Holotype. IDSSE-2018-0612-HS01, collected from the Xisha Trough of the South China Sea, station SY86-HS01 (18°16.11'N, 113°25.32'E), depth 2985 m, 12 Jun. 2018, preserved in absolute alcohol. Paratypes. Two specimens. IDSSE-2018-0531-HS01, collected from the Xisha Trough of the South China Sea, station SY84-HS01 (18°2.70'N, 114°3.51'E), depth 3156 m, 31 May 2018, preserved at -80 °C. IDSSE-2020-0917-HS01, collected from the northern slope of the South China Sea, station SY283-HS01 (17°23.20'N, 115°32.32'E), depth 3806 m, 17 Sep. 2020, preserved in absolute alcohol.

In the Xisha Trough, which is located in the northern slope of the South China Sea, depth 2985 m.

Body elongated and cylindrical, color yellowish-white. Tentacles 15. Ventrolateral tube feet up to 40–50 pairs, in alternating two or three rows. Dorsal papillae 18–20 on each side, in single rows. Ventrolateral papillae 9–12 on each side, in single rows. Midventral tube feet two and rudimentary. Dorsal deposits irregular perforated plates and varying types of crosses. Perforated plates and crosses with open ramifications ventrally. Papillae deposits slender and sturdy rods with spatulated ends, and crosses with open ramifications. Spatulated rods and irregular deposits in tube feet. Sturdy spatulated rods in tentacles.

External morphology. Body elongated and cylindrical, ventrum flattened. 29 cm long and 9 cm wide before fixation (Fig. 2A, B). Color in vivo yellowish-white, tentacles, papillae, and tube feet often darker (Fig. 2C, D). Mouth anteroventral, anus posteroventral. Tentacle discs with rounded knobs at the edges, and the tentacles partially retracted into the mouth, making the number of tentacles difficult to calculate. Ventrolateral tube feet 40 pairs, arranged in alternating two or three rows on each ventrolateral ambulacrum. Midventral tube feet two and rudimentary, scattered along the mid-ventral ambulacrum, one positioned in the front third of the body, the other is positioned in the back third of the body, the anus surrounded by few small tube feet. Dorsal papillae roughly 18–20 on each body side, evenly distributed, measuring 4–10 cm in length, and placed in single rows along the dorsal radii. Ventrolateral papillae ~ 9–12 on each side, generally shorter than dorsal papillae, measuring 2.5–7 cm in length, arranged in single rows along the ventrolateral radii. All papillae slender and tapering from base to end.

Figure 2. 

Oneirophanta idsseica sp. nov. (Holotype: IDSSE-2018-0612-HS01) A in situ image B holotype in live C dorsal view D ventrolateral view. Scale bars: 5 cm.

Ossicle morphology. Dorsal deposits contain (1) perforated plates with open ramifications (Fig. 3A3, A9, A10), 0.3–0.8 mm in diameter, bearing 0–5 processes on the surface, central holes smaller towards the edge. (2) crosses two types, the first with dichotomously ramified ends, one side bearing 2–5 processes and the other side smooth and without spines (Fig. 3A5–A8); the second irregular, with arms slender than the first type and numerous bifurcated spines throughout the length (Fig. 3A1, A2). (3) spatulated rods with enlarged ends (Fig. 3A4). Papillae deposits contain (1) extremely slender or sturdy spatulated rods, 0.8–1.6 mm long (Fig. 3B1, B7–B10); (2) spatulated crosses with 2–3 processes on the surface (Fig. 3B2, B5, B6); (3) crosses with open ramifications resembling dorsal deposits (Fig. 3B3, B4). Deposits in tentacles only spatulated rods (Fig. 3C), more robust than spatulated rods in papillae. Tube feet deposits mainly robust spatulated rods (Fig. 3D1, D2), 0.6–0.8 mm long, and irregular deposits, possibly in the developmental stage of perforated plates (Fig. 3D3, D4). Deposits in the ventrum same as dorsal (Fig. 3E1–E5), except for large spatulated rods with enlarged ends (Fig. 3A4).

Figure 3. 

SEM images of different tissues from Oneirophanta idsseica sp. nov. (Holotype: IDSSE-2018-0612-HS01) A1–A10 dorsal body wall B1–B10 papillae C tentacles D1–D4 tube feet E1–E5 ventral body wall. Scale bars: 50 μm (A1, A2, A7, B3, B4, D3, D4, E4); 100 μm (A3–A6, A8, A10, B2, B5, B6, E3, E5); 300 μm (A9, B1, B7–B10, C, D1, D2, E1, E2).

Consists of IDSSE and the Latin suffix icus (belonging to), to honor IDSSE’s contributions and efforts to the field of deep-sea exploration.

A seamount in APEI 4, Clarion Clipperton Zone; Xisha Trough, the northern slope of the South China Sea, at depths of 2985–3806 m.

Oneirophanta idsseica sp. nov. is characterized by the arrangement of ventrolateral tube feet in two or three rows that number up to 40–50 pairs with distinctive cross-types in dorsal deposits. O. idsseica sp. nov. is distinct from Oneirophanta setigera (Ludwig 1893) due to the presence of small, perforated plates and crosses with open ramifications that are usually bifurcated. O. idsseica sp. nov. differs from Oneirophanta conservata Koehler & Vaney, 1905 and Oneirophanta mutabilis Théel, 1879 by the arrangement and high number of ventrolateral tube feet, and the absence of large, perforated plates on dorsum.

The phylogenetic trees showed that O. idsseica sp. nov., together with an unnamed species (Oneirophanta stet. CCZ_100, see below), formed a sister group that included Oneirophanta cf. mutabilis and O. mutabilis. From a morphological point of view, O. idsseica sp. nov. mostly resembled O. stet. CCZ_100 with ventrolateral tube feet arranged in two or three rows, two rudimentary midventral tube feet, spatulated crosses and small, irregular perforated plates on dorsum, and crosses with open ramifications in different stages of development on the ventrum. From a molecular point of view, the COI pairwise distance between O. idsseica sp. nov. and O. stet. CCZ_100 was 0.6% (Suppl. material 1). Compared with the description of O. stet. CCZ_100, our study provides more details of ossicle morphology of tentacles, tube feet, and papillae.

Holotype. IDSSE-2018-0612-HS01, collected from the continental slope of the South China Sea, station SY157-HS01 (18°51.18'N, 114°24'E), depth 1340 m, 1 Jul. 2019, preserved in -80 °C

On the continental slope of the South China Sea, depth 1340 m.

Body elongated, color reddish brown, with darker tentacles and tube feet. Mouth and anus ventral. Tentacle 20. Ventrolateral tube feet ~ 37 pairs, each tube foot end with a brown ring, arranged in alternating three rows, bilateral symmetry. Dorsal papillae 23–26 on each body side, arranged in single rows. Ventrolateral papillae 9–11 on each body side. Midventral tube feet two and rudimentary. Deposits perforated plates, rods of varying shapes and few spatulated crosses.

External morphology. Body elongated, dorsum convex, ventrum flattened. 20 cm long, and 5 cm wide before fixation (Fig. 4A, B). Skin reddish brown, with darker coloration on tentacles and tube feet, a brown ring at the end of each tube foot. Tentacle 20, never with ramified processes, circum-oral papillae absent. Ventrolateral tube feet ~ 37 pairs, placed in alternating three rows, bilateral symmetry. Midventral tube feet two and rudimentary, one placed on half the body, the other placed on a rear quarter of the body, and several small tube feet surrounding anus. Dorsal papillae 23–26 on each body side, arranged in single rows along dorsal radii, measuring 5–6 cm. Ventrolateral papillae 9–11 on each side, arranged in single rows, comparatively shorter than dorsal papillae, measuring 2.3–4.6 cm.

Figure 4. 

Oneirophanta brunneannulata sp. nov. (Holotype: IDSSE-2018-0612-HS01) A dorsal view B ventral view. Scale bar: 6 cm.

Ossicle morphology. Dorsal deposits only robust perforated plates present (Fig. 5A1–A3), 0.6–1.1 mm in diameter, with central holes tapering from the center to the end, some perforated plates with irregular central apophysis (Fig. 5A1). Papillae contain (1) robust spatulated rods (Fig. 5B1–B3), 0.8–1 mm in length and partially connected at the ends (Fig. 5B1); (2) spatulated crosses (Fig. 5B4); (3) Perforated plates with 2–4 large central holes, rather slender than dorsal, measuring an average 0.9 mm in diameter (Fig. 5B5, B6). Tentacle deposits slender and sturdy rods with open ramifications (Fig. 5C), 0.4–0.6 mm in length. Tube feet with varying types of rods: (1) slender rods with open ramifications, 0.5–0.7 mm long (Fig. 5D1); (2) smooth spindle-shaped rods without spines, 0.7 mm in length (Fig. 5D2); (3) sturdy spatulated rods (Fig. 5D1, D3, D4), with rudimentary or enlarged ends, 0.5–0.9 mm in length. Numerous amorphous shaped, irregular broken deposits on ventrum (Fig. 5E1, E2).

Figure 5. 

SEM images of Oneirophanta brunneannulata sp. nov. (Holotype: IDSSE-2018-0612-HS01) A1–A3 dorsal body wall B1–B6 papillae C tentacles D1–D4 tube feet E1–E2 ventral body wall. Scale bars: 300 μm.

The specific epithet brunneannulata in Latin means brown rings. It is here used as a noun in apposition and refers to the distinctive brown rings around the tube feet.

Only in the type locality.

Oneirophanta brunneannulata sp. nov. differs from other species in the genus in possessing brown rings at the end of tube feet that are arranged in three rows along ventrolateral radii. O. brunneannulata sp. nov. is relatively similar to O. mutabilis in possession of plates and spatulated rods, but there are differences: (1) different diameters of perforated plates, 0.6–1.1 mm in O. brunneannulata sp. nov., but 2–3 mm in O. mutabilis, and some perforated plates in O. brunneannulata sp. nov. possess a central apophysis. (2) different types of deposits in tube feet, O. brunneannulata sp. nov. has various forms of spatulated rods and a few spindle-shaped rods, but lacks perforated plates, and there are perforated plates in addition to sturdy and spatulated rods in O. mutabilis. O. brunneannulata sp. nov. differs from O. setigera in having perforated plates on the dorsum and the ventrum, lacked spatulated crosses, and had a large number of tube feet that were arranged in three rows. Larger central perforations on perforated plates were in papillae, and the presence of spatulated rods and papillae were arranged in single rows along the dorsal radius (double rows along dorsal radius in O. conservata) distinguished O. brunneannulata sp. nov. from O. conservata (Table 2).

Holotype. IDSSE-2023-0208-HS02, northern slope of the South China Sea, station SY530-HS02 (16°28.60'N, 110°18'E), depth 1389 m, 8 Feb. 2023, preserved in absolute alcohol. Paratypes. Three specimens. IDSSE-2023-0204-HS02, northern slope of the South China Sea, station SY529-HS02 (16°28.20'N, 110°43'E), depth 1393 m, 4 Feb. 2023, preserved in absolute alcohol. IDSSE-2023-0208-HS01, northern slope of the South China Sea, station SY530-HS01 (16°28.30'N, 110°43'E), depth 1389 m, 8 Feb. 2023, preserved in absolute alcohol. IDSSE-2023-0208-HS03, northern slope of the South China Sea, station SY530-HS03 (16°28.18'N, 110°43'E), depth 1392 m, 8 Feb. 2023, preserved in absolute alcohol.

Northern slope of the South China Sea, depth 1389 m.

Body elongated, skin smooth, color orange in vivo. Mouth anteroventral, anus posteroventral. Tentacle 19 or 20. Ventrolateral tube feet 11–14 on each body side, arranged in single rows. Midventral tube feet two and rudimentary. Dorsal papillae 15–27 on each side, placed in single rows along dorsal radius. Ventrolateral papillae 7–10 on each side, arranged in single rows. Dorsal deposits spatulated crosses, spatulated rods, and widely scattered spinous rods with branched spines. Papillae deposits with spinous rods and spatulated rods. Spatulated crosses with the arms twice divided and two types of spinous rods are in ventrum. Spinous rods and sturdy spatulated rods with open ramifications in tentacles and tube feet.

External morphology. Body elongated, dorsum inflated, ventrum flattened, slightly narrowed anteriorly (Fig. 6). Length in vivo 22 cm (Fig. 6B), in ethanol 20 cm. Maximum body width 6 cm in vivo, 5.7 cm in ethanol. Color in vivo orange (Fig. 6C), tentacle and tips of papillae and tube feet more pigmented than the body skin. Tentacle 20. Circum-oral papillae absent. Mouth anteroventral, anus posteroventral terminal. Ventrolateral tube feet 14 pairs, placed in single rows on ventrolateral ambulacrum, partly projecting horizontally from the body (Fig. 6D). Midventral radius naked. Midventral tube feet two and rudimentary, with one placed on half the body, while the other placed on a rear quarter of the body, and several smaller tube feet close to anus. Left dorsal papillae 27, right dorsal papillae 21, measuring 1.5–5.2 cm, arranged in regular single rows along dorsal radii (Fig. 6C). Ventrolateral papillae 9–10 on each side, measuring 2.2–3.2 cm, placed in single rows along the ventrolateral radii.

Figure 6. 

Oneirophanta lucerna sp. nov. (Holotype IDSSE-2023-0208-HS02) A, B in situ images C dorsal view D ventral view. Scale bars: 5 cm.

Ossicle morphology. Dorsal deposits contain spatulated crosses, spinous rods and spatulated rods. Spinous rods with irregular spines, 0.1–0.2 mm in length (Fig. 7A1, A2); spatulated crosses with arms 0.2–0.8 mm in length (Fig. 7A3); spatulated rods up to 1 mm (Fig. 7A4). Papillae with spinous rods 0.2–0.3 mm in length (Fig. 7B1, B2), and spatulated rods 0.4–0.9 mm in length, some with an extra branch from central part (Fig. 7B3), some bifurcated at the end (Fig. 7B4). Ventral body wall spatulated crosses with the arms twice divided (Fig. 7C1, C2) and the spinous rods of two types, one with more complex and irregularly placed spines (Fig. 7C3), the other with fewer and more regularly arranged spines (Fig. 7C4), up to 0.4 mm long. Tentacles and tube feet with similar ossicle types, spinous rods of two types, a few regularly distributed spines, 0.3–0.35 mm in length (Fig. 7D1, E1), irregularly shaped (Fig. 7D5, E2), and somewhat sturdy spatulated rods with perforated extremities that occasionally bifurcated (Fig. 7D2–D4, E3, E4), 0.2–0.7 mm in length.

Figure 7. 

SEM images of Oneirophanta lucerna sp. nov. (Holotype IDSSE-2023-0208-HS02) A1–A4 dorsal body wall B1–B4 papillae C1–C4 ventral body wall D1–D5 tube feet E1–E5 tentacles Scale bars: 50 μm (A1–A2, B1–B2, C2); 100 μm (C3, D1–E5); 200 μm (A3, A4; B3–C1).

The species was named after the Latin word lucerna to commemorate the traditional Chinese Lantern Festival, which was relatively close to the time these specimens were collected.

Northern slope of the South China Sea, depths of 1389–1393 m.

The new species conformed to the genus Oneirophanta characterized by uncontracted the tentacles, the absence of oral papillae and tentacle discs without ramified processes. Oneirophanta lucerna sp. nov. differed from other species of Oneirophanta in possessing highly variable shaped spinous rods and ventrolateral tube feet that are only arranged in single rows, whereas, in other species (Table 2), they are arranged in two or three rows along ventrolateral ambulacrum: O. conservata, O. setigera, and O. mutabilis in double rows, O. idsseica sp. nov. in alternating two or three rows, and O. brunneannulata sp. nov. in three rows. The three accepted species of Oneirophanta and the other two new species in this study all have perforated plates that are absent in Oneirophanta lucerna sp. nov. This is the first record of a species in the genus Oneirophanta with mainly irregular spinous rods.

One specimen. IDSSE-2020-1203-HS01, in the Mariana fore-arc area, western Pacific Ocean, station SY310-HS01 (11°41.42'N, 140°58.56'E), depth 3394 m, 3 Dec. 2020, preserved in absolute alcohol.

External morphology. Body cylindrical, nearly equal in width throughout the whole length and tapering anteriorly. 15 cm long and 4.5 cm wide after fixation with 95% alcohol for several days (Fig. 8A, B). Skin white, calcified and brittle. Tentacles 20, unretractile, 0.6–1.6 cm in length, with 7–9 marginal digits. Mouth and anus ventral. Ventrolateral ambulacra with 16 tube feet on each side, arranged in two irregular rows. Dorsal papillae 6 pairs, the maximum length up to 10 cm, placed in single rows along dorsal radii. Ventrolateral papillae 13 on each side, the maximum length up to 6 cm. Midventral tube feet not found due to damage to the mid-abdomen of the specimen, and only approximately six small tube feet observed near the anus.

Figure 8. 

Oneirophanta mutabilis mutabilis Théel, 1879 (IDSSE-2020-1203-HS01) A dorsal view B ventral view. Scale bar: 3 cm.

Ossicle morphology. The body wall and papillae with perforated plates (Fig. 9A1–C3), 0.9–1.5 mm in diameter. The entire periphery of fully developed plates usually surrounded by closed holes, with large central holes (especially in ventrum) and small peripheral holes. Some plates bearing 8–14 small, vertical spines (Fig. 9B1, C3), but no formation of a secondary layer of meshwork. Tube feet with rods up to 0.8 mm long (Fig. 9D1, D2, D4) and incompletely developed plates with open ramifications (Fig. 9D3, D5). The irregular rods in tentacles with variable bending angles (Fig. 9E1–E5).

Figure 9. 

SEM images of Oneirophanta mutabilis mutabilis Théel, 1879 (IDSSE-20201203-HS01). A1, A2 dorsal body wall B1, B2 dorsal papillae C1–C3 ventral body wall D1–D5 tube feet E1–E5 tentacle. Scale bars: 300 μm (A1–C3); 100 μm (D1–E1, E3–E5); 50 μm (E2).

Cosmopolitan, depth 2515–6000 m (Hansen 1975, 1967; Thandar 1984).

Oneirophanta mutabilis was first described west of the Crozet Islands (H.M.S. Challenger station 146: 46°46'S, 45°31'E) at depths of 2514 m (Théel 1879). It was divided into two subspecies, Oneirophanta mutabilis mutabilis Théel, 1879 and Oneirophanta mutabilis affinis Ludwig, 1893, based on differences in tentacle shape, the type of deposit, and the number of dorsal papillae (Hansen 1967). There are four main distinctions between them (see Table 2): (1) O. m. affinis was restricted to a relatively small area in the eastern Pacific where it replaced the otherwise cosmopolitan O. m. mutabilis. (2) tentacles with marginal digits were found in all specimens examined of O. m. mutabilis, but tentacle discs of O. m. affinis were smooth or had incised edges, and they lacked marginal digits. (3) the perforated plates of O. m. affinis were almost completely devoid of vertical spines, whereas those of O. m. mutabilis had vertical spines. (4) numerous rods of O. m. affinis were usually present in the tentacle discs, which was an additional difference from O. m. mutabilis. The morphological characteristics of our specimens were in accordance with the description of O. m. mutabilis (Hansen 1975). This was the first record of O. m. mutabilis from the Mariana fore-arc area.

Genus Deima Théel, 1879

Two specimens. IDSSE-2019-0630-HS01, collected from the northern slope of the South China Sea, station SY155-HS01(17°43'N, 114°13'E), depth 3451 m, 30 Jun. 2019, preserved in absolute alcohol. IDSSE-2018-0531-HS02, collected from the Xisha Trough of the South China Sea, station SY84-HS02 (18°2'N, 114°5'E), depth 3404 m, 31 May 2018, preserved at -80 °C.

External morphology. Body ovate, dorsal vaulted, ventral flattened. 9–10 cm long and average 5.5 cm wide in vivo. Skin rigid, body wall brittle and easily broken. Color orange in vivo (Fig. 10A, B). Tentacles retracted into the mouth, resulting in uncountable quantities (Fig. 10C, D). 12 pairs of ventrolateral tube feet contractible based on morphology of different stages, single-rowed. Five pairs of rigid and conical dorsal papillae placed in two rows. Ventrolateral papillae three pairs, half the length of the body. Pre-anal tube feet absent.

Figure 10. 

Deima validum validum (IDSSE-2018-0531-HS02 and IDSSE-2019-0630-HS01) A in situ image (IDSSE-2018-0531-HS02) B in vivo image (IDSSE-2018-0531-HS02) C dorsal view (IDSSE-2019-0630-HS01) D ventral view (IDSSE-2019-0630-HS01). Scale bars: 5 cm.

Ossicle morphology. Basal layer and several additional layers amount in the center of the perforated plates on the body wall and dorsal papillae, in diameter 0.6–1.8 mm (Fig. 11A1–B3, D1, D2), with regular holes and a large, reticulated knob, ~ 0.2 mm high. The tube feet with some types of developmental stages towards perforated plates (Fig. 11C1, C3) and perforated plates with one layer (Fig. 11C2), rods bearing spines (Fig. 11C4).

Figure 11. 

SEM images of Deima validum validum (IDSSE-2019-0630-HS01). A1–A3 dorsal body wall B1–B3 papillae C1–C4 tube feet D1–D2 ventral body wall. Scale bars: 300 μm (A1–B3); 50 μm (D1, D2)

This subspecies probably has a cosmopolitan distribution, except for the Arctic and Southern Ocean, at depths of 724–5426 m (Théel 1882, 1886b; Sluiter 1901a; Hérouard 1902, 1923; Koehler and Vaney 1905; Ohshima 1915; Grieg 1921; Deichmann 1940; Hansen 1975; Sibuet 1977; O’Loughlin 1998; Bohn 2006).

Deima validum was first described by Théel (1879) in the preliminary report of the exploring voyage of H. M. S. Challenger under Sir C. Wyville Thomson., with a detailed description of another novel species, Deima fastosum. Four new species were reported subsequently by different taxonomists: D. atlanticum Hérouard, 1898; D. blakei Théel, 1886; D. mosaicum Ohshima, 1915 and D. pacificum Ludwig, 1894. The family Deimatidae underwent a thorough revision by Hansen (1975), who regarded D. fastosum, D. atlanticum, D. blakei, D. mosaicum and D. pacificum as junior synonyms of D. validum. Hansen (1967) separated the species into two subspecies, cosmopolitan Deima validum validum and the eastern Pacific Ocean Deima validum pacificum Ludwig, 1894, based on differences in the number of dorsal papillae and the type of deposit in which they were found.

The two specimens examined here are consistent with the diagnosis of subspecies D. v. validum as described in detail by Hansen (1975). Perforated plates were like those of D. fastosum, with a large, reticulated, conical knob, which hardly ever approached in other known specimens that were used to investigate D. v. validum. This unique feature represents an extreme case of plate variation, where the spinous appearance of the skin is due to these very high and often vertically rising knobs on the plates. Hansen (1975) hypothesized that additional mesh structures on perforated plates varied with depth into two types: one-layered plates were characteristic of bathyal specimens, and many-layered plates were typical of abyssal specimens. However, it also exhibited a transitional type of plate in the abyssal Tasman Sea specimens and the bathyal Japanese specimens (perforated plates that were completely or almost completely devoid of additional meshwork), which prevented a clear distinction between a bathyal and an abyssal type of deposit.

Only in the Bay of Bengal (depth 1224–3365 m) did the development of additional layers of meshwork increase progressively with depth. In this research, high-knobbed plates were present in the abyssal South China Sea specimens (depth > 3000 m), but they were absent from the South China Sea specimens first reported (depth 1100 m) by Liao (1997). This was the second time that Deima validum has been recorded in the South China Sea. The proposition that the development of additional meshwork on perforated plates increased with depths needs to be investigated on more specimens at different depths in the South China Sea.