Oneirophanta idsseica, Xiao & Zhang, 2024

Xiao, Yunlu & Zhang, Haibin, 2024, Three new species and one new record of Deimatidae (Echinodermata, Holothuroidea, Synallactida) discovered in the South China Sea and the Mariana fore-arc area using integrative taxonomic methods, ZooKeys 1195, pp. 309-335 : 309

Oneirophanta idsseica sp. nov.

Figs 2 , 3

Oneirophanta stet. CCZ_100, Bribiesca-Contreras et al. 2022: 64-65, fig. 40.

Type material.

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.

Type locality.

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

Diagnosis.

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.

Description of holotype.

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.

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 ).

Etymology.

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

Distribution.

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

Remarks.

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.

Figures

Figure 2. Oneirophanta idsseica sp. nov. (Holotype: IDSSE- 2018 - 0612 - HS 01) A in situ image B holotype in live C dorsal view D ventrolateral view. Scale bars: 5 cm.

Figure 3. SEM images of different tissues from Oneirophanta idsseica sp. nov. (Holotype: IDSSE- 2018 - 0612 - HS 01) A 1 - A 10 dorsal body wall B 1 - B 10 papillae C tentacles D 1 - D 4 tube feet E 1 - E 5 ventral body wall. Scale bars: 50 μm (A 1, A 2, A 7, B 3, B 4, D 3, D 4, E 4); 100 μm (A 3 - A 6, A 8, A 10, B 2, B 5, B 6, E 3, E 5); 300 μm (A 9, B 1, B 7 - B 10, C, D 1, D 2, E 1, E 2).