Montinumen cryptovium, Zhao & Huang & Ma, 2025

Zhao, Jun-Da, Huang, Chao & Ma, Kayan, 2025, A new genus and new species, Montinumen cryptovium gen. nov. et sp. nov. (Crustacea: Decapoda: Potamidae), the first cave-dwelling freshwater crab in Yunnan, Southern China, Zootaxa 5717 (1), pp. 115-126 : 119-124

publication ID	https://doi.org/10.11646/zootaxa.5717.1.7
publication LSID	lsid:zoobank.org:pub:2DA30496-6B59-451C-9845-F5238C35AE21
DOI	https://doi.org/10.5281/zenodo.17890158
persistent identifier	https://treatment.plazi.org/id/2B53D526-FF84-FFB0-8FB1-FF48E3EABCF2
treatment provided by	Plazi
scientific name	Montinumen cryptovium
status	gen. nov. et sp. nov.

Treatment

Montinumen cryptovium gen. nov. et sp. nov.

[Chinese name: NJỄƜṘDz]

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

Type material. Holotype: SYSBM002156 , male (28.6 × 19.8 mm), Maguan county , Wenshan Zhuang and Miao Autonomous Prefecture, Yunnan Province, China, shallow pool in unnamed cave, 22°43’N 103°59’E, 500–550 m a.s.l., coll. Jun-Da Zhao, July 2022 GoogleMaps .— Paratype: SYSBM002157 , female (32.4 × 21.9 mm), Maguan county , Wenshan Zhuang and Miao Autonomous Prefecture, Yunnan Province, China, shallow water pool in unnamed cave, 22°43’N 103°59’E, 500–550 m a.s.l., coll. Xiang-Jin Liu, Aug 2022 GoogleMaps .

Description. Carapace broader than long; subtrapezoidal, width 1.4–1.5 times length (n = 2), width of front half remarkably boarder than back half; regions indistinct; dorsal surface slightly convex, covered with small granules ( Fig. 1 View FIGURE 1 ). Frontal margin slightly ridged, almost forming a straight line with anterolateral margin in dorsal view ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Epigastric cristae low, blunt, fused with postorbital cristae, divided by a shallow groove ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Postorbital cristae slightly raised ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Branchial regions relatively flat; cervical groove shallow; H-groove visible ( Fig. 1 View FIGURE 1 ). Cornea relatively reduced, eye stalk short ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). External orbital angle relatively reduced, very low, fused with external orbital margins, separated from anterolateral margins by inconspicuous gaps ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Epibranchial tooth blunt, very small, exorbital angle lined by one fused granule ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Anterolateral margin slightly ridged, lined with numerous fused granules ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Posterolateral margin straight, slightly rugose ( Fig. 1 View FIGURE 1 ). Orbits small; supraorbital and infraorbital margin smooth, ridged ( Figs. 1 View FIGURE 1 , 2A View FIGURE 2 ). Sub-orbital, subhepatic and pterygostomial regions divided by sutures; surfaces covered with small, rounded granules, ( Fig. 2A View FIGURE 2 ). Epistome median lobe narrowly triangular ( Fig. 2A View FIGURE 2 ).

Maxilliped III merus width about 1.1 times length; ischium width about 0.7 times length; merus subrectangular; ischium trapezoidal, with median sulcus; exopod reaching to approximal one-third of merus height, with long flagellum, length slightly longer than merus ( Fig. 3A View FIGURE 3 ).

Chelipeds unequal, surface slightly rugose, covered with small, rounded granules ( Fig. 3D, E View FIGURE 3 ). Merus cross section triagonal, inner margins crenulated, surface rugose, covered with small, rounded granules ( Fig. 1 View FIGURE 1 ). Carpus inner distal angle with sharp spine, surface rugose ( Fig.1 View FIGURE 1 ). Major cheliped palm length about 1.7 times height of male and female; dactylus 0.8 times palm length of male and female ( Fig. 3D, E View FIGURE 3 ). Palm surface rugose, covered with small, rounded granules; inner margin of fingers lined with large and small blunt and sharp teeth, with small gap when closed ( Fig. 3D, E View FIGURE 3 ).

Pereiopods II–V (first to fourth ambulatory legs) very slender, surface rugose. Pereiopods III the longest; Pereiopods V propodus length 4.6 times as long as broad in male, and 4.9 times as long as broad in female; dactylus slender, with small, sharp spines on margins ( Fig. 1 View FIGURE 1 ). Male thoracic sternum glabrous, pitted; sternites I–IV relatively wide, width 1.8 times length. Sternites I, II fused, forming a wide triangular shape; sternites II, III fused, separated by a horizontal sulcus; sternites III, IV fused, with indistinct sulcus ( Fig. 2B View FIGURE 2 ). Male sternopleonal cavity deep, long, reaching anteriorly to imaginary line joining medial part of cheliped coxae ( Fig. 2B–D View FIGURE 2 ); median longitudinal suture between sternites VII and VIII deep ( Fig. 2D View FIGURE 2 ). Male pleonal locking tubercle positioned anterior to sternite suture V/VI but not exceeding mid-length of sternite V ( Fig. 2D View FIGURE 2 ). Female vulva subovate, very large, positioned close to each other, located within sternite VI, upper margin runs along suture between sternites V/VI, reaching to suture between sternites IV/V.

Pleon and telson narrowly triangular in males ( Fig. 2C View FIGURE 2 ), broadly ovate in females ( Fig. 2E View FIGURE 2 ). Male pleonites III–VI gradually narrower anteriorly, lateral margins almost straight; pleonite VI 2.3 times as broad as long; telson 1.4 times as broad as long, with blunt apex ( Fig. 2C View FIGURE 2 ).

G1 generally slender, reaching beyond the pleonal locking tubercle, almost to sternites suture IV/V in situ ( Fig. 2D View FIGURE 2 ). Subterminal segment 2.6 times × as long as terminal segment, twisted towards middle, inner margin slightly concave. Terminal segment elongated, distal end pointing anterolaterally, with large, semicircular basal-dorsal flap on proximal third ( Fig. 3C, F, G View FIGURE 3 ). G2 subterminal segment 1.6 times as long as the flagellum-like terminal segment ( Fig. 3C View FIGURE 3 ).

Etymology. The species name is an arbitrary combination of the Latin words crypt and via, meaning “hidden” and “pathway”, alluding to its elusive nature and the difficulty in locating its habitat.

Distribution. Unnamed cave in Gulinqing town, Maguan County, Wenshan Zhuang and Miao Autonomous Prefecture, Yunnan Province, China.

Color in life. Overall light pinkish to purple, abdomen ivory yellow ( Fig. 5B View FIGURE 5 ).

Habitat. Presently, the habitat and ecology of Montinumen cryptovium gen. nov. et sp. nov. remains largely unknown. The only two specimens, collected a month apart, were the sole crabs present at their respective times of collection and were discovered approximately 200 meters from the entrance of an unnamed cave, within a small, isolated water pool in the dark zone. This pool, the sole water body within the cave, measures no more than 5 meters in diameter and 10 cm in depth. No visually obvious connections to other water bodies or cave cavities were observed within the entire cave, and no other aquatic life or evidence of such was found within the pool. These observations suggest that the primary habitat of Montinumen cryptovium gen. nov. et sp. nov. likely extends beyond the immediate pool area.

Phylogenetic analysis and discussion. For the phylogenetic analyses, the BI and ML methods produced similar topologies ( Fig. 4 View FIGURE 4 ). As expected, the basal relationships in this tree could not be resolved using the 16S alone ( Zhao et al. 2022), but for the purpose of this study, the results show that Montinumen gen. nov. is phylogenetically distinct and sister to Barbamon , with the K2P genetic distance (5.8%) between them comparable to those between other sister potamid genera (e.g., Diyutamon and Chinapotamon : 5.17%, Mediapotamon and Tenuilapotamon : 3.04%), and thus supporting the proposal of a new genus. The two form a clade that is likely related to the “ China clade” ( Shih et al. 2009), Parapotamon De Man, 1907 and Tortomon Huang, Wang & Shih 2020 , but the real relationships between them will have to be recovered using additional markers and is outside the scope of this study. In addition, the results show that the new genus is not closely related to other cave-adapted and cave-associated potamid genera in China, ie., Phasmon , Chinapotamon , Diyutamon , Calcipotamon and Tiwaripotamon , all of which belong to the “China-East Asia Islands” clade ( Huang et al. 2017; Huang et al. 2020b; Huang et al. 2020c).

The new species exhibits some possibly stygomorphic traits, such as elongated legs and reduced eye size, though pigmentation and the cornea appear normal. The new species’ only known occurrence in a karst cave and elongated legs suggest that it is at least obligatorily associated with the karst environment. According to the local who first discovered this new species, it was the first time he had seen this crab in the decades he has been visiting the mountains in this area. Our further examination of the epigeal environment surrounding the cave yielded no additional specimens or traces of Montinumen cryptovium gen. nov. et sp. nov. It would seem that the main population inhabits the more secluded areas of the cave system that is connected to the observable water pool or located within nearby subterranean streams accessible through small fissures in the cave. However, this hypothesis would not be able to explain why the species still possesses fully functional eyes and pigmentation as these characters often regress relatively quickly in response to full immersion in the cave environment ( Klaus et al. 2013). Montinumen cryptovium gen. nov. et sp. nov. does not seem to be a recent incursion from what we currently know, as taxa that have recent incursed usually have extent epigean counterparts, like Chinapotamon clarkei and some cave Sundathelphusa species ( Ng 2017; Klaus et al. 2013). In conclusion, Montinumen cryptovium gen. nov. et sp. nov. is an incredibly rare and mysterious species that represents the first known cave-associated crab from Yunnan Province, south China.