Gordius wulingensis Zou & Liu 2025, sp. nov.

Gordius wulingensis Zou & Liu sp. nov. Type material. Holotype. • One male, China, Hunan Province, Wuling Mountains, Jiuzhaiping Cave (28°17'17"N, 109°38'56"E), ca. 380 m a. s. l., 19 November 2022, collected by Ya-Zhen Zou, Hunan Wuling Mountain Biological Science Museum (HWMBSM), voucher number: JSUJIU 01. Paratypes. • 1 ♂, same collection information as holotype, HWMBSM, voucher number: JSUJIU 02. 1 ♀ and 2 ♂♂: same locality and collectors as holotype; collected on 24 August 2023 (HWMBSM, voucher number: JSUJIU 03), 10 April 2024 (JSUJIU 04), and 25 April 2024 (JSUJIU 05). Further specimens. • 1 ♂ and 1 ♀: China, Hunan Province, Wuling Mountains, Xiaodong Cave (28°40'52"N, 109°56'52"E), ca. 320 m a. s. l., 2 July 2023, collected by Zhi-Xiao Liu, HWMBSM, voucher number: JSUXIAO 01, JSUXIAO 02. • 1 ♀: China, Hunan Province, Wuling Mountains, Baihu Cave (28°25'44"N, 109°44'31"E), ca. 250 m a. s. l., 23 July 2023, collected by Zhi-Xiao Liu, HWMBSM, voucher number: JSUBAI 01. • 1 ♀: China, Hunan Province, Wuling Mountains, Jinji Cave (28°46'52"N, 110°14'6"E), ca. 910 m a. s. l., collected by Shi Li and Yan Tang, 13 August 2024, HWMBSM, voucher number: JSUJINJI 01. • 1 ♀: China, Hunan Province, Wuling Mountains, Huangni Cave (28°48'57"N, 110°14'19"E), ca. 1080 m a. s. l., collected by Shi Li and Yan Tang, 14 August 2024, HWMBSM, voucher number: JSUHUANG 01. Etymology. The specific epithet, “ wulingensis ”, is derived from its habitat within the karstic caves in the Wuling Mountains. Description of adult males (n = 5) (Figs 1 – 3, 5, 7). Body length 231.6 ± 77.51 (98–285) mm, width (widest, after dehydration) 0.45 ± 0.13 (0.22–0.53) mm, creamy white, smooth, with rainbow-like reflection (Fig. 7 B, C). The anterior end is columnar and spherical, with a nearly transparent white region between the epidermis and internal structures, and it lacks a dark-brown collar (Fig. 1 A). Under SEM, the surface of the anterior end appears smooth (Fig. 1 B) or wrinkled (Fig. 1 C); short bristles (4.59 ± 1.89 (2–7.5) µm in length) are scattered except on the tip in most samples (Fig. 1 D). Several sharp ridges were seen on the dorsal and ventral sides (Figs 2 A; 7 A). SEM reveals that the epidermis of the mid-body region in the male is smooth and lacks bristles, while the ventral region exhibits several ridges (Fig. 2 B). The posterior end of males contains two tail lobes (Fig. 3 A, B, C, E); each lobe is 384 ± 59.6 (300–449) μm long and 164.6 ± 33.8 (125–200) μm wide; length-to-width ratio is 2.41 ± 0.56 (1.50–2.84). Concentrated bristles are present at the tips of the inner lobes and scattered along the posterior half of the caudal lobe (Fig. 3 B, C, D, E). The postcloacal crescent (Fig. 3 A, B, C, E) is 243.4 ± 30.9 (200–280) μm long and 27 ± 10.4 (20–45) μm wide, short and roughly arc-shaped, located on ventral side near base of tail lobes. In the posterior end, the cuticle around the cloacal opening and the postcloacal crescent is pigmented browner than the surrounding cuticle, indicating a stronger cuticularization (Fig. 3 A). The cloacal opening is circular, 17.2 ± 5.1 (12–23.8) μm in diameter and anterior to postcloacal crescent (Figs 3 A, B, C, E). The wall inside the cloacal opening exhibits honeycombed areoles (Fig. 3 F); no circumcloacal spines or bristles were observed in the region next to the cloacal opening. Description of adult females (n = 5) (Figs 2, 4, 5, 7). Body length 429.2 ± 228 (145–745) mm, width (widest, after dehydration) 0.65 ± 0.21 (0.32–0.83) mm, creamy white, smooth. Under SEM, the anterior and posterior ends are columnar, rounded at their tips, and do not exhibit scattered bristles (Fig. 4 A, C, E, H). The surface of the anterior end is smooth and may have concave infoldings, which probably are dehydration artefacts. Scattered short bristles, 4.70 ± 1.89 (2.81–7.35) µm in length, are present (Fig. 4 B, D). The surface of the anterior end appears smooth (Fig. 4 E) or wrinkled on the tip of one sample (Fig. 4 H), with scattered short bristles, which are 6.11 ± 0.42 (5.81–6.4) µm in length (Fig. 4 F, I). Notably, on the anterior end are scattered specialized spine-like bristles (indicated by the dashed box in Fig. 4 E) (Fig. 4 G). The head tapers, and the tail is rounded (Fig. 7 A). The cloacal opening is small and circular (Fig. 4 E, H). Under SEM, the cuticle in the mid-body is smooth with rare bristles (Fig. 5 C). In contrast, the density of bristles increases near the anterior and posterior ends of the body (Fig. 5 A, B). Overall, bristles become progressively less common towards the mid-body. Distribution, habitat. Based on our field collections and the historic specimens, Gordius wulingensis are mainly distributed in the karstic caves in the Wuling Mountain area of Hunan, China. This species can also be found in Yongshun County, Guzhang County, and Jishou City (Fig. 6). This species is usually observed in puddles, beneath stones, or sometime on the moist soil of the caves (Fig. 7 C). Occasionally, they are found on the cave walls at elevations exceeding 1 m above ground level (Fig. 7 D). Phylogeny. This analysis used 22 terminal taxa of the genus Gordius and three out-group species to build molecular phylogenetic trees (Table 3, Fig. 8). The average length of the COI gene was 668 bp, with a maximum of 692 bp and a minimum of 554 bp. The genetic distances between COI sequences of Gordius wulingensis specimens collected from different caves ranged from 0.000 to 0.012. Due to these minimal genetic distances, we concluded that they represent the same species. The mean interspecific genetic distances between Gordius wulingensis and other Gordius species or clades were in the range of 0.162 –0.264 (Table 3). No subgroup was detected because the polytomic topology exhibited low bootstrap values and short genetic distances. G. wulingensis clusters closest with G. paranensis and G. sp. 5 and 6. Diagnosis. All specimens exhibit a rainbow-like reflection on the skin, white body color, lacking of a dark-brown collar. Distinct longitudinal sharp ridges visible on the ventral and dorsal surfaces. Males exhibit concentrated bristles at the tips of the inner lobes and scattered bristles along the posterior half of the caudal lobe, extending downward to the same length. Honeycombed areoles present on the inner wall of the cloacal opening. Dense bristles in the anterior and posterior ends, decreasing in the mid-body. Immature worms found in Tachycines sp. (collected on 25 April 2024), species identity confirmed by COI gene.