & Molodtsova, 2021). Based on these results we postulate that the small morphological differences seen among some B. pseudoalternata sp. nov. specimens (particularly in the shape of the spines) in the absence of genetic differences, represents only intraspecific variation.
In a more recent phylogenetic study, Bilewitch & Tracey (2020) sequenced three mitochondrial gene regions and two nuclear gene regions for a number of Bathypathes specimens collected from the seas around New Zealand. The mt gene regions were 16S, TrnW-igr-ND2 and ND5-igr-ND1. One of the specimens sequenced by Bilewitch & Tracey (2020) was identified as Bathypathes alternata (NIWA 64561); however, examination of photos of the specimen (provided by J. Horowitz) indicate that the gross morphology of the colony matches the morphotype of B. pseudoalternata. In their phylogenetic reconstructions using TrnW-igr-ND2 and ND5-igr-ND1, Bilewitch & Tracey (2020) included the sequence data for North Atlantic specimens of Bathypathes "alternata" (= B. pseudoalternata) from the Brugler, Opresko & France (2013) study. The resulting phylogenetic reconstruction using TrnW-igr-ND2 revealed that the New Zealand specimen had the same haplotype as the four North Atlantic specimens. In the phylogenetic reconstruction using ND5-igr-ND1 (Bilewitch & Tracey, 2020, Appendix Table B2), NIWA 64561 was indistinguishable from six North Atlantic specimens of B. pseudoalternata sp. nov., one of which was YPM IZ 028566 (field number MAN 201-1), the same specimen that Chery et al. (2018) showed had a nad5-IGR-nad1 haplotype identical to that of three Hawaiian specimens of B. pseudoalternata. In both of the phylogenetic reconstructions of Bilewitch & Tracey (2020), the New Zealand and North Atlantic specimens of B. pseudoalternata sp. nov. formed a separate subclade that fell within a larger clade containing subclades of specimens of Stauropathes, Bathypathes, and Telopathes MacIsaac & Best, 2013 in MacIsaac et al., 2013 as well as two subclades that the authors considered unknown genera.
It should be noted, however, that depending on the methodologies employed, phylogenetic analyses using a limited number of mitochondrial regions may not always be adequate to distinguish between closely related species. For example, in their analysis, and using TrnW-igr-ND2 data taken from GenBank, Bilewitch & Tracey (2020) did not find any differences between B. alaskensis Opresko & Molodtsova, 2021 (KF054475), B. ptiloides Opresko & Molodtsova, 2021 (KF054479), and numerous New Zealand specimens identified as B. patula. However, in Brugler, Opresko & France (2013), the holotype of B. ptiloides was found to have different haplotypes from B. alaskensis across 3 mt gene regions, IgrW, IgrN, and cox3-cox1 (none of the New Zealand specimens identified as B. patula in Bilewitch & Tracey were sequenced by Brugler, Opresko & France (2013)). In some cases, limited mitochondrial gene data are even insufficient to separate three different genera in the family Schizopathidae (see Brugler, Opresko & France, 2013 regarding the 'trigeneric complex').
Although the available genetic studies provide supporting data that B. pseudoalternata is cosmopolitan, the absence of genetic differences in a limited number of mt gene regions in geographically separate populations does not preclude the possibility that new genetic data will reveal differences in these populations; therefore, our conclusions are largely based on the available morphological evidence. The situation may change with development of new genetic approaches and the use of different mitochondrial and nuclear markers. Therefore, the geographical variations described here, particularly in the morphology of the spines, may eventually prove to be indicative of separate species. However, for the time being there is no reliable morphological or molecular evidence to distinguish between specimens from the Pacific and Atlantic oceans and New Zealand described here as Bathypathes pseudoalternata. Similar results showing a wide geographic distribution based on both morphological and molecular analyses was recently reported for the shallow and mesophotic antipatharian species Antipathes grandis Verrill, 1922 in the family Antipathidae (Gress et al., 2020).
It is noteworthy that many of the records of Bathypathes pseudoalternata sp. nov. identified as part of this study (Fig. 7) are from seafloor areas that are known to contain high concentrations of commercially-valuable seabed minerals, specifically cobalt-rich ferromanganese crusts in the Central Pacific and North Atlantic (see Clark et al., 2011;Miller et al., 2018). This has important implications for the international seabed mining regulations that are currently being developed (Miller et al., 2018). While the growth rate and lifespan of B. pseudoalternata has not been studied to date, black corals include some of the slowest growing and longest living organisms on Earth, with lifespans of individual species ranging from centuries to millennia (reviewed in Wagner, Luck & Toonen, 2012). Given the reported slow growth rates that are characteristic for black corals and recognizing the ecological importance of these species, areas inhabited by them should be avoided by extractive industries, including seabed mining activities.

CONCLUSIONS
The new species Bathypathes pseudoalternata, common at mid-and lower bathyal depths of the Pacific, Atlantic and Indian oceans, and previously misidentified with the abyssal species Alternatipathes alternata (Brook, 1889), is formally described based on morphological data. The new species is virtually cosmopolitan and has been reported from continental slopes, ridges and seamounts, including areas covered by cobalt-rich ferromanganese crusts at lower bathyal depths in six deep-sea provinces (Watling et al., 2013) in the Pacific, Atlantic and Indian oceans. Available genetic (MacIsaac et al., 2013;Brugler, Opresko & France, 2013;Chery et al., 2018;Bilewitch & Tracey, 2020) and morphological data (this study) are not sufficient to distinguish between geographically distant populations, therefore leading to the hypothesis that these geographically dispersed populations all belong to the same species. We demonstrated a critical need for local faunal reviews of deep-sea black corals. Existing data on the distribution of the deep-sea black corals are critically scarce, uneven and reflect sampling effort, thereby masking actual distribution patterns. Also, we warn that the identifications of corals in underwater photographs by machine learning need to be checked by experts before being used as distribution records. Finally, the finding of a new species of black coral, which creates habitat for various other associated species and is likely slow growing, indicates that areas where it occurs should be avoided by extractive industries, particularly seabed mining.
information on the material in their collection and/or for their assistance during visits to their respective institutions. We also would like to thank T. Britayev of the Severtsov Institute of Ecology and Evolution RAS, D. Martin and E. Macpherson of CEAB for determination of associated fauna. The specimen from the Great Australian Bight was collected under the Great Australian Bight Deepwater Marine Program (GABDMP), a CSIRO led research program sponsored by Chevron Australia, with data generated to be made publicly available. Photomicrographs of NMNH specimens were prepared in the SEM Laboratory at the NMNH with the aid of S. D. Whittaker. Special thanks to R. McGuinn for providing invaluable help in retrieving photo records from the NOAA National Database of Deep-Sea Coral and Sponges. Also, we would like to thank sincerely T. Hourigan of NOAA and two anonymous reviewers for their helpful comments and suggestions. D. M. Opresko is a Research Associate of the NMNH, and gratefully acknowledges that affiliation.