New species, redescriptions and new records of deep-sea brittle stars (Echinodermata: Ophiuroidea) from the South China Sea, an integrated morphological and molecular approach

. Deep-sea ophiuroids were collected by the manned submersible ‘Shenhaiyongshi’ from the South China Sea at depths of 500–3550 m, in 2017 to 2020. A total of 18 species were identi ﬁ ed, including three new species and eight new records, increasing the total number of species known from the South China Sea to 304. Most of the ophiuroids recorded from the South China Sea were found in shallow waters (77.9%) and a few of them occurred only in deep water (20.4%). Three new species are described as Ophiacantha aster sp. nov., Ophiomoeris petalis sp. nov. and Ophiopristis shenhaiyongshii sp. nov . We provide comprehensive descriptions of morphological features, including characteristics of the arm skeletons, and a phylogenetic analysis based on COI and 16S sequences. Overall intraspeci ﬁ c and interspeci ﬁ c genetic distance variations among the families found in this study were 0.5% to 2.47% and 1.16% to 44.16%, respectively, along the South Paci ﬁ c region to the South China Sea. Our phylogenetic analysis suggested that COI partial genes resolved the interspecies complexity in the class Ophiuroidea better than 16S partial genes. The order Euryalida had low interspecies genetic distance variation within the class Ophiuroidea. The present study suggests a high probability that species of Asteroschema and Gorgonocephalus are more widely spread around the Indo-Paci ﬁ c region than previously expected. deep-sea ophiuroid examined in this study identi ﬁ ed to 18 species, three new species and eight new species represent 12 within seven families in the class Ophiuroidea. The most common deep-water ophiuroid from our


Introduction
The class Ophiuroidea contains more than 2100 accepted species (Stöhr et al. 2021) and new species are discovered frequently. This class includes 259 genera, 34 families and six orders. Ophiuroidea are widespread across all oceans and occur in various physiochemical environments. They have a range of lifestyles, adapted to a wide range of marine environments. The majority of ophiuroids are bottom dwellers on the seafl oor, buried in mud or hidden in crevices of rocks and corals, but some species are epizoic, living on various kinds of hosts, such as corals, gorgonians and glass sponges . It has been suggested that ophiuroids and corals may have a parasitic, commensal association or a mutualistic relationship (Girard et al. 2016). Deep-sea ophiuroids represent a high percentage of the biomass in deep-sea environments (Frensel et al. 2010).
DNA barcoding is an eff ective tool for the identifi cation of ophiuroid species and has become useful for rapid assessment of biodiversity in combination with fi eld surveys (Ward et al. 2008;Hoareau & Boissin 2010). Previous molecular studies indicated that interspecies genetic distances within the class Ophiuroidea range approximately from 2.2% to 31.6% for the COI partial gene (Muths et al. 2006(Muths et al. , 2009Boissin et al. 2008Boissin et al. , 2011Boissin et al. , 2017Pérez-Portela et al. 2013;Richards et al. 2015;Sands et al. 2015) and intraspecies genetic distances approximately range from 0.5% to 6.4%, with a mean of 2.2% (Boissin et al. 2017). Recently, advanced molecular technology and a large amount of molecular data being collected have become a major advantage in the identifi cation of ophiuroid species (Okanishi et al. 2011a;Okanishi & Fujita 2013;O'Hara et al. 2014O'Hara et al. , 2016O'Hara et al. , 2019Boissin et al. 2017;Christodoulou et al. 2019). However, no molecular analysis of deep-sea ophiuroids in the South China Sea has been done yet.
The South China Sea covers 3.5 million km 2 and is part of the Western Pacifi c Ocean. It includes over 200 islands and islets with coral reefs in waters shallower than 200 m, to deep-sea basins, reaching nearly 5000 m in depth (Teh et al. 2019). The ophiuroids in the South China Sea have still not been completely studied and most of the previous expeditions hardly covered the area. The deep-sea exploring voyage of HMS 'Challenger' (1873-1876) was the fi rst major expedition that explored the South China Sea, but only at two localities. The South China Sea ophiuroids from the 'Challenger' and 'Albatross' voyages were described by several authors (Lyman 1882;Koehler 1922a). Deep-sea exploration began in China by launching the fi rst manned submersible, 'Jiaolong', used to discover the biodiversity of deep waters from Chinese seas and the Mariana trench and fi nding new deep-water fauna and fl ora communities in recent years (Li 2017).
Recent studies done in the Japan Sea indicated new species of Euryalida and new records of species that had previously only been reported from the South Pacifi c region, Australia and New Zealand (Okanishi et al. 2011a(Okanishi et al. , 2011b(Okanishi et al. , 2001cKim & Shin 2015;Okanishi & Fujita 2018a, 2018bOkanishi et al. 2018). Previous explorations of the ophiuroid community in the South China Sea compiled three comprehensive species checklists (Lane et al. 2000;Liao 2004;Putchakarn & Sonchaeng 2004), but few studies have been published since 2004 Chen et al. 2020;Li et al. 2021). Therefore, the checklist of the Ophiuroidea of the South China Sea is considered outdated.
The present study covers deep waters around Hainan Island, the Xisha Islands and the Zhongsha Islands complex in the South China Sea. Here, we present an account of the ophiuroids collected, with descriptions of new records and new species. Our goal is to present a diagnosis of the morphological features of these species combined with molecular details, to complement the limited original descriptions and the lack of fi gures, with many morphological variations presented in the literature. Three new species are described and 15 species are redescribed, including eight new records from the South China Sea; all are richly illustrated.

Morphological analysis
To document morphological characters and color patterns, most of the specimens were photographed through a dissecting stereo microscope (OLYMPUS SZX7) and larger specimens with a digital camera (Canon EOS 6DII). Micro-morphological features and smaller specimens were examined with a Phenom ProX scanning electron microscope (SEM). For SEM examination, household bleach was used to remove the outer integument (skin) from the specimens. The animal was held with tweezers and submerged in NaOCl (household bleach) diluted with water (1:1) for 20 seconds. This was repeated until the skin of the specimen had been removed. The specimens were then gently washed in distilled water and left to dry, before mounting them on a stub with carbon tape. Ethanol was used to dissolve Table 1. Deep-sea ophiuroids found in the South China Sea; damaged unidentifi ed specimens recorded under generic name. New records for South China Sea marked with an asterisk. the carbon tape to remove the specimen safely from the stub after SEM and for further analysis of the specimen. Skeletal elements of specimens were prepared by dissolving the soft tissue of part of an arm in undiluted NaOCl. The excess NaOCl was removed from the dissociated skeletal elements (ossicles) by repeated fl ushing with distilled water, to avoid formation of crystals on the surface of the ossicles. After drying, the ossicles were mounted on a stub using dissolved carbon tapes.
All sequences were aligned using the Clustal W algorithm in MEGA X (Kimura 1980;Thompson et al. 1994;Kumar et al. 2016Kumar et al. , 2018. The best-fi t substitution model of each gene in the ML trees was estimated by the 'Find Best DNA/Protein Models' option of MEGA X. The GTRGAMMA (GTR + G) model and the GTRGAMMAI (GTR + G + I) model were selected for the 16S and COI sequences, respectively (Tavaré 1986;Rodriguez et al. 1990;Edler et al. 2021). Phylogenetic trees were reconstructed for each partial 16S and COI gene by using the maximum likelihood bootstrap method. ML analysis was run with RaxML GUI ver. 2.0 by using raxmlHPC-PTHREADS-SSE3 (Edler et al. 2021). ML trees were constructed for COI and 16S with a rapid bootstrap likelihood analysis, including 1000 bootstrap replicates. Phylogenetic trees were visualized by FigTree ver. 1.4.4.
The genetic distances were analyzed according to the Kimura 2-parameter model (Kimura 1980) by using MEGA X. The overall genetic distance between and among clades that was clearly visible on the reconstructed phylogenetic trees was calculated. The standard error of each group was discovered by performing 1000 bootstrap replications.

Results
A total of 80 deep-sea ophiuroid specimens were examined in this study and identifi ed to 18 species, including three new species and eight new records. These species represent 12 genera within seven families in the class Ophiuroidea. The most common deep-water ophiuroid species from our collection was Ophientrema scolopendrica Lyman, 1883 (Table 1).
seas, at depths of 510-1100 m. Asteronyx loveni was fi rst reported from the South China Sea by Chang et al. (1962).
Distribution 62-4721 m depth. Global, except Arctic and Antarctic (Olbers et al. 2019;OBIS 2021).   Döderlein, 1927 Figs 4-6 Asteronyx luzonicus Död erlein, 1927: 64, pl  D . Flat, pentagonal. Entire dorsal disc covered by smooth, transparent, naked skin, but some small calcifi ed scales between radial shields. Skin on center of dorsal disc becoming mesh-like and less smooth than at periphery of disc. Radial shields narrow, elongate, separate, covered by skin and extending to near disc center, but not meeting in center (Fig. 4A). Ventral (oral) surface of disc also covered by smooth skin (Fig. 4B). Oral shields slightly triangular. Adoral shield large, wider than long and slightly concave at distal margin, bordering proximal end of a depressed area near genital slit. Two tooth papillae, large and pointed, three smaller pointed lateral oral papillae, teeth also pointed. Two small, short, well separated genital slits in each ventral interradius ( Fig. 4C-D).

Asteronyx luzonicus
A . Simple, similar in size and length, fi ve in number, with no abrupt change in width proximally, gradually tapering toward arm tip. No dorsal arm plates, arm covered with smooth translucent skin, leaving vertebrae visible (Fig. 4E). Ventral and lateral arm plates concealed by thick skin but slightly visible on proximal arm (Fig. 4F). Lateral arm plates meet at ventral midline, spines at ventrolateral margin extending onto ventral surface (Figs 4F-G, 5A-B). Starting from fi rst free arm segment beyond disc, fi rst two to three arm segments without spines at tentacle pores ( Fig. 4B-C); third to fourth arm segments with one or two short arm spines, becoming hook-shaped or with one secondary tooth (Fig. 4C). Following tentacle pores with two to fi ve hook-shaped arm spines and only uppermost arm spine elongated to simple hook shape, but ventralmost spines with one secondary tooth (Fig. 5D). Length of all arm spines decreasing to approximately half length of corresponding arm segment in middle of arm. On distal third of arm, arm spines half as long as corresponding arm segment and transforming into hooks without secondary tooth, except ventralmost arm spine. Ventralmost arm spine becoming cylindrical, blunt with thorny tip, as long as one arm segment ( Fig. 5A-F). Tube foot elongate, cylindrical, slender, as long as an arm segment and positioned close to ventralmost arm spine ( Fig. 5A-C).

C
. Dorsal disc light brown in center, naked skin between radial shields darker, radial shields whitish brown; ventral interradial areas light brown; arms whitish brown above and below (Fig. 4).

O
. Lateral arm plate curved around vertebrae, bearing fi ve spine articulations, each with small muscle opening, but lacking a separate nerve opening ( Fig. 6A-C). Arm spine articulations at distal edge of lateral plate, raised outwards, pointing distalwards. A depression on inner side of lateral arm plate (Fig. 6B). Vertebrae with streptospondylous articulation, ventral side with a longitudinal groove along midline and no oral bridge ( Fig. 6D-H).

Remarks
The here examined specimens were collected on a deep-sea seamount, attached to a gorgonian. Asteronyx luzonicus was fi rst described by Döderlein (1927) from the Philippines. However, there are few published descriptions of A. luzonicus (Baker et al. 2018) and the species has not been imaged well before. Döderlein (1927) mentioned an elongated, slender ventralmost arm spine, as long as an arm segment, parallel to the longitudinal arm axis, as a distinguishing morphological feature of A. luzonicus, because in other species of Asteronyx, the ventralmost spine is usually placed transversally to the longitudinal axis of the arm and often reaches twice the length of an arm segment. Dried specimens Döderlein, 1927 (IDSSE EEB-SW0003). A. Dorsal disc. B. Ventral disc. C. Ventral arm base (start of fi rst arm spine highlighted). D. Oral frame. E. Dorsal arm. F-G. Ventral arm (proximal and distal region). Abbreviations: ars = arm spine; as = adoral shield; cs = calcifi ed scales; gs = genital slit; tp = tentacle pore. Scale bars: A-B = 2 mm; C-E, G = 1 mm; F = 500 μm.

Fig. 4. Asteronyx luzonicus
of A. luzonicus are particularly strikingly dark and a main distinguishing morphological feature of A. luzonicus is the presence of black spots on the disc of sexually mature specimens (Baker et al. 2018). Specimens from our collection were not sexually mature and only had darker skin on both the dorsal and ventral disc, but they concur with a Baker et al. (2018) in the calcifi ed scales.
Recent studies on species of Asteronyx suggested that features of the dorsal disc surface, as well as the position and length of the genital slit may be important characters to distinguish the six species in this genus ). Döderlein did not describe the dorsal disc surface and mentioned few morphological characters to distinguish his new species from other species in the genus Asteronyx, i.e., in characters of the tentacle scale and darker disc spots in sexually mature specimens. Specimens of Asteronyx in our collection are immature but similar in size. Therefore, a comprehensive morphological analysis can be used to distinguish A. loveni from A. luzonicus such as: hook-shaped arm spines with at most one secondary tooth along the arm in A. luzonicus, whereas A. loveni has hook-shaped arm spines with more than one secondary tooth (Figs 2F,4F,5); in A. luzonicus the dorsal disc surface is fl atter than in A. loveni and becoming rough, mesh-like in the center (Figs 2A, 4A); the fi rst two to three arm segments in A. luzonicus have only a tentacle foot and no arm spine, but in A. loveni only the fi rst arm segment is without a single spine (Figs 2B,4B). The ventralmost arm spine is as long as one arm segment, cylindrical, tapered, with a blunt, thorny tip ( Fig. 5H-J). Asteronyx reticulata Okanishi, Martynov & Fujita, 2018 is similar to A. luzonicus, but diff ers in having mesh-like skin on its ventral disc .    D . Flat, interradially deeply excavated and small in relation to total body size of specimen. Disc covered with small, tumid, irregular scales, but larger on ventral than on dorsal disc. Conically tumid scales on radial shields, forming tubercles with fi nely thorny tip ( Fig. 7A-B), both shields and tubercles decreasing in size towards disc center. Radial shields thick, swollen, widely separated distally, convergent proximally and with additional smaller scales and tubercles. Genital slits short, vertical on ventral interradii (Fig. 7D). Seven blunt, spearhead-shaped teeth. Jaws covered with swollen scales and no true oral papillae ( Fig. 7C-D). Adoral shield obscured by swollen scales, oral shield absent and oral tentacle pore covered by small tube-shaped scales. Entire oral area covered with tumid scales, higher and pointier towards margin of ventral disc (Fig. 7C).
A . Narrow, more cylindrical and slightly swollen in fi rst 14-15 free arm segments (Fig. 7E). Dorsal and ventral arm also covered with more trapezoid tuberculous scales, distally tubercles more rounded and less elongated and arms with increasingly banded appearance distalwards ( Fig. 7E-I). Tubercles on ventral arm slightly lower and pointier than on dorsal surface. First tentacle pore with zero to one arm spine, on second to fi fth arm segments one arm spine at tentacle pores, thereafter two arm spines per pore ( Fig. 7G-H). Arm spines cylindrical with thorny tip, innermost spine slightly swollen, longer than arm width, twice as long as outermost spine, with thorny tip. Outermost spine not swollen, half as long as innermost spine, tapered (Figs 7I-J, 8C).

C
. Pale reddish-brown in alcohol specimen (Fig. 7), slightly stronger color in live specimen.
O . Lateral arm plates curved around vertebrae, bearing two strongly outwards curved arm spine articulations with large muscle and nerve openings ( Fig. 8A-B). Vertebrae with a streptospondylous articulation, with deep slope between proximal and distal end, ventrally with longitudinal groove along midline, no oral bridge ( Fig. 8D-H).

Remarks
Asteroschema horridum was fi rst described by Lyman (1879), with the Kermadec Islands as the type locality (HMS 'Challenger' Expedition). Baker (1980) and McKnight (2000) redescribed it. The specimen from our collection was similar to Lyman's holotype description, but diff ers slightly from the descriptions in Baker (1980) and McKnight (2000) in having the start of the second arm spine at the fi rst few arm segments (beyond segments 3-6, usually beyond segment 2). This character varies among species of Asteroschema (Baker 1980). Another morphological variation was shown as the extent of swelling on the proximal arm. In the present study, the small specimen (8.2 mm) had a swollen arm for fi ve to six segments, which is similar to Baker (1980) and McKnight (2000), but they did not mention the diameter of the disc (size of the specimen). The character of having a small, fl at disc with tubercleshaped scales is one that distinguishes A. horridum from other species of Asteroschema. Asteroschema horridum is considered a close relative of A. tumidum Lyman, 1879 according to morphological aspects (Baker 1980;McKnight 2000). Previously published descriptions suggested that the morphological characters of A. horridum may vary with the size and maturity of the specimens (Baker 1980;McKnight 2000).
A . Branched at least six to seven times, fl exible dorso-ventrally, fl at ventrally, high rounded dorsally (Fig. 9A). Ventral arm along fi rst branch and near base covered by smooth skin, distalwards scattered with smaller granules (Fig. 9G). Dorsal arm surface covered by domed scales, on proximal segments  with naked scales and naked areas corresponding to pedicellarial bands. Dorsal arm surface densely covered by compact granules after second arm fork. Pedicellarial bands start at second arm fork with three to four isolated clumps on each side of arm and becoming continuous across arm. From sixth arm fork, raised pedicellarial bands give an annulated appearance to arm ( Fig 9H). First arm segment lacks spines, next three with two arm spines, next nine to ten with three arm spines and thereafter four arm spines per segment, decreasing to three after fi fth branch (Fig. 9G, I-J). Ventral arm spines smaller, slightly fl attened, unevenly pointed, distally turning into multi-toothed hooks ( Fig. 9J-L). Pedicellariae with small secondary tooth ( Fig. 9K-L).

C
. Creamy white in alcohol specimen, with light brown disc and darker arms when alive ( Fig. 9A-B).

O
. Pedicellarial band formed by approximately six articulating tubercles on middle arm region and eight articulations at curved distal end ( Fig. 10A-B). Tubercles form two parallel rows with single foramen (Fig. 10A-B). Ventral arm spines distally transformed into hooks with two or three secondary teeth (Fig. 10C). Pedicellariae on pedicellarial band with single secondary tooth and apophysis (Fig. 10D). Pedicellariae on pedicellarial band diff er from ventral arm spine by smooth apophysis. Vertebrae with hourglass-shaped streptospondylous articulation with smooth lateral furrows ( Fig. 10E-I). Paired openings in lateral side of vertebrae for lateral water canals, no oral bridge ( Fig. 10F-G).

Remarks
The subspecies G. chilensis novaezelandiae is currently unaccepted and considered a junior synonym of G. chilensis (Philippi, 1858) (Stöhr et al. 2021). Our molecular results (see below) place this specimen with other New Zealand material in a sister clade to G. chilensis from Antarctica and the Southern Ocean. We consider the New Zealand clade suffi ciently diff erent from the Antarctic clade to reinstate the subspecies name for it, pending further investigation that may result in raising it to full species status. For the Antarctic clade, the name G. chilensis chilensis should be used for the time being, although the type locality is in Southern Chile, and if future molecular data fi nd the Chilean population to be in the same clade as the New Zealand and South China Sea material, a new name would need to be found for the Antarctic clade. The only morphological diff erence between the two subspecies mentioned by Mortensen (1924) is a sparser distribution of the dorsal disc granules in G. chilensis novaezelandiae. The specimen from the present study is smaller and thus probably younger than the holotypes of both subspecies and the New Zealand specimens described by Baker (1980) and McKnight (2000). However, McKnight (2000) also reported two specimens of the same size as ours (16 mm disc diameter) with a dense cover of disc granules. This character may either be variable or age-related. We consider it highly likely that the specimens studied by Baker (1980) and McKnight (2000) represent G. chilensis novaezelandiae. The morphological characters of our specimen varied slightly from their descriptions, particularly in the number of branches in the arms, which is size dependent. According to Baker (1980) the shields indirectly contributed to thickening the periphery of the disc, but other species of Gorgonocephalus showed a distinct gap in granulation at the end of the shields and granules on the periphery of the disc. Therefore, this morphological feature can be used to distinguish G. chilensis within the genus Gorgonocephalus. Considering other morphological characters, G. tuberosus Döderlein, 1902 is similar to G. chilensis by having a dense cover of coarse, conical, or hemispherical granules on the disc, small and closely arranged in the dorsal angle of the interradial space (Döderlein 1902).
H.L. Clark (1923), Seno & Irimura (1968) and Mortensen (1936) reported on younger individuals that were attached to the mature adult individuals, which prompted H.L. Clark (1923) to consider G. chilensis as viviparous. Mortensen (1936) suggested that it is not viviparous and these younger specimens simply attached to mature specimens as a host, like larger specimens attach to gorgonians (Olbers et al. 2019). This is the fi rst record of G. chilensis novaezelandiae from the Indo-Pacifi c region and far from previously recorded occurrences, which may suggest that other non-Antarctic records also belong to this subspecies. Its true bathymetric and geographic distribution is currently unclear, because most previously recorded specimens were reported as G. chilensis and need to be re-examined, preferably with molecular methods. The taxonomic value of the disc granulation should be tested by examination of a large number of specimens, which could also fi nd other distinguishing characters.

Remarks
The disc diameter was 67 mm. Our specimen is similar to the holotype description by Döderlein (1911) and the description in Liao & Clark (1995), but showed some morphological variations, especially on the disc (Fig. 11). Therefore, we hesitate to fully associate our specimen with G. dolichodactylus. The original description and the description in Liao & Clark (1995) mention small granules on both the dorsal and ventral disc, but the present specimen is completely naked except for the radial shields

Remarks
We recorded three specimens from the South China Sea at 917 m to 1911 m depth. Disc diameter ranges from 17 to 20 mm. The specimens are similar to the holotype description by Wyville Thomson (1873) and to the fi rst description from the South China Sea in Liao (2004)

Remarks
We collected a damaged specimen that could only be identifi ed to genus level as Ophiomusa. It diff ers from O. lymani by the coverage of irregular scales on the disc center, the radial shields being smaller and more triangular, large ventral disc scales connected to the oral shield entirely covering the interradial space between the genital plates and having an ventral arm plate present on up to seven arm segments from the arm base ( Fig. 13H-L). Ophiomusa sp. forms a sister clade to O. lymani on our molecular tree (see below). In the phylogeny by Christodoulou et al. (2019), this sister clade is composed of O. faceta (Koehler, 1922a) and O. facunda (Koehler, 1922a), which suggests that our specimen may also belong to one of these species. Ophiomusa faceta is similar to Ophiomusa sp. by having irregular disc scales on the disc center and radial shields separated by a single layer of disc scales, but diff ers by lacking a ventral arm plate beyond the second arm segment, smooth dorsal disc surface and 6-7 arm spines on the lateral arm plate. Ophiomusa facunda is similar to Ophiomusa sp. by having a rough dorsal disc surface and radial shields separated by a single layer of disc scales, but diff ers in having large dorsal disc scales, the entire ventral surface occupied by a large interradial scale, in lacking a ventral arm plate beyond the third arm segment and having 6-7 arm spines on the lateral arm plate. The diff erences between those species and our specimen prevent us from associating it with either of them.
D . Circular, dorsal side covered by overlapping scales with one or mostly two spines ( Fig. 15A-B). Disc spines tall, 0.4-0.6 mm high, 10-11 times as high as wide, slender, thorny, with two to three small, non-fl aring terminal thorns ( Fig. 15C-G). Few disc spines scattered on fi rst dorsal arm segment ( Fig. 15E). Radial shields covered by disc scales and skin ( Fig. 15A, E). Ventral disc completely covered by disc spines, two small disc spines at ventralmost margins of three oral shields. Genital slits conspicuous, extending from oral shield to periphery of disc ( Fig. 15B-F). Oral shield much wider than long, broadly triangular, with obtuse proximal angle and lobed or convex distal edge. Adoral shield long and narrow, mostly not separated. Adoral shields enclosing proximal edges of oral shield and separating it from fi rst lateral arm plate, reaching just beyond lateral angle of oral shield (Fig. 15F). Jaws longer than wide. Cluster of papillae on jaw with three to fi ve large, pointed, spiniform tooth papillae on dental plate and two rows of pointed papillae on apex of jaw (on oral plates), with large, pointed tip, fi ve to seven lateral oral papillae and distalmost two fl at and enlarged ( Fig. 15F).
A . Dorsal arm plates as long as wide, fan-shaped, triangular with straight proximal end, slightly curved distal margin, covered with minute spines and consecutive plates completely separated (Fig. 15J). V entral arm plates pentagonal, as wide as long except on fi rst one to two arm segments, straight obtuse triangular proximal end, distal margin truncated to concave, along arm plates thickened distalwards, consecutive plates well separated ( Fig. 15H-I). Lateral arm plates meeting above and below (Fig. 15H). 10-12 arm spines at each lateral arm plate; fi ve to six upper arm spines four to six arm segments in length, smooth and meeting across dorsal midline (Fig. 15G); fi ve to six lower arm spines short, two to three shortest, half arm segment in length, thorny (Fig. 15K). Lowermost arm spine on lateral arm  segments slightly fl attened with curved shape and straight, elongated thorns along tip of curved surface (Fig. 15H). One elongated, large tentacle scale with thorny apex, longer than ventral arm plate but becoming narrow and pointed distalwards on arm (Fig. 15K). First arm segment with two tentacle scales covering tentacle pore (Fig. 15F).

C
. When alive, grey to whitish on disc and arms, mouth region and spines light brown ( Fig. 15A-B).

O
. Arm spine articulations well developed and placed at small angle in relation to distal edge of lateral arm plate, separated from proximal part of plate by wavy ridge. Ventral and dorsal articular lobes fused into a single volute-shaped articular structure (with sigmoidal fold, see Martynov 2010), dorsal lobe perforated by small holes, ventral lobe smooth and non-perforated (Fig. 16A). Large muscle opening and small nerve opening, but dorsalmost arm spine articulation without sigmoidal fold and ventral ones entire but connected to main part of lateral arm plate by short ridge (Fig. 16A-B). Vertebrae with long zygospondylous articulation, distally abruptly truncated, dorsal median furrow moderately expressed and podial basins long and large ( Fig. 16C-G).

Remarks
The disc diameter of our specimen is within the range of the type specimens (11-21 mm). Koehler (1904) divided these specimens according to morphological variation of the arm plate and arm spines, but these variations fall within the morphological type series of O. eximia, and little morphological variation is expressed in the number of arm spines, height of the disc spines and number of oral papillae. The size of the specimens aff ects the morphological variation in O. eximia such as the number of oral papillae, the number of tentacle scales on the fi rst arm segment and disc spine length. Due to these morphological variations, it is rather diffi cult to distinguish species of Ophiotreta. Ophiotreta matura Koehler, 1904 is quite similar to O. eximia, but diff ers in having stouter disc spines with fl ared terminal thorns, thornier upper arm spines and arm plates and hook-shaped thorny ventralmost arm spines (O'Hara & Stöhr 2006). The specimen in the present study showed a unique morphological feature in two small thorny spines located at the ventral margin of the oral shields directly proximal to the genital slits (Fig. 15F).

Diagnosis
Disc spines rugose with two to three small terminal thorns on t runcated tip or minutely bifurcat ed tip. Radial shields with distal end exposed. Oral shield distal edge with series of thick, rugose thorny spines. Ventralmost arm spine with longer thorns at lateral edge, distally with some ventrally directed thorns. Tentacle scales oval and pointed, two on the fi rst three segments, thereafter one large scale.

Etymology
The specifi c name is dedicated to the manned submersible vessel 'Shenhaiyongshi', which collected the specimen.

M
. Disc diameter 6.3 mm, base of arm width 1.5 mm.
D . Pentagonal and covered with translucent, perforated, rounded, overlapping scales bearing one or two tall spines ( Fig. 17A-C). Disc spines 0.4 to 0.8 mm high, glassy, hollow, rugose with two to three small terminal thorns on truncated tip or minutely bifurcated tip. Disc spine height and shape change from center to periphery of disc ( Fig. 17C-G). Central disc spines short (0.3-0.4 mm high), smooth, with one to two terminal thorns. Peripheral disc spines and around radial shields taller (0.6-0.8 mm high), strongly rugose, with subterminal tooth and two to three terminal thorns on truncated or bifi d tip ( Fig. 17D-E). Edge of disc periphery and distal margin of radial shield with few short thorny stumps with crown of two spinelets (Fig. 17E). Proximal ends of radial shields largely covered by disc scales, distal ends exposed, triangular, pair of shields widely separated (Fig. 17G). Ventral disc covered by scales with spines, but shorter and less dense than a dorsal disc. Oral shield wider than long, triangular with obtuse proximal angle, curved lateral margins, rounded to truncated distal edge with series of short, thick, rugose, thorny-tipped spines (Figs 17H, 18A-B). Adoral shield long, with straight lateral margin, but near fi rst ventral arm plate slightly curved, pair of shields meeting proximally (Fig. 18A). Adoral shields enclose proximal edges of oral shield, curving to lateral plate of fi rst arm segment, separating oral shield from arm. Jaw slightly longer than wide, opening of second tentacle pore superfi cial (Fig. 18A).
Mostly two (one jaw has three) small, pointed tooth papillae on apex of jaw, below large pointed single column of teeth. Up to fi ve fi nger-like, tapering, pointed lateral oral papillae. Two enlarged and fl attened, pointed oral papillae arising from adoral shield and separated from other oral papillae, probably adoral shield spines (Fig. 18A). Genital slits conspicuous and extending from oral shield to periphery of disc (Fig. 18B). Oral surface covered by transparent integument, partially obscuring oral frame beneath, but only visible in live wet condition (fresh) (Fig. 17H).
A . Five moniliform arms, with glassy plates (Fig. 18C-F). Dorsal arm plates separate, as long as wide, bell-shaped, with straight proximal end, slightly wavy distal margin covered with minute spines (Fig. 18C-D). Ventral arm plates as wide as long, with convex distal end, slightly obtuse proximal end, lateral edges concave, well separated along arm, axe head-shaped proximalmost arm plates ( Fig. 18E-F). Lateral arm plates meeting above and below. Six arm spines. Three dorsal spines, three arm segments in length, laterally compressed, thorny, lateral margins with row of widely spaced, tall sharp thorns, apex truncated or bluntly rounded (Fig. 18H). Three ventral arm spines, one to two arm segments in length, laterally compressed, dense row of shorter, sharp thorns, apex truncated or blunt (Fig. 18H). Ventralmost arm spines with longer thorns at lateral edge, distally with some ventrally directed thorns, but without true hook-shape (Fig. 18H). Tentacle pores large, on up to three proximalmost segments with two scales, oval to pointed, one on lateral arm plate, other on ventral plate. Beyond third segment with single large tentacle scale on lateral plate, covering pore (Fig. 17A, E-F).

C
. In dried specimen glassy, darker in center but rest of specimen white and arm spines transparent ( Fig. 17A-B). When alive, disc glassy dark brown, arms creamy white and arm spines transparent ( Fig. 17G-H).

O
. Arm spine articulations well developed, volute-shaped, on protruding distal part of lateral plate with porous stereom, delimited from smooth middle part of lateral plate by thin wavy edge. Proximal edge of spine articulation entire, but connected with main part of lateral arm plate by short ridge. Arm spine articulation with large muscle opening and small nerve opening ( Fig. 19A-C). Dorsal arm spine laterally compressed, thorny, several longitudinal rows of perforations with widely spaced, tall thorns, apex truncated (Fig. 19D). Vertebrae with zygospondylous articulation, with moderately expressed narrow dorsal furrow, distally abruptly truncated, and podial basins moderate in size (Fig. 19E-I). Ambulacral furrow with greatly widened middle, without oral bridge (Fig. 19G).

Remarks
The genera Ophiopristis and Ophiotreta are at present poorly delimited from each other. On the phylogen etic tree in Christodoulou et al. (2019) they are both polyphyletic and several species may have to be reassigned to other genera in a future taxonomic revision. Assigning our new species to either of them is therefore diffi cult. We compared the new species to the type species of both genera, Ophiotreta lineolata (Lyman, 1883) and Ophiopristis hirsuta (Lyman, 1875). Both are present on the tree in Christodoulou et al. (2019), in diff erent clades, suggesting that they indeed represent two diff erent genera. According to the original description, O. lineolata has a cluster of tooth papillae, a dense dorsal disc cover of coarse grains intermingled with a few short spines, the radial shields are obscured by granules and there are 8-9 translucent, almost smooth arm spines and a single large tentacle scale on all but the fi rst pore (Lyman 1883). Ophiopristis hirsuta has elongated jaws with seven spiniform lateral oral papillae and no tooth papillae (although the illustration seems to show two tooth papillae), the dorsal disc is covered by short, fi ne spines (which on the illustration look rather long), the distal ends of the radial shields are exposed and swollen, the six arm spines are fl attened, glassy and strongly serrated, and there are two large tentacle scales (Lyman 1875). There was also a striking size diff erence, with O. lineolata having a disc diameter of 18 mm and O. hirsuta just 3.7 mm.
Ophiopristis shenhaiyongshii sp. nov. shares the disc spines, distally exposed radial shields, few or no tooth papillae, fl at serrated arm spines and the small size with Ophiopristis hirsuta and diff ers from Ophiotreta lineolata in all respects. Hence, we propose to assign it to the genus Ophiopristis due to its similarity to the generic type species. It can be delimited from other species currently assigned to Ophiopristis as follows: Ophiopristis dissidens (Koehler, 1905) is similar to O. shenhaiyongshii sp. nov. by having exposed distal radial shields, the oral shield wider than long and widely separated ventral and dorsal arm plates, but diff ers by having smooth disc spines, two to three tentacle scales until the twelfth segment and a brown line running discontinuously along the dorsal surface of each arm (O'Hara & Stöhr 2006). Rodrigues et al., 2011 is similar to O. shenhaiyongshii sp. nov. by having exposed radial shields, rugose disc spines with two or three thorns, the oral frame covered by a transparent integument and glassy arm plates and arm spines, but diff ers by having rounded pentagonal oral shields, contiguous dorsal and ventral arm plates, and two to three tentacle scales along the arm (Rodrigues et al. 2011).

Ophiopristis gadensis
Ophiopristis shenhaiyongshii sp. nov. diff ers from most species of Ophiopristis by having a series of thorny spines on the distal margin of the oral shield. This character is only shared by O. luctosa (Koehler, 1904) and O. procera (Koehler, 1904), and both species were fi rst recorded from Indonesian waters. Ophiopristis luctosa diff ers from O. shenhaiyongshii sp. nov. by having long, thin, sharp, hollow, smooth disc spines, the radial shield largely obscured, small rounded oral papillae, ventral arm plates twice as long as wide, rectangular and contiguous, the ventralmost spine with a hook-like appearance and one oval tentacle scale, and a yellow color with a few broad brown bands on the arms (Koehler 1904;O'Hara & Stöhr 2006). Ophiopristis procera diff ers from O. shenhaiyongshii sp. nov. by having completely concealed radial shields, the ventral arm plate twice as long as wide, rectangular and contiguous, the fi rst pair of tentacle pores covered by tentacle scales variable in size and overlapping, and the following pairs only covered by one large, elongated oval scale, equal to the length of an arm segment. Ophiopristis shenhaiyongshii sp. nov. shares similar disc spine and arm spine characteristics with O. procera (Koehler 1904 D . Disc slightly pentagonal (Fig. 20A-B). Dorsal disc covered by thin, uncalcifi ed, dark skin with surface of small projecting granules (Fig. 20A). Ventral disc smooth, with few or no smooth granules (Fig. 20B, E). Radial shields short, distally wider and widely separated (Fig. 21F). Distal half of radial shields uncovered, bearing few granules on outer margin but do not reach periphery of disc ( Fig. 21F-G).
Oral shields short, twice as wide as long and somewhat rhombic. Adoral shields short and wide, proximally meeting fully or some hardly meeting (Fig. 21A). Teeth signifi cantly larger than oral papillae. Lateral oral papillae broad, fl at, rounded, transparent, distal edges break easily (Fig. 21E). Number of lateral oral papillae varies among jaws, three per side, but some jaws have four oral papillae on one side (Fig. 21E). Genital slits conspicuous and extending from distal end of oral shield to periphery of disc (Fig. 21B).
A . Dorsal arm plate transversely oval with pointy corners, nearly twice as broad as long, covering only about half width of arm (Fig. 21C). First ventral arm plate transverse diamond-shaped, with rounded angles, next two plates square with convex outer margin. Beyond third ventral arm plate, arm segments nearly semicircular with slight peak, covering one third of width of arm segment (Fig. 21D-E). Tentacle scale absent on fi rst arm segment and starts to appear from second or third arm segment (Fig. 21E). Lateral arm plates stout, forming well defi ned spine ridge and meeting on ventral side except on fi rst two arm segments (Fig. 21D-F). Seven arm spines on an elevated ridge, four dorsal and three to four ventral. Dorsal arm spines often nearly two segments long, stout, nearly cylindrical and tapering to a blunt point (Fig. 21G). Ventral arm spines smaller, thorny with hook-shaped tip, with a few spiny points on their adoral or proximal side ( Fig. 21H-I). Ventralmost arm spines on fi rst few arm segments elongated and less hook-like, with rough surface but distalwards along arm developing into a short, little hyaline hook (Fig. 21I).

C
. Dark olivaceous green on dorsal disc, light brown color on radial shield, dorsal arm spine, and dorsal and ventral arm plate; dark reddish brown on oral papillae and ventral arm spine (Fig. 20A-B).

O
. Ventralmost arm spine short, thorny, with proximally oriented tip (Fig. 22A). Arm spine articulation well developed, with volute-shaped perforated lobe except in dorsal-and ventralmost articulations ( Fig. 22B-C), with large muscle opening and small nerve opening (Fig. 22B-C). Vertebrae with short zygospondylous articulation with a broad dorsal extension of lateral muscle fl anges, distally abruptly truncated, dorsal median furrow moderately expressed, and podial basins short and large ( Fig. 22D-G). Middle segment of ventral vertebrae with deeply expressed furrow without oral bridge (Fig. 22F).

Remarks
The genus Ophientrema includes only two species, O. euphylactea (H.L. Clark, 1911) and O. scolopendrica (Lyman 1883). H.L. Clark (1911) mentioned a band of black spots on the dorsal disc of O. scolopendrica caused by tissue on the inner surface of the skin. In the present study, we examined nine specimens and all of them concurred with the description of the holotype of O. scolopendrica. However, there are some slightly diff erent features in the color of the dorsal disc and number of lateral oral papillae, but the color of the present specimens matched the description by Koehler (1922a).
Among the specimens from this study, we identifi ed two diff erent color patterns: 1) dark olivaceous green on the dorsal disc, light brown on radial shield, dorsal arm spine, and dorsal and ventral arm plate; 2) dark reddish brown on oral papillae and ventral arm spine, dark olivaceous green and brown on dorsal disc and bright red pink on radial shield, light brown on oral papillae, dorsal and ventral arm plate, and arm spine (Fig. 20A-D). Some specimens possessed seven oral papillae on one jaw, but when we examined our specimens and previous records, we considered this as intraspecifi c variation. Ophientrema euphylactea is distinguished from O. scolopendrica by the scale density on the dorsal disc and by having spiniform oral papillae (H.L. Clark 1911).   Genus Ophiurothamnus Matsumoto, 1917 Ophiurothamnus clausa (Lyman, 1878) Figs 23-24 Ophioceramis clausa Lyman, 1878: 124, pl. 6 fi gs 161-163. Ophiothamnus stultus Koehler, 1904: 141-142, pl. 25 fi gs 9-10, pl. 26 fi g. 1.

Remarks
The disc diameter (4.5-5.0 mm), the circular disc and the morphology are similar to the description of Ophiurothamnus clausa in O' Hara & Stöhr (2006) and to the description from the South China Sea in Chen et al. (2020), but we identifi ed some variability in the number of scales on the dorsal disc and in the shape of the radial shields (Fig. 23). The arm skeleton characters of O. clausa diff er from those of other species in the family Ophiacanthidae.
The dorsal arm plate is triangular and well developed (Fig. 24A). The arm spine articulation is well developed and has a large muscle opening and a small nerve opening. A volute-shaped perforated lobe is present in most articulations, but is lacking in the dorsalmost one (Fig. 24B). A perforation is present on the inner side of each lateral arm plate (Fig. 24C). The vertebrae have a long streptospondylous articulation, with broad proximal and distal ends and a narrow middle segment. The proximal end of a vertebra has two clearly separated openings and the distal end of the podial basin becomes short and small. The dorsal side of the vertebrae is distally triangular and proximally fl attened, without a keel. The ventral side of the vertebrae has a narrow and straight ambulacral furrow without an oral bridge ( Fig. 24D-G).
Ophiurothamnus clausa has variable morphological features according to O'Hara & Stöhr (2006), especially regarding the characters of the disc, and the variation in disc covering is very striking, ranging from smooth naked plates to plates with granules, the number and shape of the scales covering the disc, the shape and position of the radial shields and the shape of the arm spines. These character diff erences cannot be explained by biogeographic distribution or size of the animal (O'Hara & Stöhr 2006) and may instead be evidence of cryptic speciation.

Remarks
The present study found only one specimen of O. plicata, with 12.5 mm in disc diameter (Fig. 25). Within Chinese waters, this species has previously been recorded only from the South China Sea.
The arm spine articulation is well developed and placed at a slight angle to the distal edge of the lateral arm plate. A volute-shaped perforated lobe is present in most articulations, but is reduced in the dorsalmost one (Fig. 26A-B). The arm spine articulation has a large muscle opening and a small nerve opening (Fig. 26A-B), and it decreases signifi cantly in size ventralwards (Fig. 26A). The vertebrae have a streptospondylous articulation, with a short, broad podial basin at the proximal end and a narrow, small distal end. The dorsal end of the vertebrae is distally triangular and proximally fl attened with a longitudinal groove along the midline (Fig. 26C-G). The ventral end of the vertebrae has a broad ambulacral groove without an oral bridge (Fig. 26E).
The genus Ophioplinthaca is clearly distinguishable from other genera within the family Ophiacanthidae by the deep interradial incisions into the disc, which are lined distally by enlarged disc scales. Ophioplinthaca plicata is morphologically variable among individuals and is diffi cult to distinguish from other species of Ophioplinthaca, especially from O. pulchra Koehler, 1904. The morphological features of the specimen of O. plicata from our collection and previous descriptions show complex morphological variation among individuals, and more specimens need to be analyzed from neighboring seas and the Indo-Pacifi c region to understand their morphological variation with respect to geographic distribution (O'Hara & Stöhr 2006). A molecular analysis using specimens from many localities is required to understand the genetic variation in this species and its impact on morphological variation.   D . Flat, pentagonal, covered by thin skin, underlying scales with short disc spines (Fig. 27A-B), 0.3-0.4 mm high, rough, with two to three sharp terminal thorns, similar on both dorsal and ventral disc (Fig. 27C-E). Radial shields long, narrow, well separated, parallel to each other, concealed by thin skin, their form and position clearly visible through skin, with thorny stumps (Fig. 27A, E). Ventral disc near oral shields with scales without thorny stumps (Fig. 27B). Oral shield much wider than long, broadly triangular, with wide, pointed proximal angle and a straight or convex distal edge (Fig. 27F). Madreporite larger, with central depression, pentagonal. Adoral shields large, three times as long as wide, narrow, not separated (Fig. 27B, F). Adoral shields enclose proximal oral shield edges and extend around it, separating it from fi rst lateral arm plate (Fig. 27C, F). Jaws wider than long, ventralmost tooth with large, blunt, pointed or fl at edge, and three pointed, spiniform lateral oral papillae with thick and rounded base (Fig. 27F). Genital slits conspicuous and extending from oral shield to periphery of disc (Fig. 27I).
A . Dorsal arm plate on fi rst few arm segments triangular, somewhat fan-shaped and distal margin more convex, but along arm becoming straighter, towards rhombic, somewhat swollen, two times as wide as long, barely separated (Fig. 27G). Ventral arm plate widely triangular, two times as wide as long, distal margin convex, proximally with low, wide angle, lateral side weakly concave and widely separated, but along arm ventral arm plate decreasing in size, as wide as long, with convex distal margin (Fig. 27H). Lateral arm plates meeting above and below, but along arm becoming separated ventrally (Fig. 27H). Six sharp and thorny arm spines (Fig. 27J). Three uppermost spines longest, two to three arm segments in length, one to two dorsal arm spines smooth but others thorny (Fig. 27K). Two to three tapering ventral arm spines with rough tip (Fig. 24L). One tentacle scale, elongated, half as long as ventral arm plate, pointed (Fig. 27I).

C
. When alive, entire specimen light red, light brown when dry ( Fig. 27A-B).
O . Arm spine articulation well developed and placed at small angle to distal edge of lateral arm plate (Fig. 28A-B). Volute-shaped perforated lobe forms most articulations, but reduced in dorsalmost articulation ( Fig. 28A-B). Arm spine articulation with large muscle opening (Fig. 28B). Ventral arm spine smooth, but with rough tip (Fig. 28C). Vertebrae with a well-developed zygospondylous articulation, with a broad dorsal surface, distally abruptly truncated, with a shallow dorsal medial furrow, podial basins slightly wider than long (Fig. 28D-H). Ambulacral groove widened in middle, without oral bridge (Fig. 28G).

Remarks
Ophiacantha bathybia was described by H.L. Clark (1911); additional specimens have been collected in , 1981(OBIS 2021. Ophiacantha bathybia from the present study is quite similar to the holotype description, but it diff ers in the arm spine and arm plate characters. Our specimens were larger than the holotype (12 mm disc diameter) and some of the morphological diff erences may be caused by this size diff erence. The lateral arm plates in the holotype description are connected at both the dorsal and ventral edges, but in our specimens they were only connected by a ligament. Another morphological variation was the number of thorny spines, which diff ers from segment to segment and arm to arm. Ophiacantha bathybia has no uniquely distinguishing characters, but appears to be diff erent from other species of Ophiacantha by the size of the disc and characters of the disc covering, dorsal arm plates, arm spines and tentacle scale. Mature specimens have narrow, long radial shields and thorny disc stumps, but diff er in arm spine and dorsal arm plate shape. It has been recorded from abyssal depths, and its morphological characters are similar to those of other abyssal species of Ophiacantha, namely O. pacifi ca Lütken & Mortensen, 1899, O. sollicita Koehler, 1922, O. cosmica Lyman, 1878, O. frigida Koehler, 1907and O. sociabilis Koehler, 1897, but these species have been recorded from Panama, the Southern Ocean and the Atlantic Ocean. Ophiacantha bathybia may be considered as a sister species to these abyssal species of Ophiacantha based on their morphological features.
Ophiacantha cosmica diff ers from O. bathybia by having up to eight arm spines, slender lateral oral papillae without a thick base and a blunt ventralmost tooth (Lütken & Mortensen 1889;Stöhr & O'Hara 2021). Ophiacantha sociabilis diff ers from O. bathybia by having four to fi ve lateral oral papillae, a less enlarged distalmost papilla and wider jaws (Koehler 1897;Stöhr & O'Hara 2021). Ophiacantha sollicita diff ers from O. bathybia by having up to eight arm spines, conical disc spines, a notably thickened distal oral papilla and non-moniliform arms (Koehler 1922b). Ophiacantha frigida diff ers from O. bathybia by having a larger distalmost lateral oral papilla and usually two tentacle scales on the fi rst tentacle pore (Koehler 1922b). Ophiacantha pacifi ca is highly similar to O. bathybia in morphological features of the oral frame, as well as the number and length of arm spines, but the ventralmost arm spines of O. bathybia have a rough tip (Lütken & Mortensen 1889) (Fig. 28C). However, these morphological diff erences vary and overlap among individuals between species of the genus Ophiacantha. Therefore, a molecular analysis from a wider range of localities is needed to understand the species boundaries. D . Sub-pentagonal, covered by skin with underlying scales, bearing four to six short, stump-like spines with crown of sharp, straight thorns and a few elongated thick, thorny stumps in the center ( Fig. 29A-F). Radial shields long, narrow, widely separated, slightly convex, distal end thickened and exposed (Fig. 29A). Radial shields concealed by thin skin and thorny stumps, but clearly visible through skin when specimen dried (Fig. 29A). Dorsal arm plate on fi rst arm segment covered by thorny stumps (Fig. 29D). Ventral disc also covered by thorny stumps, but less dense than on dorsal disc, scales clearly visible, genital slits short (Fig. 29F). Oral shield triangular, much wider than long, distal end slightly convex, proximal edges concave or straight (Fig. 29B), madreporite larger, as long as wide, distal edge strongly convex (Fig. 29G). Adoral shields narrow, curved, three times as long as wide and not separated, bordering proximal edges of oral shield, not separating it from arm (Fig. 29G). Jaws elongated, with one large pointed ventralmost tooth and three long, spiniform, pointed lateral oral papillae on each side, distalmost papilla wider than other two (Fig. 29G).
A . Dorsal arm plates triangular, distal edge convex, separated (Fig. 29I). Ventral arm plate on fi rst arm segment small, wider than long, slightly triangular with concave distal edge. Second ventral arm plate pentagonal, wider than long, with obtuse proximal angle, excavated lateral edges and slightly convex distal edge. Following plates as wide as long, distalwards becoming wider than long, slightly hexagonal, with curved distal edge and angular proximal edges, separated except on second arm segment (Fig. 29H, J). Lateral arm plates meeting above and below. Six smooth arm spines, three dorsal and three ventral. Dorsalmost arm spine one and a half to two arm segments in length, second dorsalmost arm spine longest and meeting across dorsal midline ( Fig. 29I-J). Ventral arm spines similar in length, with fi nely thorny surface (Fig. 29K). One elongated tentacle scale, large, often as long as ventral arm plate (Fig. 29H).
O . Arm spine articulations well-developed, placed at an angle on separate, protruding distal part of lateral plate, bordered by a wavy ridge, middle articulations largest (Fig. 30A-B). Arm spine with thorny surface (Fig. 30C). Volute-shaped perforated lobe in most articulations, reduced in dorsalmost articulation, with large muscle opening and small nerve opening (Fig. 30A-B). Vertebrae with short, well-developed zygospondylous articulation with a broad, shallow dorsal furrow, distally abruptly truncated, podial basins wider than long (Fig. 30D-H). Ambulacral groove widely diverging distally, without oral bridge (Fig. 30H).

Remarks
The holotype description of the dorsal disc of Ophiacantha vorax (5 mm disc diameter) is slightly diff erent from conditions in our specimen, which has no elongated disc stumps. However, this morphological feature was only found here on one of the 3 specimens collected. Variability in morphological characters (arm spines, extent of thorny stumps on radial shields, tentacle scale) is low in O. vorax (Koehler 1922a). Koehler (1897) considered one of the distinguishing features of O. vorax to be the presence of only six arm spines, but H.L. Clark (1911) and Liao (2004) documented eight to nine arm spines in their specimens. Therefore, the number of arm spines is not a suitable character to distinguish O. vorax from other species of Ophiacantha.
Ophiacantha pentagona Koehler, 1897 is related to O. vorax, but diff ers in having long and thicker oral papillae, strongly moniliform arms and longer ventral arm plates in the middle region of the arm. Ophiacantha vorax closely resembles O. longidens Lyman, 1878 in the shape of the oral papillae and oral shields, as well as in the disc shape, but diff ers in the arrangement of arm spines, and the shape of the arm plates and tentacle scales. Ophiacantha longidens has fl attened arm spines, blunt with a thorny surface. Another species resembling O. vorax is O. duplex Koehler, 1897, recorded from deep waters in Japan, Philippines, Indonesia, Australia and Madagascar. It can clearly be distinguished by its large tentacle scale, less thorny, long, thin arm spines, large dorsal arm plate and the presence of large disc spines, as well as the smaller thorned spines.

Diagnosis
Disc slightly pentagonal, interradially excavated and covered by conical pointed granules with wide, round base. Single small, broad, triangular, pointed tentacle scale. Adoral shields larger than oral shield. Jaw with one large, blunt, wide ventralmost tooth and four to fi ve lateral oral papillae. Top of radial shields and center of disc creamy white, dark brown lines radiate from disc center to arms.

Etymology
The specifi c name is derived from the Greek word for 'star', alluding to the shape and color of the dorsal disc.
D . Disc slightly pentagonal, interradially excavated, covered by oval disc scales, each bearing a conical pointed granule with wide, round base ( Fig. 31A-B). At disc periphery and lateral disc, granules lower and less pointed, near oral shields no granules on scales ( Fig. 31C-F). In disc center, granules spine-like (Fig. 31C). Ventral disc reduced to oral frame (Fig. 31B). Radial shields long, narrow, parallel to each other, but well separated, distal ends exposed (Fig. 31A, E). Genital slits conspicuous, short, extending from oral shield to periphery of disc (Fig. 31E). Oral shield broadly triangular, with acute proximal angle, with straight, convex or wavy distal edge. Adoral shields larger than oral shield, trapezoid extending along concave proximal edges of oral shield, not separating it from arm (Fig. 31F). Jaw with one large, blunt, wide ventralmost tooth, four to fi ve lateral oral papillae, fi rst three elongated, spiniform, fi rst possibly infradental papilla, distal two papillae larger, distalmost one scale-like (possibly adoral shield spine) (Fig. 31F). Dorsal teeth square, with straight proximal edge (Fig. 31F).
A . Moniliform. Dorsal arm plates fan-shaped, distal edge convex, acute proximal angle, straight to slightly concave proximolateral edges, completely separated (Fig. 31G). Ventral arm plates pentagonal to broadly rectangular, wider than long, concave lateral edges around tentacle pore, distal edge straight with small inward curve, separated (Fig. 31H). Lateral arm plates with a high spine-bearing ridge, meeting dorsally and ventrally (Fig. 31I-K). Eight transparent, rounded arm spines, more or less thorny (Fig. 31J-K). Four dorsalmost spines longest, with rough surface, as long as three arm segments, meeting at dorsal midline, length gradually decreasing ventralwards. Ventral arm spines tapering, rougher than dorsal arm spines, one to two arm segments in length (Fig. 31I-K). Tentacle pore on proximalmost and second arm segments half to two-thirds arm segment in length (Fig. 31B). One to three arm segments covered by disc and only possessing ventral arm spines (Fig. 31I). Arm spines on fourth to sixth arm segments longer than those on middle or dorsal arm segments (Fig. 31I-J). Arm spines on fi rst six arm segments tapered, smooth, but distal to sixth segment spines rough or thorny (Fig. 31K). One small, broad, triangular, pointed tentacle scale, one third as long as ventral arm plate (Fig. 31B).

C
. Top of radial shields and center of disc creamy white, but dark brown lines radiate from disc center to arms, outside radial shields, a third line between each pair of radial shields continues along center of each arm. Similar longitudinal, lighter brown line on ventral arms (Fig. 31).

Variations in paratypes
Paratypes generally similar to holotype, disc diameters 3.9-9.0 mm, but some morphological variations were observed. Only dorsal arm spines tapered or thorny in some specimens (Fig. 33). One paratype had black dots scattered near periphery of disc (Fig. 33A). Disc color of some paratypes slightly lighter than in holotype and interradials not as strongly excavated as in holotype, radial shields more widely separated (Fig. 33B). Smallest paratype with 3.9 mm disc diameter and similar to holotype, except fi fth arm segments with thorny spines. Lateral oral papillae four to six, distalmost one varies from scalelike, fl at to narrower and pointed, ventralmost tooth much larger than oral papillae (Fig. 33C-D). Disc granules varied in shape; some specimens with taller spines in disc center (Fig. 33E).

Remarks
The arm spine articulation is here interpreted as a variation of the zygospondylous type, not uncommon in the family Ophiacanthidae, typical of epizoic species. The deeply interradially excavated disc is a character used by Paterson (1985) to distinguish the no longer valid subfamily Ophioplinthacinae Paterson, 1985(Martynov 2010, but the narrow, long radial shields and the oral frame fi t with the genus Ophiacantha. Ophiacantha aster sp. nov. diff ers from most species of the genus by having deep incisions in the disc along with fi ve (more or less) lateral oral papillae. Ophiacantha antarctica Koehler, 1900 resembles O. aster sp. nov. by having long narrow radial shields with a more or less excavated interradial disc, four elongated lateral oral papillae and tapering long uppermost arm spines, but it diff ers in having a thickened integument that covers the disc scales, conical granules on the scales with two to fi ve fi ne spinules, and near the periphery of the disc the granules are less elongated and more cylindrical. Ophiacantha antarctica has seven arm spines with a thin pointed tip (Koehler 1900), frequently has disc spines on the dorsal arm plates and transversely split dorsal arm plates dividing them into a small proximal and larger distal section (observation from anonymous reviewer).
Another species that resembles Ophiacantha aster sp. nov. is O. veterna Koehler, 1907, known from the Atlantic Ocean (2200 m), which has similar morphological characteristics in its disc shape, oral shield, adoral shield, oral papillae and arm spines, but diff ers mainly in having dense small granules on both ventral and dorsal disc and in the shape of the ventral arm plate along the arm (Koehler 1907a). The new species is unusually colorful among deep-water species.

Distribution and habitat
Found on a seamount near the Zhongsha Islands, South China Sea (516 m) as a colony attached to a dead coral branch. The vertebrae of Ophiacantha aster sp. nov. are almost streptospondylous and suggest an epizoic life-style, typical of suspension feeders.

Diagn osis
Disc with deep interradial incisions with seven lobes and ring of irregular scales bearing a few granules around centrodorsal plate and overlapping with radial shields. Seven thin arms curled under disc.

Etymology
The specifi c name is derived from the Latin word for fl ower petals, alluding to the shape of the disc.
D . Disc heptamerous with deep interradial incisions and a lobe above each arm, four large lobes, three smallest lobes and one intermediate lobe (signs of regeneration after fi ssion). Arms diff er in width in groups of 3, 3, 1 arms. Lobes formed by large drop-shaped radial shields, largest one a third of disc diameter in length and two to three times as long as wide (Fig. 34A-C). Radial shields distally connected, separated proximally by l arge triangular scales, covered by small conical granules (Fig. 34E).
Center of disc sunken and covered by large, round centrodorsal plate without granules (Fig. 34A, C). Ring of irregular plates bearing a few conical granules around centrodorsal plate and overlapping with radial shields (Fig. 34A, C). Genital slits small, half as long as interradial ventral disc (Fig. 34F). Oral shields swollen, much smaller than adoral shields, as wide as long, fan-shaped with pointed proximal angle, slightly concave lateral edges and convex distal edge; size varies among radials (Fig. 34B, D, F). Adoral shields swollen, twice as wide as long, with angled, concave proximal edges of oral shield, not separating it from arm (Fig. 34F). Jaw as wide as long bearing one pointed ventralmost tooth and three spiniform lateral oral papillae, proximalmost one spine-like, other two shorter and rounded (Fig. 34F). All teeth spiniform like ventralmost tooth (Fig. 35B, D, F). A . Seven thin arms, curled under disc. Dorsal arm plates fan-shaped, two times as wide as long, with convex distal edge and always separated (Fig. 34G). Ventral arm plate much wider than long on proximal segments, but reduced to small, round, thin scales, embedded in a strand of thick skin running along entire arm distally, always present along arm and always separated (Fig. 34H). Four thick, conical, smooth arm spines, two thirds arm segment in length (Fig. 34D, H). One minute tentacle scale clearly visible on proximal arm segments (Fig. 34D).

C
. In alcohol, entire specimen creamy white.
O . Arm spine articulation formed by two thick, smooth, curved lobes, ventral lobe smaller than dorsal lobe, positioned at angle to distal edge of lateral plate, with large muscle opening and small nerve opening, the latter large in dorsalmost articulation. Volute-shape not well defi ned, absent in dorsalmost articulation, dorsal and ventral articular lobes connected but separating in ventral articulations ( Fig. 35A-C). Vertebrae elongated, with streptospondylous articulation, dorsal and lateral furrows absent, middle section much lower than proximal part and distal muscle fl anges, with straight ambulacral groove. Podial basins at distal end, tongue-like with round hole (Fig. 35D-F).

Variations in paratypes
Five heptamerous paratypes, with disc diameter ranging from 1.45 mm to 1.87 mm. All with similar morphological features as holotype, having conical granules on disc plates/scales and large, drop-shaped radial shields. Most paratypes with arms curled under disc.

Remarks
The oral frame arrangement and the lobe-like shape of the disc concur with the genus Ophiomoeris, which was recently transferred to Ophiacanthidae (O'Hara et al. 2018). The arm spine articulation of this genus is interpreted as a variation of the volute-shape typical of the family Ophiacanthidae. Previously only four species of Ophiomoeris were recognized and all of them are pentamerous: O. obstricta (Lyman, 1878), O. exuta Stöhr, 2011, O. nodosa (Koehler, 1905) and O. tenera (Koehler, 1897). The seven-fold symmetry is consistent in all our specimens. This is the fi rst record of a sevenarmed (or indeed non-pentamerous) species in the genus Ophiomoeris, and this distinguishes it from all co ngeners. The size diff erences among the seven sections of the animals suggests that it is a fi ssiparous species. Morphological characters to diff erentiate species in the genus Ophiomoeris are otherwise diffi cult to interpret due to their high morphological variation among individuals. Taking geographic distribution into account, there may be several cryptic species complexes in the genus Ophiomoeris that can only be resolved by a molecular study (O'Hara & Stöhr 2006;Stöhr 2011).

Distribution and habitat
South China Sea (1550 m), found on a deep-sea seamount near the Zhongsha Islands.
A . Dorsal arm plates large, triangular, with straight distal edge, truncated proximal edge, twice as wide as long and contiguous (Fig. 36H). Ventral arm plates pentagonal, wider than long, completely separated (Fig. 36I). Lateral arm plate bears three arm spines with thick base, rounded, blunt tip, 1-1½ arm segment in length and middle one longest (Fig. 36J). Ventralmost spine thicker than others for fi rst few arm segments. One large, broad, rounded tentacle scale, two thirds as long as ventral arm plate ( Fig. 36I-J).

C
. Dorsal disc white with a few brown patches on scales. Disc periphery and distal part of radial shields light brown. Arms and ventral disc also light brown, but arm spines dark brown (Fig. 36A-B).

O
. Lateral arm plate with three well developed arm spine articulations, consisting of two unequal, subparallel, curved lobes (not volute-shaped). Arm spine articulation with two similar openings for muscle and nerve, but in some articulations nerve opening slightly larger than muscle opening ( Fig. 37A-C). Vertebrae with well-developed zygospondylous articulation, narrow, shallow dorsal furrow, truncated far from distal articulation (Fig. 37D-H). Ambulacral groove deep, with hole in middle, without oral bridge (Fig. 37F).

Remarks
All specimens in the present study were found attached to a glass sponge species from a deep-sea seamount. Ophiactis perplexa was fi rst described by Koehler (1897) and has not been redescribed since, but specimens were recorded in , 2003and 2007(OBIS 2021. Our specimens are similar to the holotype description, but we noticed variations in some morphological characters that prevent us from fully associating these specimens with O. perplexa. In particular, the spines on the periphery of the ventral disc only appear in two of our specimens, and most specimens have disc spines at the base of the radial shields ( Fig. 35C-E), but most of the species in the genus Ophiactis show this morphological variation among individuals. Ophiactis fl exuosa Lyman, 1879 is related to O. perplexa, but is distinguished by the presence of spines on the disc, the shape of the radial shield and the pentagonal shape of the ventral arm plate. Another similar species is O. defi nita Koehler, 1922, recorded   Ophiactis defi nita (currently accepted as O. brachygenys, see below) is clearly distinguished from O. perplexa by the absence of spines on the disc, a longer oral shield with a much smaller border and a pointed distalmost oral papilla. Ophiactis brachygenys H.L. Clark, 1911 has separated dorsal arm plates and a smaller disc border than O. perplexa, but rarely has a few spines on the disc (Fig. 38M).  Lütken & Mortensen, 1889 Figs 38-39

Remarks
Ophiactis profundi is a hexamerous, fi ssiparous species. The disc diameter ranged from 4 mm to 6 mm and all the specimens were found at one station on a deep-sea seamount in the South China Sea. Morphological descriptions of the holotype of Ophiactis profundi (Lütken & Mortensen 1889) from the eastern Pacifi c Ocean and of other specimens from the Philippine Sea (Koehler 1922a) were similar to our specimens (Fig. 38).
The dorsal arm plates are well-developed, widely triangular (Fig. 39A). The lateral arm plate has three well-developed arm spine articulations, formed by two subparallel bent lobes, not connected to each other, placed at an angle to the distal edge and equal in size, with two similar openings for muscle and nerve ( Fig. 39B-C). The vertebrae have a well-developed zygospondylous articulation, with a narrow, shallow dorsal furrow, not extending beyond the distal articulation, and the ambulacral groove is distalwards widened (Fig. 39D-F).
A . Dorsal arm plate slightly fan-shaped, large, slightly convex distally, wide proximal angle, as long as wide and mostly separated (Fig. 40G). First ventral arm plate small, triangular with truncated distal end. Second ventral arm plate tetragonal or pentagonal, wider than long, connected to fi rst ventral arm plate, then pentagonal with straight to concave distal edge, round proximal angle along arm and mostly separated (Fig. 40B, H). Three arm spines on most segments, with thickened base, rounded, blunt tip, 1-1½ arm segment in length, middle one longest (Fig. 40G-H). One large, broad, oval tentacle scale, half as long as ventral arm plate, covering pore completely (Fig. 40H).

C
. Wet specimen light brown, dry disc light grey and dorsal arm light brown or pink (Fig. 40).
O . Extracted from heptamerous specimen, lateral arm plate with three well-developed spine articulations formed by two subparallel, bent, separated lobes, unequal in size. Two similarly sized openings for muscle and nerve ( Fig. 41A-C). Vertebrae with well-developed zygospondylous articulation, narrow, shallow dorsal furrow, not extending beyond distal articulating structures, deep ambulacral groove, widened distally, large podial basins ( Fig. 41D-H).

Remarks
According to our specimens, Ophiactis cf. brachygenys is a fi ssiparous species and these are the fi rst records of specimens with more than fi ve arms (confi rmed by molecular data, see below). Six-and seven-armed specimens had similar morphological features. However, the hexamerous specimen (2.8 mm disc diameter) diff ers slightly from the heptamerous specimen in the number of arms, the widely separated radial shield, the fi rst one to two arm segments having four arm spines but only on one or two arms, and a smooth, pointed disc spine near the periphery of the disc (Fig. 42).
The holotype description of O. brachygenys is similar to both specimens from the present study, except for the number of arms and completely separated radial shields.
H.L. Clark (1911) mentioned that one of his specimens (9 mm disc diameter) had distally separated radial shields, the dorsal arm plate was shorter and wider, and both dorsal and ventral arm plates were more closely together. Also, it had a few short disc spines at the periphery of the ventral disc (H.L. Clark 1911). These variations concur with the specimens from the present study . Ophiactis defi nita was synonymized with O. brachygenys by Liao (2004). The description of O. defi nita is similar to that of O. brachygenys, but Koehler (1922a) described some morphological diff erences between the two species. Ophiactis brachygenys from the present study and the description of its holotype (H.L. Clark 1911) are morphologically distinguished from the type of O. defi nita by having separated dorsal arm plates along the arm, wider than long and not as broad radial shields, and wide as long oral shields. In addition, the radial shields are proximally and distally separated, and disc spines are present on the ventral disc (Figs 40A,42K,M). Molecular analysis of both species from diff erent localities suggests that these morphological variations may be species specifi c (see below).

Family Asteronychidae
Final 581 bp partial COI and 443 bp partial 16S sequences were obtained after removing ambiguous aligned sites and successfully used in reconstructing a Maximum Likelihood (ML) tree from 16S (8 specimens) and COI (7 specimens), respectively (Fig. 43). In the 16S ML tree, species of Asteronyx were divided into two subclades (Sub-Clades 01 and 02). Sub-Clade 01 consists of A. loveni (Japan, East China Sea and South China Sea) and A. longifi ssus Döderlein, 1927 (California). Sub-Clade 02 consists of A. reticulata (East China Sea and Japan) and A. luzonicus (South China Sea) (Fig. 43A). Similar results were found in the COI ML tree (Fig. 43B). Genetic distance values are provided in Suppl. fi le 1 and Suppl. fi le 2. Two samples of A. reticulata from Japan and the East China Sea had identical sequences and three samples of A. loveni from Japan, the East China Sea and the South China Sea also showed no sequence variation (Suppl. fi le 1).

Families Euryalidae and Gorgonocephalidae
A total of 26 COI sequences trimmed to 608 bp were obtained after removing ambiguous aligned sites and successfully used in reconstructing an ML tree (Fig. 44). Two main clades were detected, with the family Euryalidae belonging to Clade 01 and family Gorgonocephalidae belonging to Clade 02. The genus Asteroschema was detected as a sub-clade within family Euryalidae (Sub-Clade 01) and the genus Gorgonocephalus was detected as a sub-clade within family Gorgonocephalidae (Sub-Clade 02). Two inter-clades were detected among species of Asteroschema (Sub-Clade 01   Gorgonocephalus (Sub-Clade 02). Inter-Clade 2A consists of Astrodendrum sagaminum (Döderlein, 1902) from Japan, Gorgonocephalus pustulatum (H. L. Clark, 1916) from New Zealand, G. sundanus Döderlein, 1927 from Australia and G. cf. dolichodactylus from the South China Sea (depth 400-1114 m). Inter-Clade 2B consists of G. arcticus Leach, 1819 from Cornwallis Island, G. eucnemis (Müller & Troschel, 1842) from Japan, G. chilensis from the Antarctic, the Southern Ocean, New Zealand and the South China Sea (depth 398-1550 m) (Fig. 44). Genetic distance values are provided in Suppl. fi le 3 and Suppl. fi le 4.

Family Ophiotomidae
A 623 bp sequence of the COI gene was obtained after removing ambiguous aligned sites and used to reconstruct the ML tree from ten specimens for the family Ophiotomidae (Fig. 46). We failed to get COI or 16S gene sequences from Ophiotreta eximia. Therefore, only Ophiopristis shenhaiyongshii sp. nov. was added to this phylogenetic tree from our collection. Two main clades were detected within Ophiotomidae. Inter-Clade 2A represents Ophiotreta matura, O. eximia, Ophiopristis shenhaiyongshii sp. nov., O. luctosa, O. procera and Ophiacantha spectabilis G.O. Sars, 1872 (Fig. 46). Genetic distance values are provided in Suppl. fi le 6.
The clades in the COI ML tree were grouped corresponding to the 16S ML tree. Therefore, all the species represented in the COI ML belong to the main Clade 02. Ophiurothamnus clausa and Ophioplinthaca plicata from the South China Sea fell into Sub-Clade 01. Ophiacantha vorax and Ophientrema scolopendrica from the South China Sea fell into Sub-Clade 02. The genus Ophioplinthaca fell into Inter-Clade 1B within Sub-Clade 01 (Fig. 47B). The topology of the two ML trees was slightly diff erent from each other (Sub-Clade 01). Genetic distance values are provided in Suppl. fi le 7 and Suppl. fi le 8.

Family Ophiactidae
A total of 18 COI sequences trimmed to 554 bp were obtained after removing ambiguous aligned sites and were used to reconstruct the ML tree (Fig. 48). Two main clades were detected within the family. Sub-Clade 02 represented all Ophiactis species recorded from the South China Sea. Ophiactis cf. perplexa, O. cf. brachygenys and O. profundi fell into two inter-clades within Sub-Clade 02 (Inter-Clades 2A and 2B) (Fig. 48). Genetic distance val ues are provided in Suppl. fi le 9.

Taxonomy of deep-sea Ophiuroidea from the South China Sea
Our molecular phylogenetic analysis of the family Asteronychidae identifi ed two main clades as genera Asteronyx and Astrodia Verrill, 1899 (Fig. 43). We prepared two diff erent molecular phylogenetic trees using both the COI and 16S genes to represent the family Asteronychidae due to limited available published molecular data and to better understand the genetic distance variation among diff erent mitochondrial genes. Previous studies suggested that interspecifi c genetic distances for COI within the class Ophi uroidea ranges from 5.6% to 31.6%, with a mean valve of 18.9% and intraspecifi c genetic distances from 0.5% to 6.4%, with a mean value of 2.2% according to the Kimura 2-parameter (K2P) (Boissin et al. 2017). The genetic distances from the COI of Asteronyx loveni and Asteronyx reticulata in this study fell within the ranges of these published values. We detected a more signifi cant diff erence in genetic distance from the COI gene than from the 16S gene, and these results may be caused by a smaller substitution rate and lower base pair variation in the 16S gene than in the COI gene (Palumbi 1996b). We conclude that the COI gene was more suitable for interspecies identifi cation than the 16S gene for the family Asteronychidae (Suppl. fi le 1 and Suppl. fi le 2). Cryptic speciation in the genus Asteronyx was diffi cult to recognize using only morphological details and the usefulness of some morphological characters may vary with the ontogenetic stage of a specimen. Recent studies suggested key morphological features to distinguish species of Asteronyx, such as features of the dorsal disc, and position and length of the genital slit . The holotype description of Asteronyx luzonicus by Döderlein (1927) mentioned distinguishing morphological features, such as characteristics of the ventralmost arm spine and sexually mature specimens appeared to have dark spots on the disc. Specimens in the present study were not mature, but appeared to have calcifi ed scales on the disc, as mentioned by Baker et al. (2018). This study identifi ed some morphological characters distinguishing A. luzonicus (13.5 mm disc diameter) from A. loveni (11 mm disc diameter), such as hook arm spines having a maximum of one secondary tooth, the disc surface being fl attened, with a mesh-like center and calcifi ed scales, and the fi rst two to three ventral arm segments without an arm spine. Our molecular analysis of the family Asteronychidae was helpful to distinguish A. luzonicus from A. loveni, A. longifi ssus and A. reticulata. Asteronyx luzonicus is the second species of the genus Asteronyx recorded from the South China Sea. Previously, A. luzonicus had been recorded from neighboring seas and distant localities, such as Madagascar.
The molecular phylogenetic tree of the families Gorgonocephalidae and Euryalidae based on the COI gene (28 specimens) appear to be similar in topology and well in agreement with previous studies (Okanishi & Fujita 2013;Christodoulou et al. 2019;O'Hara et al. 2019). In this study, the genetic distance in Asteroschema horridum from Reunion Island and the South China Sea, and in Gorgonocephalus chilensis from New Zealand, the Antarctic, the Southern Ocean and the South China Sea, fell within the intraspecifi c genetic distance range of published values on Ophiuroidea and was well supported by the ML tree (Boissin et al. 2017) (Fig. 44). However, the genetic distance of COI was signifi cantly lower in Gorgonocephalus than in Asteroschema. Therefore, we conclude that species of Gorgonocephalus had a slower genetic evolution, but comprehensive molecular analysis with more molecular data from diff erent localities and species are needed to confi rm this hypothesis (Suppl. fi le 3 and Suppl. fi le 4). The molecular phylogenetic analysis of Gorgonocephalidae identifi ed two sub-clades within the genus Gorgonocephalus (Fig. 44). These clades were clearly distinguished by morphological diff erences. Gorgonocephalus dolichodactylus, G. sundanus and G. pustulatum fell into one clade and were distinguished morphologically from the other clade by having a wider, thin, intrabrachial peripheral ring, sparse, low granulation and a maximum of four arm spines (Baker 1980). The granulation pattern on the disc was not a suitable morphological feature to distinguish the species, due to signifi cant variation among individuals (Baker 1980). As an example, G. cf. dolichodactylus from the present study had naked skin on both sides of the disc, except on the radial shields (covered by a few scattered granules). Morphological analysis of G. chilensis s. lat. from diff erent localities has indicated only a few diff erences and those depended on the size of the specimen (Döderlein 1927;Baker 1980;McKnight 2000;Olbers et al. 2019). Gorgonocephalus chilensis from New Zealand waters and the South China Sea had a signifi cantly low genetic distance, but showed higher genetic distance from Antarctic/ Subantarctic specimens of G. chilensis. According to the molecular phylogenetic tree, G. chilensis from the South China Sea and New Zealand fall into one clade that is sister to another clade with Antarctic/ Subantarctic specimens (Fig. 44; Suppl. fi le 3), which prompted us to adopt a subspecifi c division. The molecular analysis indicated that Astrodendrum sagaminum falls within Gorgonocephalus, and genetic distances within Astrodendrum sagaminum, G. pustulatum, G. cf. dolichodactylus and G. sundanus ranged from 1.16% to 4.42%, but the genus Astrodendrum diff ers from Gorgonocephalus by having variously shaped external ossicles on the disc, a madreporite placed on the inner edge of the interradial lateral disc and by lacking calcareous scales on the lateral disc margin (Baker 1980;Okanishi & Fujita 2018a). This study suggests that the sequence of A. sagaminum downloaded from GenBank may be based on a misidentifi ed specimen or the species belongs in the genus Gorgonocephalus. We suggest that the genus Astrodendrum should be revised using a combination of molecular and morphological data and material from diff erent localities.
The molecular phylogenetic analysis of Euryalidae identifi ed a sub-clade that belongs to the genus Asteroschema (Fig. 44). The genetic distance of A. horridum from Reunion Island to the South China Sea was higher than that of Gorgonocephalus, but they fall within previously published values (Okanishi et al. 2011a;Okanishi & Fujita 2013) (Suppl. fi le 3 and Suppl. fi le 4). Intraspecifi c and interspecifi c COI genetic distances were much lower in Gorgonocephalus than in Asteroschema (Suppl. fi le 3 and Suppl. fi le 4). Therefore, the COI gene appears to be more suitable for Asteroschema than for Gorgonocephalus, for species identifi cation and classifi cation. The results of COI from the ML tree suggested that Asteroschema horridum was closely related to A. edmondsoni (12.51%) and Gorgonocephalus chilensis is closely related to G. eucnemis (4.07%). Asteroschema horridum can clearly be distinguished within the genus Asteroschema by its having a small, fl at disc with scales bearing conical granules. The present study and previous studies indicated that A. horridum may have few morphological variations, which depend on the size of the specimens. However, most species of Asteroschema have high morphological variation and cryptic species complexes occur among them (Baker 1980). Therefore, a combination of morphological and molecular analysis is needed to identify and revise the genus Asteroschema.
Our molecular phylogenetic analysis suggests three sub-clades within the family Ophiomusaidae. Ophiomusa lymani from the South China Sea fell into Sub-Clade 02 and genetic distance values between the South China Sea and New Zealand fell within published intraspecifi c genetic distance values (Ward et al. 2008;Boissin et al. 2017). Ophiomusa lymani was recently placed in the resurrected genus Ophiomusa and the new family Ophiomusaidae, because the type species of Ophiomusium (O. eburneum) was found to belong to a diff erent clade (Christodoulou et al. 2019). However, molecular analysis in the present study and recent studies suggest the presence of two main clades within the family Ophiomusaidae (Christodoulou et al. 2019;O'Hara et al. 2019) (Fig. 45, Suppl. fi le 5).
The molecular analysis of the family Ophiotomidae identifi ed an inter-clade that consisted of the genera Ophiotreta and Ophiopristis. Morphologically, Ophiopristis shenhaiyongshii sp. nov., O. luctosa and O. procera are clearly grouped together by having a series of thorny spines on the distal margin of the oral shield and by characteristics of the tentacle scale. Interspecifi c genetic distances in Ophiotomidae from this study determined O. shenhaiyongshii sp. nov. as a new species in the genus Ophiopristis (Suppl. fi le 6). Ophiopristis shenhaiyongshii sp. nov. was clearly distinguished within this clade by characteristics of the tentacle scale and the disc spines. In this study, the ML phylogenetic tree of the family Ophiotomidae suggests that Ophiacantha spectabilis belongs to Ophiotomidae within the clade for Ophiopristis, confi rming recent studies (Christodoulou et al. 2019;O'Hara et al. 2019) (Fig. 46). However, we refrain from transferring O. spectabilis to Ophiopristis without a comprehensive examination of specimens and suffi cient molecular data (only one sequence in GenBank).
The molecular phylogenetic analysis of the family Ophiacanthidae identifi ed two main sub-clades in both the 16S ML tree and the COI ML tree (Fig. 47). However, those two sub-clades did not possess clear distinguishing external morphological characters, but could clearly be distinguished by using characters of the vertebrae articulation appearance, such as streptospondylous articulation (Ophioplinthaca plicata, Ophiacantha aster sp. nov., Ophiurothamnus clausa and Ophiomoeris petalis sp. nov.) and zygospondylous articulation (Ophientrema scolopendrica, Ophiacantha bathybia and O. vorax). In this study, we prepared two diff erent molecular phylogenetic trees by using both COI and 16S genes to represent the family Ophiacanthidae due to limited published molecular data, unsuccessful molecular data generation and to better understand the genetic distance variation among mitochondrial genes. We detected signifi cantly higher genetic distances in COI than in the 16S gene at both intraspecifi c and interspecifi c levels.
Our phylogenetic analysis suggests that Ophiacantha aster sp. nov. is related to O. antarctica, Ophiomoeris petalis sp. nov. is related to O. obstricta, and Ophioplinthaca rudis (Koehler, 1897) is related to O. plicata. Ophiacantha bathybia fell between O. vorax and Ophientrema scolopendrica (Suppl. fi le 7 and Suppl. fi le 8). Ophientrema scolopendrica, Ophiacantha bathybia and O. aster sp. nov. appear to have a slightly lower variation in intraspecifi c genetic distances within the South China Sea (Suppl. fi le 7 and Suppl. fi le 8). Our molecular analysis found the genus Ophiacantha as polyphyletic, Ophioplinthaca and Ophiomoeris as monophyletic and previous studies suggest that the genera Ophiomitrella and Ophiolebes may also be polyphyletic. Previous studies have suggested that Ophiacanthidae needs to be revised to obtain a better understanding and many genera of Ophiacanthidae may be polyphyletic as well (O'Hara et al. 2018Christodoulou et al. 2019;Lee et al. 2019). Molecular and morphological analysis using literature data on Ophiacanthidae suggests that O. aster sp. nov. belongs to the Ophiacantha antarctica clade. Ophiacantha aster sp. nov. can clearly be distinguished within this clade by having a unique color pattern on the arms and dorsal disc, four to fi ve spiniform lateral oral papillae, sparse disc spines and deeper interradial disc excavations.
Species of the genus Ophiomoeris are diffi cult to identify when relying only on morphological analysis due to high morphological variation among individuals. Therefore, a combined molecular and morphological analysis of specimens from diff erent localities will help to better understand the species limits within this genus (O'Hara & Stöhr 2006;Stöhr 2011). In this study, we identifi ed O. petalis sp. nov. with the help of both molecular and morphological analysis. Ophiomoeris petalis sp. nov. falls within the clade of Ophiomoeris and genetic distance values support its diff erentiation from O. obstricta and Ophiomoeris sp. (Fig. 47A, Suppl. fi le 7). Morphologically, O. petalis sp. nov. can be distinguished within Ophiomoeris by having seven arms, large radial shields with a straight distal margin and a few low granules on the disc scales around the centrodorsal plate.
The genus Ophientrema was represented by only two species, O. euphylactea and O. scolopendrica, but O. euphylactea has not been recorded since its original description. Therefore, understanding the genus Ophientrema has been limited to O. scolopendrica in recent molecular and morphological studies. Ophientrema is clearly distinguished within Ophiacanthidae by having thin skin with small granules and by characteristics of the ventral arm spines along the arm. The molecular analysis indicates that Ophiomitrella conferta Koehler, 1922 may be closely related to the genus Ophientrema, but interspecifi c genetic distances between these two species were higher than previously published values (Ward et al. 2008;Boissin et al. 2017). The morphological analysis in this study and previous studies also indicate signifi cantly diff erent morphological features between those species (O'Hara & Stöhr 2006) (Suppl. fi le 8).
According to the molecular analysis in the present study, Ophiacantha bathybia and O. vorax fall into the same clade within Ophiacanthidae. Genetic relationships between these species cannot yet be understood completely, due to the lack of molecular data for the species of Ophiacantha. Ophiacantha bathybia was the fi rst abyssal species of the genus recorded from the South China Sea (depth 3563 m). The combination of morphological and molecular analysis in the present and previous studies suggest that O. bathybia is related to O. pacifi ca, O. sollicita, O. levispina Lyman, 1878 and O. frigida (Christodoulou et al. 2019;O'Hara et al. 2019), as well as O. sociabilis and O. cosmica (Stöhr & O'Hara 2021). Species of Ophiacantha from this clade have only been recorded from abyssal depths and appear to have similar morphological features. Stöhr & O'Hara (2021) proposed to apply the name O. pacifi ca to the Northern/Central/East Pacifi c populations and the name O. cosmica to the Southern Pacifi c/Antarctic/South Atlantic populations.
In this study, the genus Ophioplinthaca had diff erent topologies in the COI gene and 16S gene ML phylogenetic trees. This may be caused by the evolution rates of 16S and COI and by the accuracy of sequences used in the phylogenetic tree (Palumbi 1996b). However, the inter-clade of Ophioplinthaca was in the same sub-clade on both trees (Fig. 47). Our phylogenetic analysis suggests that O. rudis (Koehler, 1897) may be related to O. plicata (4.12 ± 0.96% SE). The genetic distance within O. plicata using COI fell at the high end of published intraspecies values (Boissin et al. 2017). Ophioplinthaca plicata had a signifi cantly high genetic distance among specimens from nearest localities at 0.9% (Tasmania and New Zealand). Therefore, the genetic distance values of O. plicata from the South China Sea to Tasmania and New Zealand (2.26%) suggest that the genus Ophioplinthaca has higher intraspecifi c genetic distances or that O. plicata may be a species complex. Previous studies suggest that O. plicata is diffi cult to identify and is often misidentifi ed as O. pulchra, due to highly variable and overlapping morphological characters in these species. The present morphological analysis and previous studies indicate that the morphology of O. plicata tends to vary somewhat across biogeographic regions. We suggest a combination of molecular and morphological analysis of O. plicata from diff erent localities to understand whether these morphological variations indicate a single species or a cryptic species complex (O'Hara & Stöhr 2006) (Suppl. fi le 7 and Suppl. fi le 8).
The molecular analysis of the Ophiactidae indicates four sub-clades within the family. Species of Ophiactis from the South China Sea fell into Sub-Clade 02 (O. cf. perplexa, O. profundi and O. cf. brachygenys) (Fig. 48). Species of Ophiactis were diffi cult to distinguish by morphological analysis alone, due to high morphological similarity between species and high morphological variation among individuals (Lyman 1879;H.L. Clark 1911;Koehler 1922a). Therefore, combining morphological and molecular analysis will help to understand the species complexity. The morphology of O. perplexa varied among individuals, especially the characteristics of disc spines (Fig. 36). Molecular analysis suggests two sister clades within O. perplexa due to a wide range of intraspecifi c genetic distances along the South China Sea to New Zealand waters (0.55% to 2.39%). Ophiactis perplexa from the South China Sea and Tasman Sea fall into one sister clade with another clade around New Zealand. However, we could not determine whether specimens in these two clades belong to a single species or a species complex without suffi cient molecular data collection of O. perplexa from diff erent localities. Ophiactis brachygenys from this study was the fi rst record of a heptamerous and hexamerous species that was previously known only from pentamerous individuals. This fi nding added another problem to the taxonomy of Ophiactis. Ophiactis defi nita and O. brachygenys were synonymized by Liao (2004). Morphologically both species share similar features, except separated arm plates, but this character can vary due to the size of the specimen or the appearance of the arm (Fig. 42G-H). Species of Ophiactis were harder to identify by key morphological characters that delimit species, when considering morphological variation among individuals. The molecular analysis of Ophiactidae identifi ed two sister clades among O. defi nita and O. brachygenys due to broad intraspecifi c genetic distances from the South China Sea to Australian waters (1.09% to 2.02%). However, whether this result indicates a single species or a species complex could not be determined from this study, due to insuffi cient data collection and the small number of localities (Suppl. fi le 9). We suggest a population genetic study for the genus Ophiactis to better understand the species complexity.
Among morphological characters, arm skeleton characters have become key features to identify families among Ophiuroidea, especially vertebrae in the families Ophiotomidae, Ophiactidae, Ophiomusaidae (Ophiomusa) and some Ophiacanthidae (Ophientrema, most species of Ophiacantha) with zygospondylous articulation and in the families Asteronychidae, Euryalidae, Gorgonocephalidae and some Ophiacanthidae (Ophiurothamnus, Ophiomoeris, Ophioplinthaca and some species of Ophiacantha) with streptospondylous articulation.
The overall intraspecifi c genetic distance variation among families included in this study was 0.5% to 2.47% along the South Pacifi c region to the South China Sea. Genetic distances among families suggested that COI was better than 16S to understand interspecifi c molecular complexity (Suppl. fi le 10). The order Euryalida had a low interspecifi c genetic distance variation among all Ophiuroidea (Suppl. fi le 10). Our molecular analysis suggests that species from the genera Ophiacantha and Ophiactis have to be revised by constructing a concatenated ML tree with additional mitochondrial and nucleic genes (COI, 16S, 18S and 28S) to better understand the species complexity and evolutionary radiation among families (Liu et al. 2018).

Biogeography and species diversity of ophiuroids in the South China Sea
The ophiuroid fauna of the South China Sea is widespread across several biogeographic regions, but most of them are tropical Indo-Pacifi c species. A total of 304 species have been recorded from the South China Sea, including our study (Lane et al. 2000;Liao 2004;Putchakarn & Sonchaeng 2004;Sirenko et al. 2019;Chen et al. 2020;Li et al. 2021) (Suppl. fi le 11). Of the South China Sea ophiuroids, 44.55% (135) have been recorded from the Philippines and Indonesian seas, 69.97% (212) from Australia and New Zealand waters, 44.55% (135) from the Northwest Pacifi c region and only few of them (15) have been recorded from South Africa and the Indian Ocean. The South China Sea may be recognized as one of the species richest localities in the Indo-Pacifi c region. The recorded species belong to all six orders, 29 out of 33 families and 93 out of 259 genera within the class Ophiuroidea and represent 14.37% of global ophiuroid diversity (Table 3). The South China Sea harbors a great diversity of ophiuroid species, most of them in the order Amphilepidida, but also Ophiacanthida Table 3. Species diversity in all six current orders of Ophiuroidea in the South China Sea (sources: Lane et al. 2000;Liao 2004;Putchakarn & Sonchaeng 2004;Sirenko et al. 2019;Chen et al. 2020;Li et al. 2021). Note: some species were removed due to uncertainty of location, especially near Philippine seas; see Lane et al. (2000).  (Lane et al. 2000;Liao 2004;Putchakarn & Sonchaeng 2004;Liu 2008;Sirenko et al. 2019;Chen et al. 2020;OBIS 2021;Stöhr et al. 2021;Li et al. 2021) (Table 3). The rate of endemicity in the South China Sea is high with 7.59% (23) of the ophiuroid fauna, and 37.96% (112) are endemic to the Indo-Pacifi c region. A possible reason for the prominence of endemics in the South China Sea may be that it has periodically been enclosed by land during glacial low sea level stands (Lane et al. 2000). Most of the high biodiversity areas within the South China Sea are still lacking in data (Nansha Islands reef system), and given the complex species diversity of ophiuroids, further sampling from those areas should record additional taxa, some of which may increase the number of endemic ophiuroids in the South China Sea.
According to the depth gradient, 168 (55.4%) species were recorded only from shallow waters, 60 (19.8%) species only from bathyal depths, 69 (22.4%) species from both shallow and bathyal waters, and two (0.7%) species from both bathyal and abyssal waters in the South China Sea. A high percentage of the shallow water species was recorded in the order Amphilepidida (Lane et al. 2000;Liao 2004;Putchakarn & Sonchaeng 2004;Sirenko et al. 2019;Chen et al. 2020;Li et al. 2021). A signifi cant number of species was recorded from both shallow and bathyal depths, which shows that most of the ophiuroid species are eurybathic (e.g., Ophiomusa lymani: 200-4000 m), or they may harbor unrecognized cryptic diversity. In general, most of the species have been recorded from shallow waters (77.9%), few have been recorded only from deep waters (20.4%), and a signifi cant number of species have been recorded from a wider depth range (22.4%) in the South China Sea (Lane et al. 2000;Liao 2004;Putchakarn & Sonchaeng 2004;Sirenko et al. 2019;Chen et al. 2020;Li et al. 2021).
There is a high chance of fi nding new records on the deep-sea seamounts in the South China Sea, and more deep-sea expeditions are required to analyze the biodiversity of deep-sea ophiuroids. However, the sample collection in the present study was restricted to only a few localities from the South China Sea, including near the Xisha and Zhongsha Islands, and Hainan Island. The actual number of species recorded from the deep waters of the South China Sea will likely increase, because some of the specimens in our collection were impossible to identify.
Station records and observations from the present study suggest that Ophientrema scolopendrica has a parasitic or mutualistic relationship with glass sponge species (Porifera: Hexactinellidea), but this needs to be studied further by repeated observation or gut content analysis to better understand their relationship (Henkel et al. 2008;Girard et al. 2016). Previous literature and our observations suggest that O. scolopendrica may be epizoic at depths of 1000-2000 m, because water depth is considered to have a high impact on biogeographic patterns on seamounts (O'Hara 2007). Ophiacantha aster sp. nov. was found on a seamount, living as a colony of more than a hundred specimens attached to a dead coral branch, but its relationship with coral species and its feeding behavior remain unknown.
Gorgonocephalus chilensis and Asteroschema horridum were recorded from deep waters in the South China Sea (1070-1550 m). Asteroschema horridum had previously been recorded as a deep-water species, but Gorgonocephalus chilensis had been recorded from a wide bathymetric range without any signifi cant morphological variations (Olbers et al. 2019) The present study suggests a high possibility of recording species of Euryalida and Gorgonocephalus from the South China Sea that have previously been recorded from the South-Pacifi c region and confi rms a high probability of a wider distribution of species of Asteroschema and Gorgonocephalus around the Indo-Pacifi c region than previously expected.