Ngan mayla, Guinot & Rodríguez Moreno, 2024

Guinot, Danièle & Rodríguez Moreno, Paula A., 2024, Ngan mayla gen. et sp. nov, a new blind potamid cave crab from Borneo, Kalimantan (Crustacea: Decapoda: Brachyura: Potamoidea, Potamidae), and three other unidentified cave crabs from the same region, Zootaxa 5476 (1), pp. 115-137 : 119-124

publication ID	https://doi.org/ 10.11646/zootaxa.5476.1.13
publication LSID	lsid:zoobank.org:pub:9959FB71-BE4C-42E1-AE73-710A58C4168D
DOI	https://doi.org/10.5281/zenodo.12731235
persistent identifier	https://treatment.plazi.org/id/7CBCFCE4-84D6-40E7-AA89-89F4784D672B
taxon LSID	lsid:zoobank.org:act:7CBCFCE4-84D6-40E7-AA89-89F4784D672B
treatment provided by	Plazi
scientific name	Ngan mayla
status	sp. nov.

Treatment

Ngan mayla sp. nov.

urn:lsid:zoobank.org:act:7CBCFCE4-84D6-40E7-AA89-89F4784D672B

( Figs. 2 View FIGURE 2 , 3 View FIGURE 3 , 6A View FIGURE 6 , 9A View FIGURE 9 )

Type material. Holotype: female, 12.2 × 15.3 mm, Indonesia, Kalimantan (East Kalimantan), Borneo, Panaan, Lubang Gedung Cave , 1°38’1”N – 117°24’14”E, 850 m, 18 August 2010, MZB Cru 5710. GoogleMaps

Description of holotype. Carapace subquadrate in outline, anteriorly rounded, flat, widest in anterior quarter; anterior part curved. Front rather flat, entirely visible in dorsal view, with straight margin although barely concave in the middle, smooth, slightly carinate. Surface of entire carapace appearing smooth; epigastric ridges and postorbital ridges distinct; H-shaped groove in centre of carapace indistinct. Anterolateral margin short, not bidentate or incised, about half as long as posterolateral margin, reaching anteriorly to orbital margin and merging with it, both with granules giving a serrated aspect. Antennule with large basal article. Antenna reduced, very small, with extremely short flagellum, filling entire inner gap of orbit.

Orbit large. Eyes strongly reduced, not mobile, eyestalk with few short setae and filling less than half of orbit, bullet-shaped, and with only a tiny trace of light brown pigment, so that the species is blind. Frontal triangle very small, as enlarged proepistome. Epistome with straight upper margin; its lower margin ending in small median triangle, with rather sharp apex; on each side, lower margin with two concavities, one for reception of palp of third maxillipeds, the other forming anterior margin of efferent opening. Efferent opening large and occupying outer quarter of anterior margin of buccal field.

Mandibular palp three-articulated; last article simple, large, pallet-shaped, with short setae.

Third maxilliped with well-developed exopod, without flagellum.

Chelipeds equal, remarkably spiny: merus with ventral surface bordered on each margin by acute short teeth (some with corneous tip) and spines, some surounded by short simple setae; carpus inner angle with very long acute tooth bearing several smaller acessory spines, simple setae and spinelike setae; palm with sharp spines on both superior and inferior borders; fingers closing over full length and ending in sharp curved tips, with small spines and short simple setae; superior border of dactylus with some spaced spines; extensor margins of fixed finger and dactylus with corneous teeth and short stiff setae.

Ambulatory legs extremely long and slender; P4 and P5 longest. Merus with distinctly serrated aspect on superior margin, inferior side more finely, corresponding to short acute teeth mixed with simple setae, especially on P2, P3; carpus mostly toothed on inferior side; dactylus very elongated, with numerous longer simple setae on both superior and inferior margins.

Thoracic sternum anteriorly covered with small dense setae. Sternal sutures 4/5 to 6/7 interrupted. Median line at level of sternites 7 and 8. Vulvae as two large, rounded openings, close to suture 5/6 and occupying about two-thirds length of sternite 6.

Colouration. Pale on whole carapace and legs.

Etymology. Dedicated to Mayla Soubzmaigne, the young daughter of Sébastien Soubzmaigne who made all the photographs of cavernicolous crabs collected by Josiane Lips and French team in various caves of the world. Mayla, as her father, likes Nature and crabs. Used as a noun in apposition.

Remarks. In Ngan mayla gen. et sp. nov. from Lubang Gedung Cave, the pale colouration of the body, the long and slender legs ( Fig. 2 View FIGURE 2 ), the nearly immobile and greatly reduced, tapering eyes ( Figs. 3A, C, D, F View FIGURE 3 , 9A View FIGURE 9 ), the length of which is less than half width of orbit, with a short bullet-shaped stalk and a vestigial unfaceted cornea, devoid of integument pigment except for a trace of pale brown pigment at tip, all are indicative of adaptation to a cavernicolous life making it a true troglobite. It is not known in which part of the cave Ngan mayla gen. et sp. nov. was observed and collected.

Ngan mayla gen. et sp.nov. is immediately distinguished from all the other potamids by its multi-spined chelipeds, which additionally bear short simple setae and spinelike setae ( Figs. 2 View FIGURE 2 , 3A–E View FIGURE 3 ). The troglobitic characteristics of Ngan mayla gen. et sp. nov. are similar to those of the type species of Cerberusa , C. caeca .

Cerberusa caeca , with all type material deposited at RMNH ( Fransen et al. 1997: 138), was recorded in 1978 in the Gunung Mulu National Park, situated in the extreme northern part of Borneo, Sarawak, at about 4°N – 115°E, covering more than 500 km 2 and comprising many caves. It was collected in three caves: Green Cave (small stream at 600 m from north entrance), with the holotype (RMNH.CRUS.D.31983) and two paratypes (RMNH.CRUS. D.31984); Deer Cave, known to contain the largest genuine cave passage in the world (Crab Inlet pool, small pools with gravel and guano on bottom), with a paratype (RMNH.CRUS.D.31965)); and Clearwater Cave, with the volume of Clearwater system calculated at 49 million cubic metres and the average passage diameter being 16 m along the entire 255 km length (Inflation Passage, gravel floored pool in dribbly stream), with two paratypes (RMNH.CRUS. D.31959) ( Holthuis 1979: 16, fig. 3, pls. 2, 8; 1986: fig. 15; Guinot 1988: fig. 7, pl. 2, figs. 5, 6; 1990: fig. 6B; 1994: fig. 2C, pl. 1, figs. 4, 5; see Grinang 2016: fig. 7). McFarlane et al. (2011: 12, fig. 2) found it in a pool of Lagang ’ s Cave, connected to the Deer Cave. C. caeca was again collected by K. Lim et al. in 2006 from another cave of the Gunung Mulu National Park, the Stone Horse Cave, near the Deer Cave, and both specimens are deposited at ZRC (ZRC 2013.1220). Cerberusa caeca prefers to dwell in water. It is blind and long-legged; its carapace is pale yellowish or white ( Fig. 4 View FIGURE 4 ), thus it is fully adapted to living exclusively in the complete dark part of the caves.

Cerberusa caeca is at least present in three massifs of Gunung Mulu National Park: Gunung Api, Southern Hills and Gunung Benerat, and is therefore a very widespread troglobitic species. Due to its extreme adaptations to a troglobitic life, it presumably has a long evolutionary history and probably diverged from its epigean ancestor before the separation of the Gunung Mulu Uplands into distinct massifs, whereas C. tipula , with fewer troglobitic adaptations (the eyes and the colouration), can be hypothesised to have arisen more recently ( McFarlane et al. 2011).

The second species of Cerberusa , C. tipula ( Fig. 5 View FIGURE 5 ), was originally found in two caves in the Gunung Api massif of Gunung Mulu National Park: Clearwater Cave (15 m inside northwest entrance, on stone outside water), with the holotype (RMNH.CRUS.D.31968); and Wonder Cave (10 m from entrance, pool with bottom sediment of guano), with a Paratype (RMNH.CRUS.D.31969) ( Holthuis 1979: 13, fig. 2, pl. 1, pl. 3, fig. 1; Guinot 1988: fig. 6A, B, pl. 2, figs. 1 – 4; 1994: fig. 2B; see Grinang 2016: fig. 8); then in a third cave, the Cave of Winds ( Holthuis 1986: 613); and, later, in an additional cave located within the Southern Hills, the Fruit Bat Cave (limestone pools 150 m from the west entrance southwest massif) ( McFarlane et al. 2011: 12, figs. 2, 3). As C. caeca , C. tipula was again collected in another cave of Gunung Mulu National Park, the Stone Horse Cave, near the Deer Cave, by K. Lim et al. in 2006, with two female specimens (ZRC 2013.1219). No epigean records are known. Cerberusa tipula can be considered a true troglobite despite its cylindrical eyes still retaining distinct pigmentation and carapace colouration.

Cerberusa tipula and C. caeca cohabit in three caves: Clearwater Cave, Deer Cave and Stone Horse Cave. C. tipula is an amphibious species, with a colourful (blue grey) dorsal surface of carapace and depigmented or lightly coloured (pale lemon-yellow or orange-yellow) legs, with practically unmodified eyes and a distinct rounded cornea containing dark pigment ( Fig. 9C View FIGURE 9 ). Living specimens of C. tipula from Fruit Bat Cave and from Agong Cave in Gunung Mulu National Park ( McFarlane et al. 2011: fig. 3 and Grinang 2016: fig. 7, respectively) show spectacular colouration: carapace grey, chelipeds and legs pale yellow. Guinot (1988: fig. 6A, B) observed that, in the Cave of Winds, a male C. tipula 12.5 × 16 mm had the eyestalk somewhat thinned distally and filling a good part of the orbit, whereas in a female of 16 × 21.8 mm it was less developed, barely thinned distally and occupying a smaller part of the orbit, and with a small, pigmented cornea ( Fig. 9C View FIGURE 9 , male and female).

In the 1980s, the first author had the opportunity to examine the type material of the two species of Cerberusa deposited at RMNH and published photographs of all of them together with sketches of the degenerated eyes ( Guinot 1988, 1994). These illustrations are all deposited in the MNHN, but, as the photographs are silver, they are not reproduced in the present paper. Therefore, photographs of females specimens of C. caeca ( Figs. 4 View FIGURE 4 , 6B View FIGURE 6 , 9B View FIGURE 9 ) and C. tipula ( Figs. 5 View FIGURE 5 , 6C View FIGURE 6 , 9C View FIGURE 9 ) collected by K. Lim et al. in 2006 in Stone Horse Cave (ZRC 2013.1220 and ZRC 2013.1219, respectively), near the Deer Cave where the type series of C. caeca was collected ( Holthuis 1979) and kindly made and sent by P.K.L. Ng, are published here.

The genera Ngan gen. nov. and Cerberusa share a similar habitus: carapace widest anteriorly; short, arcuate and carinate antero-lateral margin granulated to gently serrated, without teeth or epibranchial incision, the carina curving onto the carapace dorsal surface; front flat and rather straight; and posterolateral margins long, with tranverse ridges. Both are long-legged species. The pigmentation varies from white or pale yellow in Ngan mayla gen. et sp. nov. and C. caeca , which, with their completely degenerated eyes, would be the most troglomorphic potamids known, to a mixed colouration (as is often the case in cave animals) in C. tipula , which has slightly reduced eyes, but the longest legs.

Ngan mayla gen. et sp. nov. differs from C. caeca mainly by the numerous spines and simple setae on the chelipeds ( Fig. 2 View FIGURE 2 , 3A–E View FIGURE 3 ) (chelipeds unarmed, without spinelike setae in C. caeca : Fig. 4A–C, E View FIGURE 4 , as well as in C. tipula : Fig. 5 View FIGURE 5 ); by the more elongated, thinner legs ( Fig. 2 View FIGURE 2 ), in particular, the carpus, propodus and dactylus being longer and slender; by the P5 being as long as P4 (P5 shorter than P 4 in C. caeca : Fig. 4A, B, E View FIGURE 4 ); by certain margins of legs being finely serrated and with erect stiff, short setae (smoother, only serrated on inferior margin of propodi in C. caeca ). The legs of C. tipula are very long and thin ( Fig. 5A View FIGURE 5 ), even longer and thinner than those of Ngan mayla gen. et sp. nov. ( Fig. 2 View FIGURE 2 ) (except that P5 is proportionally shorter in C. tipula ), and also longer than those of C. caeca , which, of the three, is the species with the proportionally shorter legs ( Fig. 4A View FIGURE 4 ). The serrated margins and numerous stiff short erect bristles of the legs of Ngan mayla gen. et sp. nov. ( Fig. 2 View FIGURE 2 ) are somewhat similar to those of C. tipula ( Fig. 5A View FIGURE 5 ), whereas the legs are smoother in C. caeca ( Fig. 4A View FIGURE 4 ), with only the posterior margin of the propodus bearing spinules. The female thoracic sternum, covered with dense setae in Ngan mayla gen. et sp. nov. ( Figs. 3E View FIGURE 3 , 6A View FIGURE 6 ), is bare in C. caeca ( Figs. 4E View FIGURE 4 , 6B View FIGURE 6 ) and C. tipula ( Figs. 5E View FIGURE 5 , 6C View FIGURE 6 ). The efferent openings are enormous in C. tipula ( Fig. 5B View FIGURE 5 ), normal in C. caeca ( Fig. 4B, D View FIGURE 4 ) and Ngan mayla gen. et sp. nov. ( Fig. 3A, F View FIGURE 3 ).

The Lubang Gedung Cave hosting Ngan myla gen. et sp. nov. is around 370 km southeast of the Gunung Mulu National Park, home to both species of Cerberusa . The ranges of the two genera belong to two different hydrographic drainage systems, which implies that they are well separated geographically from each other.

Many cave species occur in Indonesia ( Leclerc et al. 2001), and several freshwater crabs are known from the Malaysian State of Sarawak: two troglobites ( Cerberusa caeca and C. tipula ) in Gunung Mulu; now, another troglobite, Ngan mayla gen. et sp. nov., from Merabu Karst. The potamid Thelphusula styx Ng, 1989 , from Long Pala Bat Cave, western corner of Gunung Mulu National Park ( Ng 1989), with a pale brownish carapace, dirtycream coloured walking legs, and small but relatively well-developed eyes, with distinct black pigmentation of the cornea, could be a troglobitic species ( Promdam et al. 2022: table 2). In contrast, Isolapotamon collinsi Holthuis, 1979 , collected from the sinkhole of the Clearwater Cave River, Hidden Valley, Gunung Mulu National Park, does not show obvious adaptations to hypogean life ( Holthuis 1979: 25; McFarlane et al. 2011: 12). The same is true for Sundathelphusa tenebrosa Holthuis, 1979 , known from several cave records in Gunung Mulu National Park and with an epigean record ( Holthuis 1979: 42; McFarlane et al. 2011: 12, fig. 4).