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Research Article
Description of two species of the genus Astrodia Verrill, 1899 (Ophiuroidea, Euryalida, Asteronychidae), including a new species from seamounts in the West Pacific
expand article infoXiaojun Xie, Bo Lu, Jie Pang, Dongsheng Zhang§|
‡ Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, China
§ Southern Marine Science and Engineering Guangdong Laboratory, Zhuhai, China
| Shanghai Jiao Tong University, Shanghai, China
Open Access

Abstract

Five specimens of Ophiuroidea from deep-sea seamounts in the West Pacific were collected and identified as two species, Astrodia duospina sp. nov. and Astrodia abyssicola. The new species, Astrodia duospina sp. nov., can be distinguished from its congeners by having indistinct or underdeveloped oral papillae, relatively short genital slits, crescent-shaped lateral arm plates, and plate-shaped external ossicles on the aboral surface of the disc. One specimen was identified as Astrodia abyssicola, which has been reported in the north-western Pacific and the north-eastern coast of Japan. The most recent tabular key of Astrodia was revised with two more key characteristics added, the shape and presence of oral papillae and the number of arm spines. The phylogenetic relationship of Astrodia and Asteronyx was analyzed based on 16S and COI sequences. The discovery of the two species further expanded the geographical distribution of the genus Astrodia.

Keywords

Deep sea, molecular phylogeny, morphology, ophiuroids, taxonomy

Introduction

Class Ophiuroidea, as the largest group among echinoderms, with 2126 valid species (Stöhr et al. 2022), are widely distributed from the tropics to polar seas, and from the intertidal to the deep ocean. The Indo-Pacific, North Pacific, and South Pacific regions are reported to have relatively high ophiuroid species richness (Stöhr et al. 2012). Due to the technical limitations of deep sea exploration, the deep-sea ophiuroid fauna remains poorly known (Rodrigues et al. 2011). Seamounts are often of volcanic origin, with elevated topography from the deep-sea floor, which alters the flow of ocean currents and provides highly heterogeneous habitats serving as “hotspots” for deep-sea animals, especially for suspension-feeding epibenthic organisms (e.g. corals, sponges, and ophiuroids) (Yesson et al. 2011). Understanding the biodiversity of ophiuroids from seamounts will provide key information for the protection of this vulnerable ecosystem.

The order Euryalida Lamarck, 1816 comprises about 200 species from three families, Euryalidae Gray, 1840, Asteronychidae Ljungman, 1867, and Gorgonocephalidae Ljungman, 1867 (Stöhr et al. 2022). Among these, Asteronychidae is the smallest family with only 12 extant species from four genera (Asteronyx Müller & Troschel, 1842, Astrodia Verrill, 1899, Astronebris Downey, 1967 and Ophioschiza H.L. Clark, 1911). The genus Astrodia was erected by Verrill, in 1899 and currently comprises four species, Astrodia abyssicola (Lyman, 1879), Astrodia excavata (Lütken & Mortensen, 1899), Astrodia plana (Lütken & Mortensen, 1899) and Astrodia tenuispina (Verrill, 1884). Astrodia tenuispina was first described by Verrill (1884) under the name Asteronyx tenuispina, and was transferred to Astrodia by Verrill (1899). Koehler (1922) described a new species, Astrodia bispinosa, which was later regarded as a junior synonym of Astrodia tenuispina (Baker 1980). The most recent description of Astrodia plana was published by Döderlein (1927). Recently, Okanishi and Fujita (2014) reviewed this genus and transferred Ophiocreas abyssicola Lyman, 1879 to Astrodia. In their review, Okanishi and Fujita (2014) provided interspecific distinguishing characteristics including the shape and arrangements of external ossicles on the aboral surface of the disc, length of genital slits in relation to the height of the disc, the shape of the lateral arm plates, presence or absence of a projection of the lateral arm plates on the middle to the distal portion of the arms. Additionally, the geographical distribution of the four species was summarized (Okanishi and Fujita 2014).

In this study, we describe a new species, Astrodia duospina sp. nov., and redescribe Astrodia abyssicola, from seamounts of the West Pacific. New interspecific diagnostic characteristics were identified, and the tabular key of Okanishi and Fujita (2014) for the genus Astrodia was updated. DNA sequences were used to infer the phylogenetic relationship of the two species with their congeners.

Materials and methods

Sample collection

Five specimens of Astrodia were collected by ROV HAILONG III, ROV HAILONG IV, and HOV JIAOLONG, from seamounts in the Philippine Sea and the Northwest Pacific, during several COMRA’s cruises in 2013, 2020, and 2021 (Fig. 1). All specimens were preserved in 95% ethanol on board the vessels and photographed using a digital camera (Canon EOS 5D), then deposited in the repository of the Second Institute of Oceanography, Hangzhou, China (RSIO).

Figure 1. 

Sampling sites of two species in the Philippine Sea and the Northwest Pacific (red circles represent the sampling sites of Astrodia duospina sp. nov., the blue circle represents the sampling site of Astrodia abyssicola).

Morphological analysis

Morphological characters were examined and photographed using a stereoscopic microscope (Zeiss Axio Zoom V16). Arm skeletons were examined with a Hitachi TM1000 scanning electron microscope. Skeletal elements were prepared by submerging in commercial bleach (2.5% NaOCl). Washed in distilled water and ethanol, air-dried, and mounted on a stub using dissolved carbon tapes. The following literature was used as references for the morphological analysis: Okanishi and Fujita (2014), Okanishi et al. (2018), Manso (2010), Baker (1980), and Martynov (2019).

Molecular analysis

Several arm segments were dissected from each individual for genomic DNA extraction using DNeasy Blood & Tissue Kit (QIAGEN) following the manufacturer’s protocols. The COI sequences and 16S rRNA sequences were amplified with primers listed in Table 1. The PCR procedures were as follows: an initial denaturation step at 95 °C for 4 min followed by 35 cycles of 94 °C for 15 s, 50 °C for 30 s, and 72 °C for 1 min, and a final extension step at 72 °C for 10 min, for COI; an initial denaturation at 95 °C for 4 min, followed by 35 cycles of 94 °C for 15 s, 50 °C for 30 s, and 72 °C for 30 s, and a final extension at 72 °C for 7 min, for 16S. PCR reactions were performed using 25 µL volumes containing: 1 µL of DNA template, 1 µL of each primer, 9.5 µL of dd H2O, and 12.5 µL of 2 × Phanta Max Master Mix (Vazyme, China). PCR products were purified with a QIAquick PCR purification kit (QIAGEN) following the protocol supplied by the manufacturer. Sequencing was performed by Sangon Biotech (Shanghai, China) on an ABI 3730XL DNA analyzer (Applied Biosystems, Foster City, CA, USA). Forward and reverse sequences were de novo assembled and edited using Geneious Prime 2021 (https://www.geneious.com), deposited in GenBank (COI: OP328780OP328783; 16S: OP325290OP325293).

Table 1.

Information on primers used for PCR programs.

Primer Sequence
Oph-COI-F TTTCAACTAATCAYAAGGAYATWGG
Oph-COI-R CTTCAGGRTGWCCRAARAAYCA
16Sar CGCCTGTTTATCAAAAACAT
16Sbr CCGGTCTGAACTCAGATCACGT

Seventy-two 16S sequences and 28 COI sequences of Asteronychidae were downloaded from the NCBI. In total, 78 16S sequences and 34 COI sequences (Table 2), including four new 16S sequences and four new COI sequences were used for phylogenetic analysis, with two species of Asteroschema as the outgroup. COI and 16S were aligned using Clustal Omega (Sievers and Higgins 2014) as a plug-in in Geneious with default settings, respectively. Maximum likelihood trees were inferred based on a concatenated alignment of 16S and COI, as well as an alignment of 16S and COI respectively. IQ-TREE was used to perform the maximum likelihood bootstrap method (http://iqtree.cibiv.univie.ac.at/) (Nguyen et al. 2015), with the substitution model GTR+I+G, bootstrap support values determined by the ultrafast bootstrap algorithm for 100,000 replicates (Hoang et al. 2018). The best substitution model was selected by ModelFinder as a plug-in in IQ-TREE websites. (Kalyaanamoorthy et al. 2017).

Table 2.

Voucher specimens and accession numbers of COI and 16S sequence data used in the phylogenetic analysis (IDSSE, Institute of Deep-sea Science and Engineering, China; MV, Museums Victoria, Australia; NSMT, National Museum of Nature and Science, Japan; RSIO, Second Institute of Oceanology, China; SIO, Scripps Institution of Oceanography, USA).

Species Locality Voucher number CO1 16S Code from Okanishi et al. (2018)
Asteronyx longifissus Monterey, California SIO: BIC: E6108 - KM014337 -
Asteronyx loveni South China Sea IDSSE-EEB-SW0002 MZ198756 MZ203264 -
Asteronyx loveni New Zealand MV F188855 KU895061 - -
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-A - LC276316 OK-226
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-B - LC276354 OK-315
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-C LC276289 LC276330 OK-256
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-G LC276290 LC276331 OK-257
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-H LC276282 LC276317 OK-227
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-I - LC276359 OK-339
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-J - LC276350 OK-295
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-K - LC276332 OK-258
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-L - LC276358 OK-337
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-R - LC276334 OK-262
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-S - LC276353 OK-314
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6904-T - LC276333 OK-261
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6951-B - LC276343 OK-281
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6951-C LC276292 LC276337 OK-269
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6951-D - LC276344 OK-284
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6951-F - LC276336 OK-268
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6951-G - LC276341 OK-279
Asteronyx loveni Off Abashiri, Hokkaido NSMT E-6951-H LC276291 LC276335 OK-267
Asteronyx loveni Off Miyako, Iwate NSMT E-6943-A LC276288 LC276329 PT-253
Asteronyx loveni Off Miyako, Iwate NSMT E-6256 AB758757 AB605076 PT-41
Asteronyx loveni Off Miyako, Iwate NSMT E-5641-A LC276284 LC276320 PT-238
Asteronyx loveni Off Miyako, Iwate NSMT E-5641-B LC276285 LC276321 PT-239
Asteronyx loveni Off Miyako, Iwate NSMT E-5641-C - LC276322 PT-240
Asteronyx loveni Off Miyako, Iwate NSMT E-5641-D LC276286 LC276323 PT-241
Asteronyx loveni Off Miyako, Iwate NSMT E-5641-E - LC276324 PT-242
Asteronyx loveni Off Miyako, Iwate NSMT E-5638-A LC276278 LC276308 PT-213
Asteronyx loveni Off Miyako, Iwate NSMT E-5638-B - LC276352 PT-306
Asteronyx loveni Off Miyako, Iwate NSMT E-5638-D - LC276357 PT-323
Asteronyx loveni Off Miyako, Iwate NSMT E-5638-E - LC276356 PT-320
Asteronyx loveni Off Miyako, Iwate NSMT E-5637-A - LC276310 PT-215
Asteronyx loveni Off Miyako, Iwate NSMT E-5637-B LC276281 LC276314 PT-220
Asteronyx loveni Shima Spur, Mie NSMT E-6360 - LC276302 PM-199
Asteronyx loveni Shima Spur, Mie NSMT E-6983 LC276280 LC276312 PM-218
Asteronyx loveni Shima Spur, Mie NSMT E-6983 - LC276347 PM-290
Asteronyx loveni Shima Spur, Mie NSMT E-6982 - LC276309 PM-214
Asteronyx loveni Off Tosa, Kochi NSMT E-1143-A - LC276318 PK-231
Asteronyx loveni East China Sea, west of Japan NSMT E-6986-A LC276273 LC276298 ECS-195
Asteronyx loveni East China Sea, west of Japan NSMT E-6986-C LC276272 LC276297 ECS-194
Asteronyx reticulata East of Hiraji Bank, Nagasaki NSMT E-6912 - LC276355 ECS-316
Asteronyx reticulata East of Hiraji Bank, Nagasaki NSMT E-6915 - LC276338 ECS-272
Asteronyx reticulata East of Hiraji Bank, Nagasaki NSMT E-7016 - LC276301 ECS-198
Asteronyx reticulata East of Naka-Kasayama Bank, Nagasaki NSMT E-6908-C - LC276293 ECS-190
Asteronyx reticulata East of Naka-Kasayama Bank, Nagasaki NSMT E-6908-D LC276271 LC276296 ECS-193
Asteronyx reticulata East of Naka-Kasayama Bank NSMT E-6931 LC276279 LC276311 ECS-217
Asteronyx reticulata West of Gajajima Isl. Kagoshima NSMT E-6354 - LC276305 ECS-204
Asteronyx reticulata East of Hiraji Bank, Nagasaki NSMT E-6910 - LC276300 ECS-197
Asteronyx reticulata East of Hiraji Bank, Nagasaki NSMT E-6911 - LC276294 ECS-191
Asteronyx reticulata East of Hiraji Bank, Kagoshima NSMT E-6926 - LC276342 ECS-280
Asteronyx reticulata East of Hiraji Bank, Kagoshima NSMT E-6929 - LC276304 ECS-203
Asteronyx reticulata West off Takarajima Isl. NSMT E-6355 LC276274 LC276299 ECS-196
Asteronyx reticulata West of Amami Ohshima Isl., Kagoshima NSMT E-6351-A - LC276325 ECS-243
Asteronyx reticulata West of Amami Ohshima Isl., Kagoshima NSMT E-6942-B - LC276339 ECS-274
Asteronyx reticulata West of Ensei Knoll, Kagoshima NSMT E-6921 - LC276349 ECS-294
Asteronyx reticulata West of Ensei Knoll, Kagoshima NSMT E-6922-A LC276287 LC276328 ECS-249
Asteronyx reticulata West of Ensei Knoll, Kagoshima NSMT E-6925-A - LC276326 ECS-247
Asteronyx reticulata West of Ensei Knoll, Kagoshima NSMT E-6925-B - LC276327 ECS-248
Asteronyx reticulata East China Sea, west of Japan NSMT E-7001 LC276276 LC276306 ECS-205
Asteronyx reticulata East China Sea, west of Japan NSMT E-7002 LC276277 LC276307 ECS-206
Asteronyx reticulata Off Amami Ohshima Isl. Kagoshima NSMT E-6352 - LC276315 ECS-223
Asteronyx reticulata West of Minami-Ensei Knoll, Kagoshima NSMT E-6916 - LC276345 ECS-286
Asteronyx reticulata West of Minami-Ensei Knoll, Kagoshima NSMT E-6923-A - LC276340 ECS-278
Asteronyx reticulata West of Minami-Ensei Knoll, Kagoshima NSMT E-6923-B LC276275 LC276303 ECS-202
Asteronyx reticulata East China Sea, west of Japan NSMT E-7003-A - LC276351 ECS-303
Asteronyx reticulata East China Sea, west of Japan NSMT E-7003-B - LC276346 ECS-288
Asteronyx reticulata East China Sea, west of Japan NSMT E-7000-A - LC276348 ECS-291
Asteronyx reticulata West of Minami-Ensei Knoll, Kagoshima NSMT E-6920 LC276270 LC276295 ECS-192
Asteronyx reticulata Off Iejima Isl., Okinawa NSMT E-6987 - LC276313 ECS-219
Asteronyx sp. Between Yakushima Isl and Tanegashima Isl., Kagoshima NSMT E-3157-B LC276283 LC276319 PSW-237
Asteronyx luzonicus South China Sea IDSSE-EEB-SW0003 MZ198757 MZ203265 -
Astrodia abyssicola Miyagi, off Onahama NSMT E-6257 AB758828 AB605077 -
Astrodia abyssicola Philippine Sea, KPR Seamount RSIO68002 OP328783 OP325293 -
Astrodia duospina sp. nov. Philippine Sea, KPR Seamount RSIO59012 OP328780 OP325290 -
Astrodia duospina sp. nov. Northwest Pacific, Ko-Hakucho-Guyout Seamount RSIO61068 OP328781 OP325291 -
Astrodia duospina sp. nov. Northwest Pacific, RB Seamount RSIO61069 OP328782 OP325292 -
Asteroschema ajax Off Lord Howe Isl. MV F99759 AB758762 AB605078 -
Asteroschema clavigerum North Atlantic haplotype 1 HM587850 HM587828 -

Results and discussion

Systematics

Class Ophiuroidea Gray, 1840

Order Euryalida Lamarck, 1816

Family Asteronychidae Ljungman, 1867

Genus Astrodia Verrill, 1899

Astrodia duospina sp. nov.

Figs 2, 3, 4, 5, 6, 7

Material examined

Holotype : China • 1 specimen; Northwest Pacific, Nazimov Guyot; 15°11.34'N, 162°49.26'E; depth 2713 m; 16 September 2020; collected by ROV HAILONG III; preserved in alcohol; RSIO61068. Paratypes: China • 1 specimen; Northwest Pacific, Nazimov Guyot; 15°11.34'N, 162°49.26'E; depth 2713 m; 16 September 2020; collected by ROV HAILONG III; preserved in alcohol; RSIO61069 • 1 specimen; Northwest Pacific, Caiwei Guyot; 15°40.61'N, 154°53.77'E; depth 2744 m; 7 September 2013; collected by HOV JIAOLONG; preserved in alcohol; RSIO31004 • 1 specimen; the Philippine Sea, Kyushu-Palau Ridge, Roischesar Peak; 13°20.85'N, 134°32.81'E; depth 1900–2000 m; 2 August 2020; collected by ROV HAILONG IV; preserved in alcohol; RSIO59012.

Diagnosis

Disc raised high above the arm. Aboral disc with plate-shaped external ossicles in the center and on the periphery. Radial shield narrow, longer than wide. Teeth triangular, oral papillae indistinct or underdeveloped. Genital slits short, approximately one-fourth of the height of the disc. Lateral arm plates crescent and not projecting on arms. Arm spines no more than two.

Description of holotype

Disc pentagonal, notched interradial edges, 14 mm in diameter, 4.7 mm in height. Aboral surface almost flat, slightly depressed in the center, entirely covered by thickened skin with plate-shaped external ossicles in the center, about 220 μm long (Fig. 3A). Peripheral disc covered with a few plate-shaped external ossicles, similar to those in the center but larger, approximately twice in length. Radial shields narrow, tumid, bar-like, without granules or spines, and almost reach center of disc (Fig. 3A, B). Approximately 7.2 mm long and 550 μm wide in the center and 1.1 mm wide at periphery.

Figure 2. 

In situ and on-board photos of Astrodia duospina sp. nov. A photo in situ (RSIO61068: the individual below, RSIO61069: the individual above, attached to an unidentified sea pen species) B photo on board (RSIO61068: the individual on the left, RSIO61069: the individual on the right) C, D photos on board (RSIO31004), aboral side (C), oral side (D). Scale bars: 10 mm (B); 20 mm (C, D).

Figure 3. 

Morphological characters of Astrodia duospina sp. nov. (holotype: RSIO61068) A aboral view of the disc B periphery of the aboral disc C oral view of the disc D genital silts E aboral view of the arms F arms spines. Abbreviations: RS radial shield; PO plate-shaped ossicle; M madreporite; T teeth; OT oral tentacle; GS genital slit; G gonad; LAP lateral arm plate; AS arm spine. Scale bars: 2 mm (A, C, E); 1 mm (B, D); 0.5 mm (F).

Oral surface flat, covered by thickened skin. Oral shield small to invisible, one madreporite. Adoral shield obscured by skin (Fig. 3C). Oral interradial surface covered with several plate-shaped external ossicles (Fig. 3C). Six teeth, triangular, forming vertical row on dental plate, each jaw covered by a pair of conical oral tentacles (Fig. 3C). Oral papillae invisible or underdeveloped. Two genital slits, small, about 1/4 as long of disc height (1.3 mm long and 260 μm wide), present on oral side of each interradius (Fig. 3D). Gonads visible on each interradius (Fig. 3C, D).

Five arms, long and slender, about eight to nine times as long as disc diameter, no abrupt change in width basally (Fig. 3E). Proximal segments 2.5 mm wide and 1.7 mm high, with arched aboral surface and flattened oral surface (Fig. 3E), gradually tapering toward tip. Arm spines only present on ventral side. First to fourth tentacle pores with one arm spine and following tentacle pores with two arm spines. Outer arm spines slightly shorter than inner ones at proximal segments, but only three-fifths as long as inner spines on middle and distal segments (Fig. 3F).

Color. Pink in situ, white in alcohol (Fig. 2).

Ossicle morphology of holotype. Vertebrae articulation streptospondylous, wider than long in proximal segments (Fig. 4A, B), longer than wide in distal segments (Fig. 5A, B). Oral side of each vertebra with longitudinal groove along midline, deeply depressed, and no oral bridge (Figs 4C, 5C). Pair of podial basins on oral side moderate in size (Figs 4C, 5C). Aboral side of each arm vertebra with longitudinal aboral groove, moderately depressed (Figs 4D, 5D). Lateral furrow of vertebrae declining obliquely from aboral to oral side (Figs 4E–F, 5E–F). Lateral arm plates crescent-shaped, each associated with one or two arm spines and spine articulations with nerve and muscle opening separated. Spine articulation bulges outward (Fig. 6A, C). A ridge on inner side of lateral arm plate, parallel to proximal edge (Fig. 6B, D). Arm spines cylindrical, never hooked, bearing fine thorns at tip throughout arms (Figs 3F, 6E–F).

Figure 4. 

Vertebrae in basal arm of Astrodia duospina sp. nov. (holotype: RSIO61068) A proximal view B distal view C oral view D aboral view E, F lateral view. Abbreviations: PB podial basin; LF lateral furrow. Scale bars: 200 μm (A–F).

Figure 5. 

Vertebrae in distal arm of Astrodia duospina sp. nov. (holotype: RSIO61068) A proximal view B distal view C oral view D aboral view E, F lateral view. Abbreviations: PB podial basin; LF lateral furrow. Scale bars: 100 μm (A–F).

Figure 6. 

Lateral arm plates and arm spines of Astrodia duospina sp. nov. (holotype: RSIO61068) A, B lateral arm plates from proximal arm, outer view (A), inner view (B) C, D lateral arm plates from distal arm, outer view (C), inner view (D) E, F arm spines from proximal (E) and distal arm (F). Abbreviations: MO muscle opening; NO nerve opening; R ridge. Scale bars: 200 μm (E); 100 μm (F); 90 μm (A, B); 60 μm (C, D).

Description of paratypes

Two paratypes (RSIO31004, RSIO61069) share the same morphological characteristics as the holotype, disc diameter 10.17 and 13.94 mm, about 1/10 and 1/9 as wide as the length of the arms, respectively. However, the radial shields of RSIO31004 are shorter than the radial shields of the holotype and of

RSIO61069 (Fig. 7F). Three arm spines exceptionally occurred only once in both paratypes (RSIO31004 and RSIO61069), the innermost arm spine of RSIO61069 is the longest and the stoutest, while the middle arm spine of RSIO31004 is the stoutest. (Fig. 7A, B). The other paratype (RSIO59012) is smaller, only 6 mm in disc diameter, about 1/3 as wide as the length of the arms and may be a juvenile of this species. The radial shields and the genital silts are much shorter than in the other three specimens (Fig. 7C, D). Likewise, the arm spines are shorter than one segment (Fig. 7E)

Figure 7. 

Morphological characters of paratypes of Astrodia duospina sp. nov. A arm spines of RSIO31004 B arm spines of RSIO61069 C–E pictures of RSIO59012, oral disc (C), aboral disc (D), arm and arm spines (E) F aboral disc of RSIO31004. These specimens have exceptionally three arm spines for an arm segment (shown by an arc in A and B). Abbreviations: AS arm spine; T teeth; PO plate-shaped ossicle; GS genital slit; G gonad; RS radial shield. Scale bars: 2 mm (F); 1 mm (C, E); 0.5 mm (A, B, D).

Etymology

The species name duo is derived from the Latin numeral word, meaning two, and Latin feminine noun, spina, meaning spine, referring to the presence of no more than two arm spines throughout the arm.

Remarks

This new species falls within the genus Astrodia by only possessing cylindrical unhooked arm spines. The new species resembles Astrodia abyssicola mostly by having plate-shaped external ossicles on the aboral disc and crescent-shaped lateral arm plates. However, the oral papillae are indistinct or underdeveloped in Astrodia duospina, which can be used to distinguish the two species from each other (Fig. 3D). Moreover, the genital slits are very short in Astrodia abyssicola, which are only one-fifth of the height of the disc, while Astrodia duospina has larger genital slits, being longer than one-fourth the height of the disc (Fig. 3C). Astrodia duospina can easily be distinguished from A. plana and A. excavata by external ossicles and lateral arm plates. External ossicles are plate-shaped on the aboral surface of the disc in Astrodia duospina (Fig. 3A, B), but are absent in A. plana. Lateral arm plates are not projecting in the new species (Fig. 3E), but are distinctly projecting from the oral surface of the arm in A. excavata. Additionally, the new species differs from A. tenuispina by having distinctly smaller genital slits (Figs 3C, 7C).

Astrodia tenuispina is a widely distributed species and was characterized by having slender unhooked arm spines, small and short oral papillae, separated genital slits (Verrill 1884). Baker (1980) compared specimens from south of Australia and the northwest Atlantic, described this species with 2 or 3 arm spines, and imbricating punctate scales on the disc surface. Okanishi and Fujita (2014) redescribed this species as with plate-shaped external ossicles on the periphery of the aboral disc, granule-shaped on the central disc, genital slits half of the height of the disc, lateral arm plates not projecting. According to these descriptions, A. duospina sp. nov. can be differentiated from A. tenuispina by having smaller genital slits and indistinct oral papillae. Furthermore, in two of the three large specimens of the new species, three arm spines were observed exceptionally at one arm segment (Fig. 7A, B), while the other three species possess three arm spines at several successive segments in the middle part of the arms. Since only a small number of specimens were examined, this characteristic was not used to distinguish the new species from its congeners, and more specimens should be examined before a robust result can be achieved.

Astrodia abyssicola (Lyman, 1879)

Figs 8, 9, 10, 11, 12

Ophiocreas abyssicola Lyman, 1879: 64–65, plate 17, figs 470–473.

Astrodia abyssicola: Okanishi and Fujita 2014: 188–192, figs 2–4.

Material examined

China • 1 specimen; Philippine Sea, Kyushu-Palau Ridge, Mugiboshi Seamount; 16.57.14'N, 134.52.7'E; depth 3225 m; 11 August 2021; collected by an HOV JIAOLONG; preserved in alcohol; RSIO68002.

Description

Disc pentagonal and almost flat, 10 mm in diameter, 3.2 mm in height, skin wrinkled under dry conditions (Fig. 9A, B). Aboral surface of disc lacks external ossicles (Fig. 9A, B). Radial shields narrow, slightly tumid, bar-like, without granules or spines, and almost reaching center of disc. (Fig. 9A). Approximately 3.8 mm long and 0.6 mm wide in center and 0.8 mm wide on periphery (Fig. 9A).

Oral surface flat, covered by thin skin, and lacking external ossicles (Fig. 9C). Oral shield triangular, one madreporite (Fig. 9D). Adoral shield big and thick, quadrangular, and longer than wide (Fig. 9D). Teeth spearhead-shaped, vertically on dental plate; each jaw bears a pair of short, conical oral tentacles (Fig. 9C). Oral papillae indistinct or underdeveloped (Fig. 9C). Two genital slits very short, 560 μm long and 110 μm wide, present on oral side of each interradius. Gonads visible in each interradius (Fig. 9D).

Five arms, long and slender, about nine to ten times as long as disc diameter, no abrupt change in width basally (Fig. 9E). Proximal portion of arm 1.8 mm wide and 420 μm high, with arched aboral surface and flattened oral surface. Arms tapering gradually toward tip. Arm spines only present in ventral part of arm. First to third tentacle pores without arm spines, fourth tentacle pores with one arm spine and following tentacle pores with two arm spines. Inner arm spines longer than outer arm spines. On middle and distal part of arm, outer arm spines three-fourths as long as inner spines (Fig. 9F). Three arm spines occurred once in two of the five arms. Lateral arm plates not projecting on arms.

Color. Bright pink in situ, entirely white in alcohol (Fig. 8B, C).

Figure 8. 

In situ and on-board photos of Astrodia abyssicola A photo in situ (RSIO68002, attached to an unidentified sea pen species) B, C photos on board (RSIO68002), aboral side (B), oral side (C). Scale bars: 10 mm (B, C).

Figure 9. 

Morphological characters of Astrodia abyssicola (RSIO68002) A aboral view of the disc B center of the aboral disc C oral view of the disc D genital silts E aboral view of the arms F arms spines. Abbreviations: RS radial shield; M madreporite; T teeth; ADS adoral shield; OT oral tentacle; GS genital slit; G gonad; LAP lateral arm plate; AS arm spine. Scale bars: 2 mm (A, C, E); 1 mm (B, D, F).

Ossicle morphology. Vertebrae articulation streptospondylous, wider than long in proximal segments (Fig. 10A, B), longer than wide in distal segments (Fig. 11A, B). Oral side of each vertebra with longitudinal groove along midline, deeply depressed, and no oral bridge (Figs 10C, 11C). Pair of podial basins on oral side moderate in size (Figs 10C, 11C). Aboral side of each arm vertebra with longitudinal aboral groove,moderately depressed (Figs 10D, 11D). Lateral furrow of vertebrae declining obliquely from aboral to oral side (Figs 10E, F, 11E, F). Lateral arm plates crescent-shaped, each associated with one or two arm spines. Spine articulations with separated nerve and muscle openings, bulging outwards (Fig. 12A, C). A ridge on inner side of lateral arm plate (Fig. 12D). Arm spines cylindrical, never hooked, bearing fine thorns at apex throughout arms (Fig. 12E, F).

Figure 10. 

Vertebrae in basal arm of Astrodia abyssicola (RSIO68002) A proximal view B distal view C oral view D aboral view E, F lateral view. Abbreviations: PB podial basin; LF lateral furrow. Scale bars: 150 μm (A–F).

Figure 11. 

Vertebrae in distal arm of Astrodia abyssicola holotype: RSIO68002) A proximal view B distal view C oral view D aboral view E, F lateral view. Abbreviations: PB podial basin; LF lateral furrow. Scale bars: 150 μm (C, D, E); 90 μm (A, B, F).

Figure 12. 

Lateral arm plates and arm spines of Astrodia abyssicola (RSIO68002) A, B lateral arm plates from the proximal arm, outer view (A), and inner view (B) C, D lateral arm plates from the distal arm, outer view (C), and inner view (D) E, F arm spines from proximal (E) and distal (F). Abbreviations: MO muscle opening; NO nerve opening; R ridge. Scale bars: 90 μm (A, B, F); 60 μm (C, D, E).

Remarks

Ophiocreas abyssicola was first described by Lyman (1879). Okanishi and Fujita (2014) transferred O. abyssicola to the genus Astrodia and redescribed it. This specimen (RSIO68002) was identical to Astrodia abyssicola by having 0~2 arm spines, rather short genital slits and crescent-shaped lateral arm plates. However, this specimen lacks external ossicles on the disc and arms, which is different from previous descriptions of Astrodia abyssicola by Okanishi and Fujita (2014) as having plate-shaped external ossicles on the periphery. Nevertheless, the genetic distance of COI and 16S (2.9% and 1.9%) between the new collected specimen and A. abyssicola are too small to justify two different species. Therefore, this specimen was identified as A. abyssicola, thus the external ossicles on the aboral surface of the disc could be plate-shaped or absent in this species.

Key morphological characters to the species of Astrodia

The key morphological characters among the five species from the genus Astrodia based on Okanishi and Fujita (2014) were revised in this study (Table 3). Three diagnostic characteristics were proposed by Okanishi and Fujita (2014) in their key for Astrodia: the length of the genital slits related to the height of the disc, external ossicles on the aboral disc surface, and shape and existence of projections of lateral arm plates. All three characteristics were useful to distinguish the new species from its congeners. The external ossicle, being absent in the A. abyssicola specimen examined in the present study but present and plate-shaped in the previous descriptions (Okanishi and Fujita 2014), might be an intraspecific variation. Additionally, we added two morphological characters, the number of arm spines and the shape of oral papillae, as key characters for interspecific discrimination of Astrodia. Astrodia abyssicola is the only species that possesses no more than two arm spines along their arms, whereas the other four species possess up to three arm spines or occasionally four. Furthermore, oral papillae are indistinct or underdeveloped in A. duospina sp. nov., but are domed granule-shaped in the four known species. Thus, we consider the number of arm spines and the shape and existence of oral papillae important characteristics for interspecific discrimination within Astrodia (Table 3).

Table 3.

Comparison of key morphological characters among species in the genus Astrodia.

Species Arm spines Genital slits External ossicles Lateral arm plates on middle to distal portion of arms Oral papillae Reference
Astrodia abyssicola (Lyman, 1879) 0–2 very short, ~1/5 (height of disc) plate-shaped on periphery shapes: crescent; projections: absent domed granule-shaped Lyman (1879), Okanishi and Fujita (2014), This study
Astrodia excavata (Lütken & Mortensen, 1899) 0–3 large, ~2/3 (height of disc) granule-shaped near radial shields and genital slits shapes: bar-like; projections: present domed granule-shaped Lütken and Mortensen (1899), Okanishi and Fujita (2014)
Astrodia plana (Lütken & Mortensen, 1899) 0–3 short, ~1/4 (height of disc) absent shapes: oblong; projections: absent domed granule-shaped Lütken and Mortensen (1899), Döderlein (1927), Okanishi and Fujita (2014)
Astrodia tenuispina (Verrill, 1884) 0–3, occasionally 4 short, ~1/2 (height of disc) plate-shaped on periphery, granule-shaped in center shapes: unknown; projections: absent domed granule-shaped, small and short Verrill (1884), Koehler (1906), Koehler (1922), Baker (1980), Gage et al. (1983), Manso (2010), Okanishi and Fujita (2014)
Astrodia duospina sp. nov. 0–2, occasionally 3 short, ~1/4 (height of disc) plate-shaped on periphery and in center shapes: crescent; projections: absent indistinct or underdeveloped This study

Molecular phylogenetic analysis

Based on the COI (583~1511 bp) and 16S (431~539 bp) sequences, the phylogenetic relationship of the two genera, Astrodia and Asteronyx, was inferred. The ML tree based on the concatenated 16S and COI sequences suggested that both Astrodia and Asteronyx were monophyletic with high bootstrap values (Fig. 13, Suppl. material 1: Fig. S1). The ML tree based on COI sequences was consistent with the tree generated from two genes (Suppl. material 1: Fig. S2), while in the ML tree based on 16S sequences, Astrodia abyssicola clustered with Asteronyx, with a low bootstrap value (Suppl. material 3: Fig. S3). Okanishi et al. (2018) suggested that the relationship of the two genera was unclear based on COI and 16S sequences. With newly sequenced DNA data added, our results indicated that the two genera are probably monophyletic. Additionally, the genetic distances of CO1 and 16S between A. duospina sp. nov. and A. abyssicola were 9.0% and 9.1%, respectively, supporting the morphological identification results. Molecular analysis also supported that the three specimens identified as Astrodia duospina sp. nov. are the same species, and the specimen identified as Astrodia abyssicola is closely related to the published sequence of this species with very small genetic distances (2.9% for COI and 1.9% for 16S) that fall into the intra-species genetic distance of Euryalida (Okanishi et al. 2018; Nethupul et al. 2022).

Figure 13. 

Maximum likelihood tree of the genus Astrodia based on concatenated sequences of COI and 16S (clades of Lin 1A, Lin 1B, Lin 2A, Lin 2B, Lin 2C and Lin 3 are from Okanishi et al. (2018), more detailed information about these clades showed in Suppl.materials. Values of each clade: SH-aLRT support (%) / ultrafast bootstrap support (%)).

Conclusion

In this study, we described a new species of the genus Astrodia collected from seamounts in the West Pacific, and another species (Astrodia abyssicola) was redescribed. Through comparing the five species of Astrodia, the tabular key of Okanishi and Fujita (2014) was revised and two additional key characteristics, the number of arm spines and the shape of the oral papillae, were identified for interspecific discrimination of Astrodia. Maximum likelihood trees supported our morphological results and suggested that both Astrodia and Asteronyx were monophyletic. This study provided both morphological and molecular information of the two Astrodia species, and the specimens reported further expanded the known geological distribution of the genus.

Acknowledgements

We thank all the scientists and crew on the RVs DAYANGYIHAO and XIANGYANGHONG, and the HAILONG and JIAOLONG team for their great work in the collection of the specimens. We also like to thank Dr Timothy D. O’Hara and Dr Masanori Okanishi and Ruiyan Zhang for their precious advice and kind help. This study was supported by the National Natural Science Foundation of China (42076135), the foundation of China Ocean Mineral Resources R & D Association (No. DY135-E2-2-06).

Reference

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Supplementary materials

Supplementary material 1 

Figure S1

Xiaojun Xie, Dongsheng Zhang

Data type: Image.

Explanation note: Maximum likelihood tree of the genus Astrodia and Asteronyx based on concatenated sequences of COI and 16S.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
Download file (4.57 MB)
Supplementary material 2 

Figure S2

Xiaojun Xie, Dongsheng Zhang

Data type: Image.

Explanation note: Maximum likelihood tree of the genus Astrodia and Asteronyx based on COI sequences.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
Download file (2.32 MB)
Supplementary material 3 

Figure S3

Xiaojun Xie, Dongsheng Zhang

Data type: Image.

Explanation note: Maximum likelihood tree of the genus Astrodia and Asteronyx based on 16S sequences.

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.
Download file (3.77 MB)
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