Endoxocrinus (Diplocrinus) kexuei, Sun & Mei & Sha, 2025

Sun, Shao’e, Mei, Zijie & Sha, Zhongli, 2025, Endoxocrinus (Diplocrinus) kexuei, a new species of stalked crinoid (Echinodermata, Crinoidea, Isocrinida, Balanocrinidae) from rotten wood in the cold seep area of the Taixinan Basin, South China Sea, ZooKeys 1241, pp. 185-204 : 185-204

publication ID	https://doi.org/10.3897/zookeys.1241.128991
publication LSID	lsid:zoobank.org:pub:B638D1B5-692B-4200-BB1E-F6F1D5CD8FC2
DOI	https://doi.org/10.5281/zenodo.15650901
persistent identifier	https://treatment.plazi.org/id/BEF603F2-8B96-54C4-B7C6-CC6AE6C89226
treatment provided by	ZooKeys by Pensoft
scientific name	Endoxocrinus (Diplocrinus) kexuei
status	sp. nov.

Treatment

Endoxocrinus (Diplocrinus) kexuei sp. nov.

Figs 1 View Figure 1 , 2 View Figure 2 , 3 View Figure 3 , 4 View Figure 4 , 5 View Figure 5 , 6 View Figure 6 , 7 View Figure 7 , 8 View Figure 8 , 9 View Figure 9

Material examined.

Holotype • MBM 287584 View Materials , collected from one rotten piece of wood in the cold seep area of the Taixinan Basin, South China Sea, at the station FX-Dive 125 (22°02.58'N, 118°46.83'E), depth 833.7 m, hard substrate GoogleMaps . Paratype • MBM 287585 View Materials (one specimen), collection data same as the holotype GoogleMaps .

Diagnosis.

Arm number 20–22; arms smooth, up to 14 cm long; lateral flanges present on all proximal brachials; IB r 1 + 2 synostosis very flat with synostosial stereom predominating; axial canal rectangular; stalk shorter than arms, 6.5 cm long; internodals per mature noditaxis 5–8; stalk stellate to pentagonal cross-section; proximalmost diameter of stalk 5 mm; less than five (down to 2) cirri per nodal; proximal cirri directed upward; cirrus sockets large and round; 30 robust cirrals per cirrus; cirri rudimentary until 3 th nodal; cryptosymplexies without marked symmorphy and with facets mainly covered by synostosial stereom axial canal usually incompletely filled with stereom needlelike network and preserving an irregular secondary lumen

Etymology.

The species name is derived from the oceanographic vessel “ Kexue ” of the Institute of Oceanology, Chinese Academy of Sciences, which made a significant contribution to the biological research in the South China Sea.

Description.

Morphological description of the holotype. Specimen MBM 287584 (Figs 1 View Figure 1 – 6 View Figure 6 ). Colour (Fig. 1 a, b View Figure 1 ) pale yellow-pink in fresh samples, and white after alcohol immersion. Several arms and all cirri broken.

Arms 22, surface smooth, whole arm length 8–17 cm; two or three divisions per ray, two of these rays have three divisions, rays closely aligned (Fig. 2 a View Figure 2 ); all brachitaxes of two ossicles, united by non-muscular joints (synostosis). On all free arms, the first two brachials are connected by a synostosis muscular articulations present on the remaining branchials. IB r 1 rectangular with distal lateral margins weakly everted. IB r 2 pentagonal with lateral margin somewhat thickened and crenulated. IIB r 1 + 2 and IIIB r 1 + 2 with lateral flanges. IIB r 1 and IIIB r 1 with exterior lateral margin longer than interior. Proximal brachials (up tp B r 6) approximately square, with varying degrees of crenulate on lateral margin, the edge regular, flat and close to straight, giving a very angular appearance to the brachials.

The non-functional ligamentary articulations IB r 1 + 2, IIB r 1 + 2 and IIIB r 1 + 2, are clearly identified as synostosis. The synostosis of IB r 1 + 2 very flat with synostosial stereom predominating and rectangular axial canal (Fig. 5 a, b View Figure 5 ). However, at IIB r 1 + 2 and IIIB r 1 + 2 (Fig. 5 c – f View Figure 5 ), while synostosial stereom predominats and, syzygial stereom forms rudimentary crenularium near the aboral edge of the facet; the syzygial stereom remains very irregular on the lateral edges.

On the free arms, non-muscular junctions are present only in B r 1 + 2, whose general morphological features are similar to the secundibrachial or tertibrachials synostosis, with synostosial stereom forming a rudimentary crenularium on the aboral margin of the articular facet (Fig. 6 a, b View Figure 6 ). The rest of the brachial articulations are typical isocrinid synarthries (Fig. 6 c, d View Figure 6 ).

Basals form quadrilateral projections, protruding and separated from each other (W / L = 1.4–1.6). Radials hexagonal or pentagon, laterally adjacent (W / L = 2.4–2.8).

First pinnule (P 1) on B r 2 (Fig. 3 a View Figure 3 ), with 9–11 pinnulars, approximately 8.1 mm long; second pinnular longest, up to 1.4 mm, penultimate segment slender, twice as long as wide. P 2 to P 4 short and wide; proximal segments relatively long, middle segments longest, and distal segments abruptly narrowed and shortened. P 10 to P 15 longest and slender, about 13.5–16.5 mm long, with 17–19 pinnulars; proximal segment wider, middle segments longer than wide (L / W = 1.4–1.6); distal pinnulars with small spines on dorsal edges. Pinnules gradually becoming shorter towards the distal of the arm. P 20 to P 29 composed of 6–15 segments, pinnulars longer than wide (L / W = 1.6–2.0), except the first pinnular.

Stout stalk about 7.8 cm long and counts 39 whole columnals, distal nodal slightly broken proximal stalk gradually turning from star-shaped to pentagonal in cross-section, 5 mm in diameter proximally and 4.3 mm distally, the longest noditaxis is the seventh (6.92 mm) with eight nodals, proximal nodals are 1.2 mm in height and distal ones being 2.1 mm tall in this noditaxis; less than 5 cirri on each nodal, usually two or three, four on the 6 th nodal and two on the 7 th nodal. Proximal cirri directed upward, forming an acute angle with the proximal stalk; rudimentary cirri on the first to third nodal. The number of interrnodal per noditaxis is 5 (Fig. 2 b View Figure 2 ). Between two nodals, the internodal exactly in the middle is slightly higher than the others. Cirri remaining rudimentary, usually before the 3 th nodal. Fully developed cirrus slender with 30 cirrals, 6.3–8.5 cm long (Fig. 2 c View Figure 2 ) (others incomplete or missing); cirrus sockets large and round, occupying approximately the entire height of nodal (Fig. 2 b View Figure 2 ). The first 3 segments are relatively short. c 1 – c 4 with W / L = 1.7–2.3; c 5 to c 9 longest, longest cirral with W / L = 0.8–1; subsequent cirrals gradually shorter, c 13 – c 15 with W / L = 1.0–1.1; terminal claw quite small, slightly longer than the penultimate segment, no opposing spine; distalmost three cirri curved downward for fixation.

Nodal and infranodal united by cryptosymplexy. The cryptosymplexy (Fig. 4 a View Figure 4 ) is pentalobate and flat, with synostosial stereom occupying most of it and syzygial stereom restricted to its outer margin. The round axial canal (Fig. 4 b View Figure 4 ) is partially filled with a relatively long spicule network separated from the perilumen and preserving an irregular secondary lumen. Petaloid zones (Fig. 4 c View Figure 4 ) pear-shaped, interpetaloid zone is dominated by syzygial stereom and without axial groove. Symplexies present between internodals (Fig. 4 d View Figure 4 ), each petaloid zone having 6–8 crenulae and open lanceolate areola and interpetaloid zone (Fig. 4 e View Figure 4 ) is dominated by labyrinthic stereom.

Morphological description of the paratype. Specimen MBM 287585 (Figs 7 View Figure 7 – 9 View Figure 9 ) incomplete, part of the stalk and all cirri are missing (Fig. 6 b View Figure 6 ). Paratype is similar to the holotype, yet smaller. Differences are listed below.

20 smooth arms, up to 7–8 cm long. Two brachials per brachitaxes and two divisions per ray. IB r 1 + 2 ax and IIB r 1 + 2 ax (Fig. 8 a View Figure 8 ). Proximal branchials with lateral edges less waved than in the holotype. Branchial surface smooth. IB r 1 + 2, IIB r 1 + 2 and IIIB r 1 + 2, have small lateral flanges.

Basals rhombic, prominent and obviously apart from each other, W / L = 1.1–1.5. Radials hexagonal or pentagon, W / L = 1.9–2.3.

P 1 on B r 2 (Fig. 8 b View Figure 8 ), with 8–9 pinnulars. Oral pinnule short; proximal pinnulars thick and wide, distal pinnulars become slender and longer. Middle pinnule longest, with 11–13 pinnulars, 13.0 mm long. At the end of arms, pinnule shorter or absent.

The remaining stalk length is about 1.7 cm, proximal column cross-section stellate to pentagonal.

Proximal infranodal columnal facet (Fig. 9 a View Figure 9 ) with slight symmorphy in inner part, pear-shaped petaloid zones, and axial canal completely filled with dense thin stereom network without secondary lumen (Fig. 9 c View Figure 9 ). Cryptosymplexies (Fig. 9 b View Figure 9 ) relatively flat without marked symmorphy and with facets mainly covered by synostosial stereom. Outer margin zone and interpetaloid zone with syzygial stereom predominating. Axial canal (Fig. 9 d View Figure 9 ) markedly pentagonal and partially filled with a relatively long spicule network separated from the perilumen and preserving an irregular secondary lumen.

Quantitative morphological characters are reported in Table 1 View Table 1 .

Distribution.

Only known from the cold seep in the Taixinan Basin, South China Sea, at a depth of 833.7 m.

Remarks.

The genus Endoxocrinus now contains four extant species. According to David et al. (2006), the new species resembles mostly E. (Diplocrinus) alternicirrus in terms of arm division series, shape of the cross-section of stalk, and the number of internodal per mature noditaxis (Table 3 View Table 3 ). In E. (Diplocrinus) alternicirrus , the arms are up to 64 in number and 10–12 cm in length, while in E. (Diplocrinus) kexuei sp. nov., the arms are only 20–22 in number and are shorter (8–14 cm). The number of arms is variable among Endoxocrinus species (Table 3 View Table 3 ). The arm number is usually considered as an adaptive trait that often depends on depth, food supply and hydrodynamics ( Roux 1987; David et al. 2006). In E. (Diplocrinus) kexuei sp. nov., the proximal cirri are directed upward and cirri are rudimentary until the 3 th nodal, differing from E. (Diplocrinus) alternicirrus , in which the proximal cirri are oriented downward and cirri are rudimentary until 5 th nodal. These characteristics are similar to E. (Diplocrinus) wyvillethomsoni , in which the proximal cirri are oriented upward and the rudimentary cirri are present to the 3 rd nodal.

Cryptosymplexies exhibit traits that are more relevant at the species level. In E. (Diplocrinus) kexuei sp. nov., the cryptosymplexies present no marked symmorphy with facets mainly covered by synostosial stereom, while in E. (Diplocrinus) alternicirrus , the interpetaloid zone of cryptosymplexies with syzygial stereom predominating but with strong symmorphy in inner portion. In E. (Diplocrinus) wyvillethomsoni and E. (Endoxocrinus) parrae , cryptosymplexies are flat or with slight symmorphy with syzygial stereom predominating on interpetaloid zones. In E. (Diplocrinus) maclearanus , the cryptosymplexies with undulating symmorphic surface, synostosial stereom predominating on interpetaloid zones. The difference in the characteristics of stalk cryptosymplexy supports our classification at the species level rather than at the subspecies level, in accordance with Roux (1977) and David et al. (2006). The axial canal structures show distinct differences among Endoxocrinus species. In E. (Diplocrinus) kexuei sp. nov., the axial canals are incompletely filled with long spicules, separated from the perilumen and preserve an irregular secondary lumen, while in E. (Diplocrinus) alternicirrus , the axial canals are completely filled with a dense mesh of long thin spicules without secondary lumen (Fig. 4 f View Figure 4 ) ( Roux 1977, David et al. 2006). In E. (Diplocrinus) maclearanus , E. (Endoxocrinus) parrae and E. (Diplocrinus) wyvillethomsoni , the axial canal is filled with short thick spicules, large meshed stereom, and irregular network, respectively. In E. (Diplocrinus) maclearanus and E. (Endoxocrinus) parrae , the secondary lumen is small or absent. Endoxocrinus (Endoxocrinus) parrae has a small to large secondary lumen.