Tayninhon nuibaden, Dang & Tien & Tu, 2025

Tayninhon nuibaden View in CoL gen. nov., sp. nov.

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( Figures 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Material examined. Holotype: male (29.2 × 25.0 mm) (IB-FC-TBx01) Vietnam, Tay Ninh Province, Tay Ninh City, Thanh Tan Commune, Ba Den Mountain , N 11°23’01.7” E 106°09’44.7”, coll. Phan DD & Tran VT, August 2020 GoogleMaps . Paratypes: 1 male (20.5 × 15.2 mm), 2 females (26.5 × 20.5 mm, 15.2 × 11.8 mm) (IB-FC-TBx02), same data as holotype. Others GoogleMaps : 1 male (23.0 × 18.2 mm) ( VNM00070944 ) , 1 male (13.7 × 11.1 mm) ( VNM00070945 ) , 1 male (18.6 × 14.5 mm) (VRTC-SB-AE10011), 1 male (14.8 × 11.9 mm) (VRTC-SB-AE10012), 1 female (15.5 × 12.7 mm) ( VNM00070946 ) , 1 female (15.5 × 12.7 mm) ( VNM00070947 ) , 1 female (16.2 × 12.7 mm) (VRTC-SB-AE10013), 1 female (15.9 × 12.7 mm) (VRTC-SB-AE10014), Vietnam, Tay Ninh Province, Tay Ninh City, Thanh Tan Commune, Ba Den Mountain , N 11°23’00.5” E 106°10’52.4”, coll. Phan DD & Tran VT, 30 January 2024 GoogleMaps .

Description. Carapace subquadrate, about 1.3 times broader than long; female slightly larger than male; dorsal surface relatively low, not inflated, regions well defined ( Fig. 1A, B View FIGURE 1 ). Frontal region almost smooth; lateral parts of anterolateral and branchial regions weakly rugose; mesogastric, urogastric, cardiac and intestinal regions very weakly rugose; orbital regions weakly rugose; suborbital and pterygostomial regions uneven, rugose covered by small granules ( Fig. 1A View FIGURE 1 ). Epigastric cristae low, not sharp, rugose, separated by broad, shallow Y-shaped furrow, separated from postorbital cristae by short, shallow groove; postorbital cristae low, not sharp, almost straight, confluent with epibranchial tooth, breaking up into rugae and granules before epibranchial tooth ( Fig. 1A View FIGURE 1 ). Cervical grooves distinct; H-shaped median gastric groove developed ( Fig. 1A View FIGURE 1 ). Frontal margin divided into two broad, medium lobes, separated by broad concavity; margin of each lobe gently convex, confluent with supraorbital margin ( Fig. 1A, C, D View FIGURE 1 ). External orbital angle broadly triangular, with outer margin longer than inner margin, demarcated from rest of anterolateral margin by small, shallow notch, lined with distinct granules; epibranchial tooth distinct, low, broadly triangular ( Fig. 1A, C, D View FIGURE 1 ). Anterolateral margins convex, cristate, lined with distinct granules ( Fig. 1A View FIGURE 1 ). Posterolateral margin almost straight, strongly convergent posteriorly ( Fig. 1A View FIGURE 1 ). Orbits subovate; eye filling most of orbital space; ocular peduncle long; cornea normal ( Fig. 1A, C, D View FIGURE 1 ). Supra- and infraorbital margins gently sinuous, lined with small but distinct granules ( Fig. 1C, D View FIGURE 1 ). Suborbital margin concave, complete, lined with small granules ( Fig. 1C, D View FIGURE 1 ). Posterior margin of epistome acutely triangular at median, each lateral margin with 2 concavities ( Fig. 1C, D View FIGURE 1 ).

Third maxilliped ischium subrectangular, about 1.6 times longer than wide, with distinct median oblique groove; merus subquadrate, about 0.7 times as long as wide, outer surface concave, anteroexternal angle nearly round, not expanded; exopod slender, with prominent anterointernal margin tooth, reaching beyond upper edge of ischium, to approximately two-fifth lengths of length of merus, flagellum distinct, shorter than width of merus ( Fig. 1E View FIGURE 1 ).

Cheliped slightly asymmetrical, relatively stout without significant difference between males and females ( Fig. 2A, B View FIGURE 2 ). Anterior margin of basis-ischium almost smooth; inner margins of merus granulated ( Fig. 2A, B View FIGURE 2 ). Outer surface of carpus rugose, inner distal angle with distinct sharp tooth. Outer surfaces of chelae rugose; chela palm in large males approximately 2.4 times as long as broad; major chela stouter, longer than minor chela ( Fig. 2A, B View FIGURE 2 ). Major chela with fingers stout, gently curved, shorter than palm, outer surface smooth; cutting edges of both fingers with different sized teeth and denticles, almost no gap when fingers closed, tips not crossing ( Fig. 2A, B View FIGURE 2 ). Minor chela with fingers slender, shorter than major chela, fingers tips relatively straight, almost no gap when closed ( Fig. 2A, B View FIGURE 2 ).

Ambulatory legs not elongated; third pair longest, first pair shortest ( Fig. 1A, B View FIGURE 1 ). Outer surface of merus slightly rugose, dorsal margin weakly serrated, length to width ratio of fourth merus 3.6; carpus slightly rugose, outer surface with submedian crista on first to third legs, absent on the fourth leg; propodus subrectangular, length to width ratio of fourth propodus 2.5; dactylus gently curved, margins with short, sharp corneous spines on both inner, outer and upper margins ( Fig. 1A, B View FIGURE 1 ).

Thoracic sternum relatively wide, surface smooth with lateral margins slightly setose ( Figs. 1B View FIGURE 1 , 2C View FIGURE 2 ). Sternites 1 and 2 completely fused to form broad triangular plate, lateral margin distinctly convex, separated from sternite 3 by complete suture; sternites 3 and 4 completely fused, without suture; sternite 8 not visible when pleon closed. Sternopleonal cavity reaching to imaginary line connecting anterior edges of cheliped coxae ( Fig. 2C View FIGURE 2 ). Male pleon narrowly triangular; telson triangular, relatively narrow, lateral margins concave, width to length ratio 1.2; proximal margin of telson slightly wider than the distal margin of somite 6; somite 6 subrectangular, width to length ratio 2.6 ( Fig. 2D View FIGURE 2 ). Male pleonal locking tubercle just below median part of sternite 5 ( Fig. 3A View FIGURE 3 ). Female pleon ovate, covering most of thoracic sternum. Vulvae ovate; low vulvar cover on extend oblique edge of vulva, occupied mostly in sternite 6 with the proximal margin appressed against the suture with sternite 5 ( Fig. 3B View FIGURE 3 ).

G1 slender, sinuous, reaching to median part of sternite 5, with terminal and subterminal articles clearly demarcated; subterminal article relatively broad, 2.9 times longer than broad, neck-like at distal part, 2.2 times as long as the terminal article; terminal article relatively long, hook shape, bent at about 10° along longitudinal axis, with fold extending at basal part of terminal article, distal part curved inwards, tip sharp, about 0.4 times length of subterminal article ( Figs. 3A View FIGURE 3 , 4A–D View FIGURE 4 ). G2 slightly shorter than G1, distal article short, about 0.74 times length of basal article ( Figs. 3A View FIGURE 3 , 4E, F View FIGURE 4 ).

Live colouration. The dorsal surface is golden brown. Ambulatory legs are orange and the colour is brighter on the dactyli ( Fig. 5 View FIGURE 5 ).

Etymology. The new species is named after the type locality, “Núi Bà Đen”. The name is used as a noun in apposition.

Habitat. These crabs have been observed in rock crevices and fissures within areas that have slow flowing water. Burrowing behaviour was not observed, and the crabs are also not arboreal. The species has been recorded at an elevation of approximately 400 m, and there have been no observations at higher elevations even though the mountain reaches 986 m. This could be due to the steep terrain in the upper part of the mountain, which causes water sources to become scarce during the dry season, while in the rainy season, the water flow becomes too strong, making the upper terrain habitat unsuitable for this crab species. The species was observed both during the day and at night, with more individuals recorded at night, possibly due to the nocturnal activity patterns typical of freshwater crabs. The best time for observation and specimen collection is at the beginning of the rainy season, around August, when environmental conditions may be more favourable for their appearance. This area is characterised by large piles of rocks and is completely devoid of soil. The local flora consists of vines and rock- dwelling plants with secondary roots ( Fig. 6A–D View FIGURE 6 ).

Remarks. Based on field surveys conducted in January 2024, this new genus and new species appears to be restricted to a single locality with ongoing habitat degradation. Ba Den Mountain may be the only remaining habitat for this species. This mountain is also home to some endemic species found only in this region. Recently, scientists have described a new species of gecko, Cyrtodactylus thuongae Phung, Van Schingen, Ziegler & Nguyen, 2014 , and lizard, Scincella badenensis Nguyen, Nguyen, Nguyen & Murphy, 2019 , from this area, highlighting its conservation importance ( Nguyen et al. 2019; Phung et al. 2019). Ba Den Mountain harbours not only biological uniqueness but also holds historical and cultural significance in southern Vietnam. The blooming of tourist activities following the construction of infrastructure on the mountain’s summit, intensive agriculture in the surrounding foothills and climate change may threaten the survival of this species.

Based on the criterial by IUCN Standards and Petitions Committee (2024), this species can be assessed at least as Vulnerable (VU) under IUCN Criterion B—Geographic Range, specifically: (1) B1. Extent of Occurrence (EOO)—the area of Bà Đen Mountain is approximately 24 km ², which is well below the threshold of 100,000 km ²; (2) B2ab—the population is severely fragmented or known from only a single location; and there is a continuing decline, observed or inferred, in: (i) extent of occurrence; (ii) area of occupancy; (iii) area, extent and/or quality of habitat ( Cumberlidge et al. 2009; IUCN Standards and Petitions Committee 2024). A targeted conservation program should be developed for this newly described species, incorporating habitat protection measures and further field surveys to assess its population status and ecological requirements. Such efforts will also benefit the broader biodiversity of this unique montane ecosystem in southeastern Vietnam.