﻿Order Euryalida (Echinodermata, Ophiuroidea), new species and new records from the South China Sea and the Northwest Pacific seamounts

﻿Abstract Ophiuroids were collected by the manned submersible ‘Shenhaiyongshi’ from the deep-sea seamounts in the South China Sea and Northwest Pacific regions at 602–1920 m depth, during 2018 to 2020. A total of nine species was identified, including two new species and seven new records from the South China Sea and one new record from the Northwest Pacific region. Two new species are described as Asteroschemashenhaiyongshiisp. nov. and Asteroschemadomogranulatumsp. nov. The seven new records included five species from the genus Asteroschema, and one species each from the genera Asterostegus and Astrodendrum. Comprehensive descriptions of morphological features are provided, including characteristics of the arm skeleton, as well as a phylogenetic analysis based on 16S and COI sequences. Intraspecific genetic distance ranges of Euryalida species from the present study were 0.34% to 1.38%, which was relatively low compared to other orders in the class Ophiuroidea. The present study suggests a high probability that species of the order Euryalida are more widely spread around the Indo-Pacific region than previously expected.


Introduction
The order Euryalida Lamarck, 1816 (basket stars and snake stars) includes the families Euryalidae Gray, 1840, Asteronychidae Ljungman, 1867, andGorgonocephalidae Ljungman, 1867, and these include the species with largest known body size in the class Ophiuroidea, the brittle stars . The majority of the Euryalida are epizoic, living attached to hosts, such as corals, gorgonians, and sponges (Baker 1980). Currently, the order Euryalida contains 193 accepted species within 48 genera . The largest families are Euryalidae and Gorgonocephalidae with 95% of all Euryalida ). These two families include 44 genera (Euryalidae 11 genera, Gorgonocephalidae 33 genera).
This study presents species in the genera Asteroschema Örsted & Lütken in Lütken, 1856 and Asterostegus Mortensen, 1933 from Euryalidae, andAstrodendrum (Döderlein, 1902) from family Gorgonocephalidae, found in the South China Sea and on Northwest Pacific seamounts.
Asteroschema is one of the largest genera in the Euryalidae, but it is still ill-defined due to limited published information and high morphological similarity between the species. Currently, 33 species are included in Asteroschema and the most recently described one was Asteroschema sampadae Parameswaran & Abdul Jaleel, 2012 from the Indian Ocean. Most of the species are pentamerous, but two hexamerous species have been recorded from New Zealand waters (Asteroschema wrighti McKnight, 2000 andAsteroschema bidwillae McKnight, 2000). Previous studies differentiated Asteroschema species based on epidermal ossicle shape and arrangement on the disc and arms, size variance and shape of inner and outer arm spines, and starting point of the second arm spine on the proximal region of the arm (Okanishi and Fujita 2009;Parameswaran and Jaleel 2012). The term epidermal ossicle has been used for small superficial, often granule-like, skeletal elements on the disc and arms (Okanishi and Fujita 2009). Echinoderm skeleton generally develops in the dermis (Byrne 1994), but it is unknown if these ossicles originate in the epidermis or in the dermis, and their possible homology with granules or spines in non-euryalid groups is also unknown. Epidermal thus does not refer to the place of origin of these ossicles, but to their position in adult specimens. Asteroschema species have been divided into three groups according to the shape of their epidermal ossicles such as: species with only granular ossicles, with conical and granular ossicles, and domed and with plate-like granular ossicles (Okanishi and Fujita 2009).
The genus Asterostegus includes only three species and is morphologically related to the genus Astroceras Lyman, 1879, but differs in having the oral shield replaced by several small interradial plates (McKnight 2003;Okanishi and Fujita 2014). Understanding morphological variations and diversity of Asterostegus is limited due to a lack of material (Okanishi and Fujita 2014). In the present study, Asterostegus maini McKnight, 2003 from the South China Sea is recorded as the first record since the holotype, but recent studies of Asterostegus included detailed descriptions of all three species (Okanishi and Fujita 2014). However, this study includes the intraspecific morphological variation among A. maini specimens between the South China Sea and South Pacific waters, and the development of morphological characters relative to size variations. Lastly, the genus Astrodendrum is widely distributed from the Indo-Pacific to South Africa, and includes six species. It differs from other genera in the family Gorgonocephalidae by having external ossicles of various shapes on the disc, and by lacking calcareous plates on the lateral disc margin .
The present study covers deep waters around the South China Sea (Xisha and Zhongsha Islands) and in the Northwest Pacific region (southwest of Guam Island). Here, we present an account of the Asteroschema, Asterostegus, and Astrodendrum species collection, with descriptions of new species and new records. Our goal is to present a detailed documentation of the morphological features of these species, to complement the limited original descriptions and the lack of figures in the literature. We present the first ever comprehensive tabular key for all species in the genus Asteroschema. Two new species are described and seven species are redescribed, including seven new records from the South China Sea and one new record from the Northwest Pacific, all richly illustrated. DNA barcoding was used to identify ophiuroid species in the past two decades (Ward et al. 2008;Hoareau and Boissin 2010;Okanishi et al. 2011Okanishi and Fujita 2013). Hence, we use barcoding to test our morphological identifications and to understand the interrelationships within genera. This study also provides biodiversity information of Euryalida species living on seamounts, which may be useful for further studies of euryalid diversity and biogeography.

Morphological analysis
Specimens were photographed through a dissecting stereo microscope (OLYMPUS SZX7) or with a digital camera (Canon EOS 6DII) to identify morphological characters. Arm skeletons were examined with a scanning electron microscope (SEM) Phenom ProX. Skeletal elements were prepared by using undiluted NaOCl to dissolve the soft tissue of part of an arm. The excess NaOCl in skeletal elements was removed by repeated flushing with distilled water. Then, the ossicles were mounted on a stub using dissolved carbon tapes. Holotypes, paratypes and all other specimens are deposited at the Institute of Deep-sea Science and Engineering (CAS), Sanya, China. The terms used to describe ophiuroids follow previous authors (Martynov 2010;Stöhr 2011Stöhr , 2012Fujita 2014, 2018;O'Hara et al. 2017;Hendler 2018;. We define granules and spines as articulated ossicles on plates or scales, but tubercles as non-articulated stereom outgrowth, following Stöhr et al. (2012) and Goharimanesh et al. (2021). Following Turner et al. (2021), we utilize the term "pedicellarial bands" for what was previously known as "girdle bands" and for the "girdle hooklets" we use the term "pedicellariae".

Molecular analysis
We extracted DNA from identified specimens by using the TIANamp Marine Animals DNA kit (TianGen, Beijing) following the manufacturer's protocol. We sequenced cytochrome c oxidase I (COI) and the 16S partial gene for phylogenetic analysis by amplifying primer sets, with suitable PCR cycle (Suppl. material 1: Table S1) (Hoareau and Boissin 2010;Okanishi and Fujita 2013). Total PCR mixture was 50 μL volume, containing 25 μL Premix Taq with 1.25 U Taq, 0.4 mM of each dNTP and 4 mMMg2+ (Ex Taq version, Takara, Dalian, China), 0.5 μM each of the primers and approximately 100 ng template DNA. We performed electrophoresis using a 1.0% agarose gel and the NanoDrop 1000 (Thermo Scientific, Waltham, MA, USA) to assess PCR product quality of the specimens. PCR products were sequenced in both directions on an ABI3730 DNA Analyzer. All new sequences were deposited at NCBI GenBank (Table 1).
We constructed two maximum likelihood (ML) phylogenetic trees to represent the families Euryalidae and Gorgonocephalidae. Family Euryalidae: to construct the ML tree, we used 12 COI and nine 16S sequences from our collection and additionally 22 COI and 14 16S sequences from GenBank (Table 1). To construct the ML tree for the family Gorgonocephalidae, we used one species from our collection and an additional eight COI sequences from GenBank (Table 1). As outgroup we used COI and 16S sequences of Astrogymnotes irimurai Baker et al., 2001 andOphiomyxa anisacantha H. L. Clark, 1911 for the ML trees.
All sequences were aligned using the ClustalW algorithm in MEGA X. When constructing the Euryalidae ML tree, we used the concatenated sequence alignment function in MEGA X to input both COI and 16S sequences. The best-fit substitution model of the COI and 16S gene in the ML trees was the General Time Reversible + Gamma Distributed (GTR + G) model, estimated by the "Find Best DNA/Protein Models" Option of MEGA X. Phylogenetic trees were reconstructed using the maximum likelihood bootstrap method. ML analysis was run with MEGA X, and ML trees were constructed, including 1,000 bootstrap replicates (Kimura 1980;Thompson et al. 1994;Kumar et al. 2016Kumar et al. , 2018. The genetic distances were analyzed according to the Kimura 2-parameter model (Kimura 1980), and the standard error of each group was discovered by performing 1,000 bootstrap replications.

Results
Seven species of Asteroschema were identified, among them two new to science that are described below. One species of each of the genera Asterostegus and Astrodendrum were identified, both of them are new to the South China Sea and described below. A tabular key to all species of Asteroschema is provided in Table 2. ML phylogenetic trees are presented in Figs 2 and 3, and genetic distances in Suppl. material 2: Tables S2 and Suppl. material 3: Table S3 of most of the species described in the study.

Molecular phylogenetic analysis
In total, 34 COI sequences trimmed to 592 bp and 25 16S sequences trimmed to 453 bp were obtained after removing ambiguous aligned sites and successfully reconstructing an ML tree for the studied Euryalidae (Fig. 2). Two main clades were detected within the ML tree of Euryalidae (clade 01: genus Asterostegus; clade 02: genera Asteroschema and Ophiocreas). Overall average genetic distances of COI between two clades were 10.04±1.34% SE (Asteroschema and Ophiocreas) and 3.08±0.75% SE Figure 2. Family Euryalidae, maximum likelihood (ML) tree based on partial COI and 16S sequences (bootstrap support values were generated with rapid bootstrapping algorithm for 1,000 replicates; blue = new species; brown = specimens from this study).
Ossicle morphology of one paratype. IDSSE-EEB-SW090: Lateral arm plate curved around vertebrae, with strong curved rib with one arm spine articular structure, with single, completely separated large muscle and nerve openings (Fig. 5A). A depression on inner side of lateral arm plate (Fig. 5B). In proximal and middle half of arm inner arm spine slightly swollen, flat, and thorny on distal arm. Outer arm spine nearly half the size of the inner one with thorny tip (Fig. 5C). Distally, both spines changing into compound hook with secondary teeth (Fig. 5D). Arm concealed by polygonal large granular or plate-like ossicles (Fig. 5E). Vertebrae with streptospondylous articulation, with deep groove between proximal and distal end, dorsally a median longitudinal furrow, ventrally with deep median longitudinal groove containing lateral ambulacral canals, no oral bridge ( Fig. 5F-J).
Paratypes variations. Disc diameter 6.5 and 8 mm, and both basically identical to holotype. However, the segment at which the second arm spine first appeared varied (14-20 free segments), but is considered intraspecific variation.
Distribution and habitat. 1742 m depth. Zhongsha Islands, the South China Sea. Attached to coral host.
Etymology. The species name is derived from the Latin words domus, meaning dome, and granulatus, meaning granulated, referring to the domed granular ossicles on the disc.
Remarks. The here examined new species was collected on a deep-sea seamount, attached to an unidentified coral species. It concurs with the group that has domed and plate-like granular ossicles, in the genus Asteroschema. This clade included only one species, prior to this study (Asteroschema igloo Baker, 1980). Large polygonal plate-like ossicles were the most significant morphological character for delimiting most of the other Asteroschema species from A. domogranulatum sp. nov. (Table 2).
Asteroschema domogranulatum sp. nov. strongly resembles A. igloo. They are similar in size according to McKnight's (2000) description (8 mm disc diameter). Therefore, here we include a comprehensive morphological analysis to distinguish A. domogranulatum sp. nov. from A. igloo such as (see also Table 2): in A. domogranulatum sp. nov. radial shields raised above the arms and disc, straight, parallel, with narrow gap, whereas in A. igloo distal ends of radial shields much wider apart, converging to center, in A. domogranulatum sp. nov. polygonal granular ossicles on dorsal disc, in center smaller than at distal edge, but in A. igloo concealed by polygonal or rounded domed ossicles, and in center large, domed, rounded ossicles, in A. domogranulatum sp. nov. teeth pointed but in A. igloo ventralmost one pointed and others blunt spearhead-shaped, in A. domogranulatum sp. nov. ventral disc covered with polygonal plate-like ossicles, and distal half of jaw naked but in A. igloo completely covered with compact polygonal or rounded domed ossicles, in A. domogranulatum sp. nov. only dorsal and lateral surface covered with plate-like or granular ossicles, dense only on few arm segments from arm base, and naked ventral arm except arm base but in A. igloo whole arm covered with dense, rounded or polygonal domed ossicles, in A. domogranulatum sp. nov. inner arm spine slightly swollen, blunt, flattened, and outer arm spine with thorny pointed tip but in A. igloo inner arm spine swollen, blunt, and outer arm spine with smooth pointed tip, in A. domogranulatum sp. nov. start of first arm spine at second arm segment, and second arm spine at nineteenth or twenty-second arm segment but in A. igloo first arm spine from third arm segment, and second arm spine starts at eighth or tenth arm segment (McKnight 2000). The most significant morphological characters of A. domogranulatum sp. nov. were the appearance of the radial shields, and the granulation of ventral disc and arms (Fig. 4).   Koehler (1914), Döderlein (1930) outer spine: conical, pointed tip Asteroschema fastosum Koehler, 1904 6-13 mm and 180-300 mm (arm base width 5 mm) high, CO including RS small CO, more rounded, close-set at disc margin small CO, more closeset than on disc, 6 or 7 grains in 1 mm flattened CO inner > outer (inner spine ≤ 4 mm) inner spine: bluntly conical at arm base, then strongly club-shaped with TP 2 (4-5) Koehler (1904), Döderlein (1927Döderlein ( , 1930, Guile (1981)

Asteroschema intectum
Lyman, 1878 5-11.5 mm and 280 mm (arm width 3 mm) fine GO, 6-7 grains in 1 mm; RS long, meeting at center, GO fine than on disc, 8-9 grains in 1 mm fine GO except jaw GO scattered, and smaller than on disc.
Disc. Disc more or less pentagonal, raised high above arm, and swollen in center (Fig. 6A). Entire disc concealed by highly dense, small, rounded, finely rugose granular ossicles (eight or nine grains in 1 mm; Fig. 6A-C). Granular ossicles similar in size and shape from center to periphery of disc (Fig. 6C). Radial shields wide, straight, close together, but not meeting in center, and completely concealed by dense granulation (Fig. 6C). Genital slits narrow, vertical at interradii and densely covered with less rounded granular ossicles (Fig. 6D). Jaws narrow, elongated, concealed by slightly larger less rounded granular ossicles (Fig. 6E). At apex of jaw a blunt, spearhead-shaped tooth, and granular ossicles all over oral plates (Fig. 6E). Ventral disc densely covered with granular ossicles similar to dorsal disc (seven or nine grains in 1 mm) but less rounded around distal end of jaw and adoral shields (Fig. 6E). Adoral shields large, with curved edge, and connected to first arm segment. Oral shields not discernible and adoral shield spine covered by granular ossicles (Fig. 6E).
Arms. Arm width comparatively large in relation to body size, not arched, subcylindrical, width unchanged from base to middle half of arm (Fig. 6A, F). From middle to distal end, arm tapering slightly and more cylindrical (Fig. 6F-H). Dorsal and lateral arm surface concealed by dense, finely rugose, rounded granular ossicles similar to disc (eight or nine grains in 1 mm), continuing to distal end of arm (Fig. 6F-H). Distal half of dorsal and lateral arm concealed by less rounded, dense granular ossicles (eight or nine grains in 1 mm; Fig. 6H). Lateral arm plates on proximal to middle half of arm concealed by granular ossicles, including on base of arm spine, but on distal end only lateral arm plates concealed (Fig. 6G-K). Ventral surface of arm concealed by dense granular ossicles, similar to ventral disc, less rounded and more polygonal, but only covering proximal half of arm (seven or nine grains in 1 mm; Fig. 6I). Middle to distal end of ventral arm surface concealed by widely separated, in size decreasing granular ossicles (three or four grains in 1 mm) to completely naked (Fig. 6J, K). First free tentacle pore without arm spine (Fig. 6I). First arm spine appears at second arm segment, with short blunt tentacle scale. Second arm spine appears at nineth or eleventh segment (Fig. 6I, J). Inner arm spine initially tapered to pointed thorny tip, one arm segment in length, middle half cylindrical, less club-shaped, with small sharp thorns on more than half of spine length (Fig. 6L-N). Outer spine smaller, with thorny tip (Fig. 6M, N). Both arm spines similar in size at distal end of arm, and turning into compound hook with 4-5 secondary teeth (Fig. 6O, P).
Ossicle morphology of paratype. Lateral arm plate curved around the vertebrae, with two arm spine articular structures, with completely separate large muscle and nerve openings (Fig. 7A). A depression on inner side of lateral arm plate (Fig. 5B, C). Inner arm spine becoming cylindrical from proximal to middle half of arm, with terminal projection, and thorny surface on upper part of spine (Fig. 7D). Outer arm spine nearly half as long as inner one with less thorny tip (Fig. 7E). Distally both spines compound hook with 3-5 secondary teeth (Fig. 7F, G). Arm and disc concealed by rounded granular ossicles (Fig. 7H). Vertebrae with streptospondylous articulation, with deep slope between proximal and distal end, dorsally a median longitudinal furrow, ventrally with median deep longitudinal groove with lateral ambulacral canals, no oral bridge, podial basins moderate in size (Fig. 7I-M).
Paratype variations. Paratype disc diameter 12 mm, similar to holotype. Second arm spine from segments 10-12 and disc slightly flatter than those of holotype; considered intraspecific variation.
Distribution and habitat. 1070-1111 m depth. Near Xisha and Zhongsha islands in the South China Sea. Attached to sponge host.
Etymology. The specific name is dedicated to the manned submersible vessel 'Shenhaiyongshi' meaning deep sea warrior in Chinese, which collected the specimen.
Remarks. Asteroschema shenhaiyongshii sp. nov. concurs with the group of Asteroschema that has only granular ossicles. This group includes nearly 20 species. Furthermore, they are divided by naked, widely separated granular ossicles on ventral disc and base of arm. Fifteen Asteroschema species have dense granular ossicles on the ventral disc. Asteroschema shenhaiyongshii sp. nov. is distinguished clearly by its unique dense, rounded, fine granular ossicles on both disc and arms, a thick sub-cylindrical proximal to middle half of the arms without annular bands, and cylindrical outer arm spine with visible thorns at middle arm segments (Fig. 7).
Asteroschema ajax and A. laeve (Lyman, 1872) are similar to A. shenhaiyongshii sp. nov. by having granular ossicles on both dorsal and ventral disc, but are easily differentiated by well-spaced annulated granular bands on the arms, and club-shaped inner arm spine (A. H. Clark 1949;Lyman 1875;FWRI 2010).
Asteroschema arenosum Lyman, 1878 is similar to A. shenhaiyongshii sp. nov. by having granular ossicles on both dorsal and ventral disc, but differs in having a swollen arm spine, sparser granular coverage (five grains in 1 mm), and a second arm spine from the fourth arm segment. Asteroschema vicinum Koehler, 1907 differs in its sub-equal arm spines and A. sulcatum Ljungman, 1872 in its highly dense granular ossicle coverage (9-15 grains in 1 mm). Asteroschema igloo and A. domogranulatum sp. nov. differ from A. shenhaiyongshii sp. nov. by large polygonal to domed granular ossicles (four or five grains in 1 mm; Table 2) (Ljungman 1872;Lyman 1878;Koehler 1907;Pawson et al. 2009).
Asteroschema glaucum Matsumoto, 1915, andA. hemigymnum Matsumoto, 1915 are similar to A. shenhaiyongshii sp. nov. by having granular ossicles on both dorsal and ventral disc, but differ by sparser granular ossicle coverage (five or six grains in 1 mm), club-shaped inner arm spine at the middle arm segments and in A. hemigymnum the ventral disc is covered with pavement-like ossicles. Asteroschema inoratum Koehler, 1906 is similar to A. shenhaiyongshii sp. nov. by having a similar density of the granular coverage on both dorsal and ventral disc, but differs in having granular ossicles on the ventral surface along the arm, a slightly club-shaped inner arm spine at middle arm segments, and both spines not transforming into a compound hook (Table 2) (Koehler 1906;Matsumoto 1915).
Asteroschema monobactrum H. L. Clark, 1917 is similar to A. shenhaiyongshii sp. nov. in having granular ossicles on both dorsal and ventral surface and in start of the second arm spine, but differs by having separate, sparser granular ossicles (seven grains in 1 mm at disc center, five or six grains in 1 mm at periphery of disc and on arm), and by the granular ossicles on both sides distalwards along the arm becoming widely separated to almost naked (H. L. Clark 1917). Asteroschema brachiatum Lyman, 1879 is similar to A. shenhaiyongshii sp. nov. in having similar density granular ossicle coverage on both dorsal and ventral disc and arms, but differs by the entire arm being covered by granular ossicles, start of the second arm spine at the fourth arm segment, and a slightly club-shaped inner spine at middle arm segments (Lyman 1879). Asteroschema salix Lyman, 1879, A. tubiferum Matsumoto, 1911, and A. rubrum Lyman, 1879 differ from A. shenhaiyongshii sp. nov. by having separated, sparser granular ossicle coverage, a club-shaped inner spine at middle arm segments, and widely spaced ossicles on the ventral arm surface (Lyman 1879;Matsumoto 1911).
Granular ossicle density of Asteroschema wrighti McKnight, 2000, A. bidwillae McKnight, 2000, and A. tenue (eight or ten grains in 1 mm) is similar to A. shenhaiyongshii sp. nov. However, Asteroschema tenue differs from A. shenhaiyongshii sp. nov. by having a club-shaped inner spine at middle arm segments, and slender long arms, and A. wrighti differs by having a club-shaped inner spine at middle arm segments, widely spaced granular ossicles on the ventral arm, irregular plate-like ossicles on the ventral disc, and smooth outer spines, and by being hexamerous (Table 2) (Lyman 1875;McKnight 2000).
Most Ophiocreas species differ from Asteroschema shenhaiyongshii sp. nov. by having naked or micro-granular ossicles in the skin in both disc and arms. (Table 2). In our phylogenetic tree of the family Euryalidae, all Ophiocreas species cluster with some Asteroschema species, but the average genetic distance between Ophiocreas species and Asteroschema shenhaiyongshii sp. nov. was 13.61% (Fig. 2, Suppl. material 2: Table S2).  Asteroschema shenhaiyongshii sp. nov. clusters among Asteroschema rubrum, A. salix, A. tubiferum, A. cf. lissum, and A. bidwillae. All these Asteroschema species have granular ossicles on the dorsal disc and dorsal surface of the arms. Asteroschema bidwillae showed a close relationship with A. shenhaiyongshii sp. nov., and was identified as a sibling species due to similar morphological characters of granular ossicles on the dorsal disc and dorsal surface of the arms, and the shape of the inner arm spines at middle arm segments, but differs by having well-spaced granular ossicles on the ventral disc, a naked proximal ventral arm surface, and by being hexamerous and fissiparous ( Description. Disc diameter 13 mm, length of arms 195 mm, arm base width 4.5-5 mm (Fig. 8).
Disc. Disc circular, hexamerous, raised above arms, deeply swollen in center (Fig. 8A, B). Disc covered with dense, small, finely rugose granular epidermal ossicles ( Fig. 8A-C). Granular ossicles dense and small in size in disc center, but slightly larger at distal edge (six or seven grains in 1 mm; Fig. 8C, D). Radial shields extending to center but proximal ends concealed by skin with granular ossicles, and distal ends raised above the disc (Fig. 8C). Granular ossicles around distal edge and periphery of disc larger and more irregular (Fig. 8C). Genital slits narrow, without ossicles and vertical on ventral interradii (Fig. 8E). Jaws elongated, mostly naked without granular ossicles (Fig. 8F). At apex of jaw flattened, pointed, and finely rugose teeth, and two to four granular tubercles that resemble lateral oral papillae (Fig. 8F). Ventral disc covered with widely separated small granular ossicles (four or six grains in 1 mm; Fig. 8B, F). Adoral shields connected to first ventral arm segment and concealed by widely separated small granular ossicles, but outline of shields clearly visible (Fig. 8F). Oral shields not discernible and adoral shield spine covered by ossicles (Fig. 8F).
Arms. Arms six, at base wide, not arched, dorsally flattened, and swollen in first few free arm segment (Fig. 8G). Arms distalwards from middle part narrowing and increasingly cylindrical (Fig. 8H). Swollen dorsal arm base covered with dense, large, irregular granular ossicles (four or seven grains in 1 mm), distalwards decreasing in size and becoming rounded (six or seven grains in 1 mm), and distally widely separated (Fig. 8G, H). Lateral arm plates covered with slightly separated granular ossicles.
Ventral arm near base covered with granular ossicles similar to ventral disc (five or six grains in 1 mm), but becoming widely separated to completely naked along the arm (Fig. 8I, J). First two or three arm segments without arm spine (Fig. 8I). First arm spine appeared at third or fourth arm segment, and second arm spine at eighteenth or twenty-first segment (Fig. 8I-K). Inner arm spine cylindrical, with blunt thorny tip, one and a half arm segment in length (Fig. 8J, K). Outer spine half as long as inner spine in middle region, with thorny, pointed tip (Fig. 8K). Both arm spines equal in length at distal end of arm, and compound hook with 3-6 secondary teeth (Fig. 8L).
Ossicle morphology. Lateral arm plate with two arm spine articular structures, with slightly separated large muscle and relatively small nerve opening (Fig. 9A). A depression on inner side of lateral arm plate (Fig. 9B). Inner arm spine from proximal and middle half of arm with cylindrical, terminal projection, and upper part of spine covered with thorns (Fig. 9C). Distally, arm spine turns into compound hook with secondary teeth (Fig. 9D). Arm and disc concealed by rounded to slightly irregular granular ossicles (Fig. 9E). Vertebrae with streptospondylous articulation, dorsally a median large longitudinal furrow, ventrally with median deep groove with lateral ambulacral canals, podial basins small (Fig. 9F-J).
Remarks. Asteroschema bidwillae was first described by McKnight (2000), with type locality New Zealand waters in the South Pacific Ocean. This is the first redescription since the original description. The specimens from our collection concur with McKnight's description, but we noticed some differences such as: granular arrangement on radial shields, irregular ossicles on arm base, ossicles on ventral arm recorded nearly to middle region, and start of second arm spine. However, some of these variations may be related to size and maturity of the specimen (the holotype had a disc diameter of 5 mm). We hesitate to fully associate our specimen with Asteroschema bidwillae due to uncertainty with the morphological variation in A. bidwillae. The genus Asteroschema contains only two hexamerous species as far as known. Therefore, the closest one is Asteroschema wrighti McKnight, 2000, but it differs in characters of the radial shields, granulation on disc and arm, innermost arm spine, and start of second arm spine (Table 2). This is the first record of A. bidwillae from the North Pacific Ocean, if it is indeed this species. Description. IDSSE-EEB-SW0072: disc diameter 12 mm, length of arms from 165-175 mm (Fig. 10).

Asteroschema rubrum Lyman, 1879
Disc. Disc flat, slightly raised above arms, swollen in center, and small in relation to total body size of specimen (Fig. 10A, B). Disc covered with smooth, dense, evenly distributed, small rounded or irregular granular ossicles, similar in size (seven or eight granular ossicles in 1 mm; Fig. 10C, D). Radial shields closely together, parallel, raised above distal disc edge, but mostly concealed by skin with granular ossicles (Fig. 10C). Radial shields do not meet in center. Genital slits wide and vertical on ventral interradii (Fig. 10E). Jaw large, long and covered with dense irregular ossicles (Fig. 10F). Spearhead-shaped teeth and granular ossicles that resemble lateral oral papillae at apex of jaw (Fig. 10F). Adoral shields large, connected to first ventral arm segment, and concealed by granular ossicles (Fig. 10F). Oral shields not discernible and adoral shield spine naked. Whole oral region swollen nearly to genital slit. Ventral disc covered with dense granular ossicles (seven or eight grains in 1 mm; Fig. 10F).
Arms. Arms at base wide, not arched, dorsally flattened, and slightly swollen in first few free segments (Fig. 10A, G). Arms distalwards from middle part narrowing and more cylindrical (Fig. 10H). Dorsal arm base covered with smooth rounded granular ossicles (six or seven grains in 1 mm), middle segments with dense granular ossicles all the way to the arm spine base (seven or eight grains in 1 mm), and distally decreasing in size and separated (seven or eight grains in 1 mm) (Fig. 10G-L). Ventral arm base covered with dense granular ossicles similar to the ventral disc (eight or nine grains in 1 mm), distally decreasing in size and separated to naked (Fig. 10H-K). First one to two tentacle pores without arm spine (Fig. 10F). First arm spine appears at second or third arm segment, and second arm spine at ninth or eleventh segment (Fig. 10I). Outer arm spine half as long as inner spine in middle region, thorny pointed tip, distally compound hook (Fig. 10K, L). Inner arm spine cylindrical, one to one and a half arm segment in length, initially tapering to a pointed thorny tip, in middle blunt, slightly swollen with thorny surface on more than half its length, distally compound hook with three or four secondary teeth (Fig. 10I-L).
Color. In live specimen, reddish brown (Fig. 10). Ossicle morphology. Lateral arm plate with two arm spine articular structures, with slightly separate large muscle and nerve openings (Fig. 11A). A depression on inner side of lateral arm plate (Fig. 11B). Inner arm spine from proximal and middle half of arm cylindrical, slightly swollen, with thorny tip (Fig. 11C). Distally arm spine turns into compound hook with secondary teeth (Fig. 11D). Arm and disc concealed by granular ossicles, slightly wider than high, round to short stumps with convex tip (Fig. 11E). Vertebrae with streptospondylous articulation, with deep slope between proximal and distal end, dorsally a median longitudinal groove, ventrally with median deep longitudinal groove with lateral ambulacral canals, podial basins moderate in size (Fig. 11F-J).
Distribution. 730-958 m depth. Near Brandella, Chile and in the South China Sea. Remarks. Asteroschema rubrum was first described by Lyman (1879), with type locality in the Southwest Pacific Ocean near South America. This is the first rediscovery after the original description. The specimens from our collection concur well with Lyman's holotype description, the only difference was the starting point of the second arm spines. However, this morphological character is highly variable among individuals. According to the holotype description, A. rubrum belongs in the clade with granular ossicles only in the genus Asteroschema, but Okanishi and Fujita (2009) considered A. rubrum in the clade with conical and granular ossicles. Although, the SEM images of granular ossicles in the skin appear as granular to somewhat small stumps with convex tip (Fig. 11E). However, the description of the holotype is identical with our specimen, and it was described as granular ossicles. The characters of the swollen oral region, smooth granulation on the disc, and innermost spine can be used to distinguish A. rubrum from other species of Asteroschema (Table 2). The dorsal disc of A. rubrum seems naked in wet condition due to its smooth granulation which can lead to misidentification as Ophiocreas species (Fig. 10A). This is the first record from the South China Sea. Matsumoto, 1911 Figures 12, 13 Asteroschema tubiferum Matsumoto, 1911: 52;1917: 44;Baker 1980: 22, fig. 4 Description. IDSSE-EEB-SW0078: disc diameter 10 mm, length of arms 200 mm (Fig. 12).

Asteroschema tubiferum
Disc. Disc flat, slightly raised above arms, swollen in center (Fig. 12A, B). Disc covered with smooth, small, closely spaced, and evenly rounded or polygonal granular ossicles, dense in disc center (seven or eight grains in 1 mm), but larger and polygonal at distal edge (six or seven grains in 1 mm) (Fig. 12C, D). Radial shields not meeting in center, but converging (Fig. 12C). Distal end of radial shields raised above disc and wider than proximal end (Fig. 12C). Genital slits narrow, vertical on ventral interradii (Fig. 12E). Jaws elongated, covered densely with granular ossicles (Fig. 12F). Flattened, pointed, and spearhead-shaped teeth and granular ossicles that resemble lateral oral papillae at apex of jaw (Fig. 12F). Ventral disc densely covered with granular and polygonal ossicles (seven or eight grains in 1 mm; Fig. 12F). Adoral shields connected to first ventral arm segment and concealed by granular ossicles but outline of shields visible. Oral shields not discernible and adoral shield spine densely covered by ossicles (Fig. 12F).
Arms. Arms at base wide, dorsally flattened, and swollen in first few free segments (Fig. 12G). Arms distalwards from middle part narrowing and more cylindrical (Fig. 10H). Swollen dorsal arm base and proximal end of arm covered with dense granular or polygonal ossicles similar to disc (five or seven grains in 1 mm), on middle segments with slightly separated granular ossicles (six or seven grains in 1 mm), and distally decreasing in size and separated (seven or eight grains in 1 mm) (Fig. 12G, H). Granular ossicles on lateral arm plates slightly separated, but continuing along arm. Ventral arm near arm base covered with granular ossicles similar to ventral disc (seven or eight grains in 1 mm), but less dense on middle half (five or six grains in 1 mm), and distally widely separated or naked (six or seven grains in 1 mm) (Fig. 12I, J). On first few arm segments, tentacle pore with extended tube (Fig. 12J). First tentacle pore without arm spine (Fig. 12F). First arm spine appears at second arm segment, and second arm spine at eighth segment. Inner arm spine cylindrical, one arm segment in length, with blunt thorny tip, and slightly club-shaped (Fig. 12K). Outer arm spine smaller in size, with smooth to thorny tip (Fig. 12K). Both arm spines equal in length at distal end of arm, and compound hook with three or four secondary teeth (Fig. 12L).
Color. In live specimen, reddish brown on dorsal disc and arm, light brown on ventral disc and arm (Fig. 12).
Ossicle morphology. Lateral arm plate with two arm spine articular structures, with large, separated muscle and nerve openings (Fig. 13A). Inner arm spine distalwards from proximal part of arm cylindrical, with terminal projection, and thorny surface (Fig. 13B). Outer arm spine cylindrical, with pointed tip with few thorns (Fig. 13C). Distally arm spine turns into compound hook with four secondary teeth (Fig. 13D). Arm and disc concealed by less dense, wider, and shorter granular ossicles (Fig. 13E). Vertebrae with streptospondylous articulation, dorsally a large longitudinal furrow, ventrally with deep median longitudinal groove with lateral ambulacral canals, no oral bridge, podial basins relatively small (Fig. 13F-J).
Distribution. 325-1800 m depth. New Zealand, Tasman Sea, Kermadec Islands, Bay of Plenty, Hawaii, Sagami Sea, the South China Sea, and Northwest Pacific seamount.
Remarks. Asteroschema tubiferum was first described by Matsumoto (1911), then redescribed by Matsumoto (1917), Baker (1980), andMcKnight (2000). These redescriptions are helpful to identify individual morphological character variation. Matsumoto (1911Matsumoto ( , 1917 mentioned that in Asteroschema tubiferum the first ten arm segments have an extended tube in the tentacle pore (a sheath around the tentacle) that is closely attached to the arm spine and this character is present in our

specimens. Previously, A. tubiferum had been recorded from both North and South
Pacific Oceans at a wide distribution range. However, this is the first record from the South China Sea. The specimens from our collection concur with previous redescriptions, but we noticed a few variations such as less densely packed ossicles on ventral and lateral arm. The A. tubiferum specimen from the South China Sea collection showed less dense granular ossicles on the ventral disc and arm. Asteroschema tubiferum strongly resembles A. rubrum, A. laeve, and A. inoratum, but the characters of the granulation pattern, tentacle scale on first few arm segments, shape of the arm and inner arm spine characters can be used to distinguish it from these species (Table 2). secondary teeth (Fig. 15D). Arm and disc concealed by wider polygonal to rounded granular ossicles (Fig. 15E). Vertebrae with streptospondylous articulation, dorsally a median longitudinal furrow, ventrally with deep median longitudinal groove with lateral ambulacral canals, no oral bridge, podial basins relatively small (Fig. 15F-J). Distribution. 341-1800 m depth. New Zealand, Tasman Sea, Kermadec Islands, Bay of Plenty, Solomon Island, Coral Sea, Timor Sea, South Africa (off Glenmore), the South China Sea. Remarks. Asteroschema salix was first described by Lyman (1879), then redescribed by Lyman (1882), Baker (1980), McKnight (2000, and Olbers et al. (2015). These redescriptions are useful to understand individual morphological character variation of A. salix. Specimens from our collection concur with previous redescriptions, but we noticed some variation such as: slightly separated granular ossicles on the disc, fewer granular ossicles on the ventral arm surface, and slightly longer arms. However, most of these morphological variations vary within individual specimens according to previous descriptions (Baker 1980;McKnight 2000). Asteroschema salix strongly resembles A. tubiferum, A. rubrum, A. laeve, A. inoratum, A. arenosum, and A. glaucum but the characters of granulations and ossicle shape on the disc and arm, radial shield, and inner arm spine can be used to delimit A. salix from these species (Table 2). Previously, A. salix had been recorded from the South Pacific Ocean, and South African waters at a wide distribution range. This is the first record from the South China Sea.
Disc. Disc flat, slightly raised above arms, swollen in center (Fig. 16A, B). Disc covered with smooth, small granular ossicles (Fig. 16C, D). Granular ossicles dense and small in disc center (six or eight grains in 1 mm), but slightly larger and separated at distal end of radial shield (five or six grains in 1 mm) (Fig. 16C). Radial shields wide, similar in size, curved, slightly raised above disc but not meeting in center (Fig. 16A, C). Radial shields clearly recognizable under thin skin embedded with ossicles (Fig. 16A, C). Genital slits narrow, concealed by polygonal granular ossicles, and vertical on ventral interradii (Fig. 16E). Jaws elongated, mostly naked without granular ossicles (Fig. 16F). Flattened, spearhead-shaped teeth, and six to seven granular ossicles that resemble lateral oral papillae at apex of jaw (Fig. 16F). Ventral disc covered with widely separated small granular ossicles (six or seven grains in 1 mm), but mostly covered by translucent thin skin (Fig. 16F). Adoral shields large, distal edge convex, concealed by thin skin with scattered small, smooth granular ossicles (Fig. 16F). Oral shields not discernible and oral tentacle pore naked without ossicles (Fig. 16F).
Arms. Arms slightly arched, circular, from middle to distal half narrower and more cylindrical (Fig. 16G). Dorsal arm base covered with granular ossicles similar to dorsal disc (six or eight grains in 1 mm), increasingly separated and decreasing in size along the middle segments of the arm (seven or eight grains in 1 mm), distally widely separated (five or six grains in 1 mm) (Fig. 16G, H). On lateral arm plates, granular ossicles widely separated but continuing to base of arm (Fig. 16H, K). Only one or three ventral arm segments near arm base covered with few granular ossicles similar to ventral disc (six or seven grains in 1 mm), then completely naked along the arm (Fig. 16I, J). First arm spine appears at second arm segment, second arm spine at ninth or thirteenth segment (Fig. 16I-K). Inner arm spine initially short, thick with thorny pointed tip, at middle arm cylindrical, taller, one and a half arm segment in length, with flattened thorny tip (Fig. 16J, K). Outer arm spine half as long as inner spine in middle region, with smooth to thorny tip (Fig. 16K). Both arm spines equal in length at distal end of arm, and compound hook with five or six secondary teeth (Fig. 16L).
Color. In live specimen, reddish brown but radial shields slightly lighter in color due to thin skin (Fig. 16).
Ossicle morphology. Lateral arm plate with two arm spine articular structures, with large muscle and nerve openings (Fig. 17A, B). Inner arm spine from proximal and middle half of arm cylindrical, with thorny tip (Fig. 17C). Distally, arm spine turns into compound hook with secondary teeth (Fig. 17D). Arm and disc concealed by less dense, wider, and short granular ossicles (Fig. 17E). Vertebrae with streptospondylous articulation, dorsally a median longitudinal furrow, ventrally with deep median longitudinal groove with lateral ambulacral canals, podial basins relatively small (Fig. 17F-J).
Distribution. 797-1515 m depth. Maldives, South China Sea, Northwest Pacific. Remarks. Asteroschema lissum was first described by H. L. Clark (1939), with type locality Maldives waters in the Indian Ocean. This is the first redescription after the original description. The specimens from our collection were close to H. L. Clark's description but we noticed some differences, such as: characters of radial shields, and granular ossicles at ventral disc and base of arm. We hesitate to fully associate our specimens with Asteroschema lissum or propose a new species, due to uncertainty of these morphological variations. Some of these variations may be affected by size, maturity, and environment (holotype disc diameter 7.5 mm). Asteroschema lissum strongly resembles A. hemigymnum, A. intectum, and A. sublaeve by having similar granular density, and almost naked ventral disc and arms but differs in characters of the radial shields, start of second arm spine, granulation pattern on the disc and arm (Table 2). This is the first record of A. lissum from the South China Sea and the North Pacific Ocean, if it is indeed this species.
Disc. Disc circular and slightly inflated radially, with sunken interradial margins (Fig. 16A). Radial shields elongated, narrow, raised above the disc, extending nearly toward the disc center (Fig. 18B, C). Distal half of radial shield periphery covered by 10-12 (0.44 to 0.75 mm in width) large, club-shaped granules (Fig. 18C). Most of these granules cluster on distal end of radial shield (Fig. 18C). Entire disc, including radial shields, covered by thick skin (Fig. 18A-C). Adoral shields with slightly ovoid outline (Fig. 18E, F). Teeth spearhead-shaped, accompanied by granular domed lateral oral papillae but not visible in wet specimen ( Fig. 18D-F). Proximally, adoral shields separated by triangular plate (Fig. 18F) Oral shields absent, and single or double row of two to six rounded, square, or irregular oral interradial plates beyond adoral shields (Fig. 18E, F). One median plate located between distal end of adoral shields, and slightly proximal to rest of oral interradial plates (Fig. 18E, F). Lateral interradial surface of disc slightly vertical and covered by thick naked skin, two conspicuous genital slits inside a large opening (Fig. 18G).
Arms. Arms simple, strong, and not branching (Fig. 18A). Cross section of arm base slightly rectangular (7.2 mm in width and 8.5 mm high), but distal half of arm more cylindrical and narrower (Fig. 18H-J). Proximal to middle region of dorsal arm slightly flattened, ventral surface slightly arched, and lateral surface vertical (Fig. 18J, K). Whole arm concealed by thick skin (Fig. 18H-K). Lateral arm plate on proximal half of arms bears three to five club-shaped granules (0.4-0.6 mm in width) (Fig.  18H), decreasing in size and number along arm and vanishing on distal half of arm (Fig. 18I). Ventral arm covered by naked skin (Fig. 18K). First one to two tentacle pores lack arm spines; second or third pore with two or three arm spines, similar in size A dorsal view B center of dorsal disc C dorsal disc (distal end of radial shields) D ventral disc E, F oral frame G lateral disc H dorsal arm (proximal) I dorsal arm (distal) J lateral arm (proximal) K ventral arm (middle) L, M arm spines (distal) N oral frame O periphery of the disc P oral frame (small transparent ossicles). Abbreviations: as adoral shield, ars arm spine, go granular ossicle, gs genital slit, j jaw, mp median plate, oip oral interradial plate, rs radial shield, t teeth, tp tentacle pore, ts tentacle scale. Scale bars: 6 mm (A); 2 mm (B-H, N); 1 mm (I-K, O, P); 500 μm (L); 200 μm (M).
( Fig. 18D). At proximal end of arms, arm spines short, thick, ovoid with more or less rounded tip with rough surface, at middle of arms club-shaped, transversely flattened (Fig. 18J, K). Arm spines turn into hook with two to three secondary teeth at distal end of arms (Fig. 18L, M).
Color. In live specimen, dorsal disc interradially dark brown but radial shields light brown. Ventral and lateral disc whitish brown, dorsal surface of proximal and middle regions of arms bright brown/red. Lateral and ventral surface of arms whitish brown, granules creamy white (Fig. 18).
Ossicle morphology. Lateral arm plate with two arm spine articular structures, middle half of arm with large and wide, separated muscle and nerve openings, depression on inner side (Fig. 19A, B). Arm spines large, short, flattened, and club-shaped with thorny surface (Fig. 19C, D). Vertebrae with streptospondylous articulation. Vertebrae on proximal to middle half of the arm large, flat with deep ventral groove, no oral bridge (Fig. 19E, F). Vertebrae on distal half of arm slightly longer, dorsally large median longitudinal furrow, and deep median longitudinal groove on ventral side, with oral bridge (Fig. 19G, H).
Distribution. 417-602 m in depth. New Zealand (Cook Island), South China Sea. Remarks. Asterostegus maini was first described by McKnight (2003), with type locality Cook Islands, South Pacific Ocean, and Okanishi and Fujita (2014) redescribed it. However, this is the first record of Asterostegus maini since the holotype. Here we recorded two specimens from the South China Sea (disc diameter 26 mm and 32.2 mm) and both are larger than the holotype (disc diameter 22 mm). Currently, three species belong to the genus Asterostegus: A. maini McKnight, 2003, A. tuberculatus Mortensen, 1933, and A. sabineae Okanishi & Fujita, 2014. Asterostegus tuberculatus differs from A. maini in granules being scattered across the whole disc including the radial shield, and only two to three stump-like granules on the dorsolateral arm plate (McKnight 2003;Okanishi and Fujita 2014). Asterostegus sabineae differs from A. maini in large stump-like granules on the radial shield, only one oral interradial plate, and one or two large stump-like granules on the dorsolateral arm plate (Okanishi and Fujita 2014). The number of oral interradial plates and their arrangement are highly variable within and between individuals (Fig. 18E, F, N). Previous studies on the holotype showed only one row with two to five interradial plates (McKnight 2003;Okanishi and Fujita 2014). However, in our specimens, these are arranged in one or two rows with two to seven interradial plates in total (Fig. 18E, F, N). Therefore, one of the key morphological characters in the genus Asterostegus, the arrangement of oral interradial plates has to be modified. One specimen from the present study (IDSSE-EEB-SW0075, 26 mm disc diameter) showed some morphological variations, such as: small granules on the periphery of the disc and on few small areas on the ventral disc ( Fig. 18O, P). These granules are extremely small compared to those on the radial shields and arms. In the specimens from the present study, first arm spines started from second or third arm segment, but in the holotype, it started from the fourth arm segment. However, except for these small morphological variations, both specimens were similar to the holotype description.
Arms. Arms branched at least eight to nine times, flexible dorso-ventrally, flat ventrally, arched dorsally ( Fig. 20I-N). Ventral arm surface covered by smooth skin; proximal half with widely scattered small, flat, polygonal granular ossicles ( Fig. 20I-K). Dorsal arm surface covered by polygonal or domed plates and between these pedicellarial bands (Fig. 20L-O) that appear after second arm fork, covering whole lateral to dorsal area of arm, creating annulated appearance (Fig. 20L). First arm segment lacks spines, next four to six with two arm spines, thereafter two or three arm spines per segment (Fig. 20I-K). Ventral arm spines similar in size, smaller, unevenly pointed, distally turning into hooks with 2-3 secondary teeth (Fig. 20K, P, Q).
Color. In live specimen, whole specimen brown, but radial shields, oral regions, and ventral arms lighter than disc (Fig. 20).
Ossicle morphology. On middle half of arm, lateral arm plates with perforations on ventral side, large muscle opening and small nerve opening (Fig. 21A). Pedicellarial bands formed by approximately 12 articulating tubercles at curved distal end of baseplate and these articulations have a single foramen per tubercle for pedicellariae with one secondary tooth (Fig. 21B, C). Ventral arm spines on distal end of arm transformed into hook with two or three secondary teeth (Fig. 21D). Pedicellariae differ from ventral arm spine by having smooth apophysis (Fig. 21C, D). Vertebrae with streptospondylous articulation with smooth lateral furrows and paired openings in lateral side of vertebrae for lateral ambulacral canals, no oral bridge (Fig. 21F-N).
Distribution. 90-1300 m depth. South China Sea, Japan, East China Sea, Sri Lanka. Remarks. The specimen is similar to the holotype description by Döderlein (1902), and the redescriptions of Döderlein (1911Döderlein ( , 1927, Liao (2004) and , but showed some morphological variations especially on the disc (Fig. 20). Therefore, we hesitate to fully associate our specimen with Astrodendrum sagaminum. All descriptions mentioned granules on both dorsal and ventral disc, but in the present specimen, the dorsal disc is covered with widely separated conical stump-like granules and the ventral disc is covered with widely scattered micro-polygonal ossicle plates in naked skin. However, Baker (1980) mentioned that the granular pattern on the disc  was not a suitable morphological feature to delimit species in the genus Gorgonocephalus. This is the first record of Astrodendrum sagaminum from the South China Sea, if this is indeed that species.

Discussion
The molecular phylogenetic trees of these species of Gorgonocephalidae and Euryalidae were in agreement with previous studies (Okanishi and Fujita 2013;Christodoulou et al. 2019;O'Hara et al. 2019). Previous molecular studies indicated that intraspecific genetic distance ranges approximately from 0.5% to 6.4%, with a mean of 2.2%, but species from the family Euryalidae usually showed less than 2% mean genetic distance (Okanishi et al. 2011Okanishi and Fujita 2013;Boissin et al. 2017). In this study, we focused on the genera Asteroschema, Asterostegus, and Astrodendrum. In addition, we included species from the genus Ophiocreas due to their similar morphology to Asteroschema species. The species in the genus Asteroschema were difficult to analyze only morphologically due to great similarity in most morphological characters. Previous studies divided Asteroschema into three groups according to ossicle shape on the disc and arms, but still many species within these groups are hard to identify. In this study, we successfully managed to obtain the COI and 16S sequences from only one of the proposed new species, Asteroschema shenhaiyongshii sp. nov., which suggests a close relationship to A. bidwillae according to genetic distance (2.56%), but A. shenhaiyongshii sp. nov. is pentamerous and shows no signs of fission. It also has dense granular ossicle coverage on the ventral disc and ventral arm surface (Suppl. material 2: Table S2; Fig. 6). Therefore, we consider these two as sibling species. According to the present study, we suggest a species complex within Asteroschema tubiferum due to morphological variations between specimens from the South China Sea and New Zealand. However, we found significantly low genetic distances between the specimens identified as A. cf. lissum, A. tubiferum, A. rubrum, and A. salix (2.79±0.66% SE) (Suppl. material 2: Table S2). Therefore, understanding key morphological differences and intraspecific genetic distance range are important to delimit Asteroschema species. Asteroschema cf. lissum was recognized here as intermediate species between A. salix and A. tubiferum due to genetic distance values between these species. In the ML tree of the family Euryalidae, all Ophiocreas species cluster with Asteroschema oligactes, A. migrator, A. edmondsoni, A. ajax, and A. horridum. All these Asteroschema species have conical ossicles or annular bands on the arms. These two clades may correspond to one of these genera each or one of them may belong to both genera (making them synonymous) and the other to a putative new genus, but since the type species of both genera have not been sequenced yet, it is impossible to decide. Thus, the present study concurs with previous molecular studies in the hypothesis that Asteroschema may be polyphyletic, but may instead be paraphyletic with the genus Ophiocreas, and the present morphological differentiation between these two genera can be questioned (Okanishi and Fujita 2013;Christodoulou et al. 2019;O'Hara et al. 2019). A com-prehensive morphological and molecular taxonomic revision, including examination of type specimens of all Asteroschema and Ophiocreas species is needed to understand the key morphological characters and genetic differences. We tentatively place our new species in Asteroschema, but acknowledge that they may later be found to belong in Ophiocreas.
The genus Asterostegus includes only three species, and is closely related to Astroceras, but a previous phylogenetic analysis recognized it as monophyletic and belonging in the family Euryalidae (Okanishi and Fujita 2013). The interspecific genetic distance (3.09±0.75% SE) within the genus Asterostegus was low and similar to other Euryalidae species (Suppl. material 2: Table S2). The genetic distance of the genus Gorgonocephalus was significantly lower than in Asteroschema. The molecular phylogenetic analysis of Astrodendrum and Gorgonocephalus showed two main clades, and previous studies showed that Gorgonocephalus may be polyphyletic (Okanishi and Fujita 2013;Christodoulou et al. 2019;O'Hara et al. 2019). In the present study, Astrodendrum cf. sagaminum clustered with Gorgonocephalus sundanus and G. pustulatum. The type species Gorgonocephalus caputmedusae (Linnaeus, 1758) clustered with G. chilensis, G. eucnemis, G. arcticus, and G. tuberosus (Fig. 3, Suppl. material 3: Table S3). We suggest that A. sagaminum should belong in the genus Gorgonocephalus, or alternatively, Gorgonocephalus could be split into two genera, but a more thorough study with more genes and more specimens should be performed, before this step is taken.
Most of the species from the present study were collected from deep water in the South China Sea. Previous studies from the South China Sea recorded only few Asteroschema species, but found no representatives of the genera Astrodendrum and Asterostegus. According to the present study, the ophiuroid diversity of the South China Sea may be higher than previously known and future expeditions to the South China Sea deep-sea seamounts may discover even more species. The present study suggests a wider distribution of Euryalida species from the South-Pacific to the North-Pacific regions than previously expected.