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.

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.

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).

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).

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.

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.

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.

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).

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.