Ngan, 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 : 117-118

Ngan gen. nov.

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Type species: Ngan mayla sp. nov. (present designation).

Diagnosis. Carapace widest anteriorly, narrowing posteriorly, flat; each postorbital and epigastric ridge consist of 1 row of granules, each ridge separated from each other, epigastric ridge transverse, placed mesioposterior to postorbital ridge, postorbital ridge short, oblique, well separated from anterolateral margin; anterolateral margin long, without teeth or epibranchial incision, forming a finely granular, even finely serrated carina, joining orbital margin anteriorly, this carina posteriorly curving onto dorsal surface of carapace; posterolateral margin longer, straight, with transverse curved ridges formed by rows of granules; front flat but slightly concave medially, directed somewhat down, with very small frontal triangle (as enlarged proepistome); cervical groove not visible in median area; orbits large, orbital margin entire. Eyes strongly reduced, filling less than half the orbit, bullet-shaped, with only tiny pale brown pigment. Antenna very reduced. Mandibular palp three-articulated; last article simple, not bifurcated, oval. Third maxilipeds filling most of oral cavity; exopod without flagellum. Epistome with straight anterior margin; its concave posterior margin ending medially in triangular apex and, at either side of this triangle, two concavities separated by sharp point, one small (for reception of the palp of third maxilliped), the other rounded (forming anterior margin of efferent openings). Efferent openings very large, circular, reaching lateral margin of oral field. Chelipeds remarkably spiny, with numerous acute teeth and spines (some horn-tipped), and short simple setae or spine-like setae on all articles, may be in groups of two or three, some teeth bearing short setae at tips; fingers closing over full length and ending in sharp curved tips, and bearing short horny spines and simple setae. Pereiopods extremely long and thin, slender with short teeth and simple setae; dactyli with numerous longer simple setae on both sides. Vulvae on thoracic sternite 6, at margin with sternite 5, with wide sternal cover, and occupying two-thirds of width of sternite 6.

Etymology. The genus is dedicated to the memory of Ngan Kee NG (1966 – 2022). She leaves a void in our scientific community, where she had a special place. She was more than a colleague, a very dear friend. She liked to stay in our Muséum, in Paris, and among her last works are those done here in winter over the last decade, notably the Annotated type-catalogue of Brachyura of the Muséum national d’Histoire naturelle, Paris, the first one dealing with Gecarcinidae and Grapsidae (N.K. Ng et al. 2019) , then Plagusiidae and Percnidae , but the last one, on the Varunidae , her favourite group, has not been completed. Gender: feminine.

Remarks. The shape of the three-articulated mandibular palp ending by a single terminal lobe ( Fig. 3F ) ( Bott 1970: fig. 133; Ng 1988: fig. 2A; Ng 2004: fig. 5I, J) indicates that Ngan gen. nov. belongs to the Potamidae . The closest genus is Cerberusa Holthuis, 1979 that consists of two species, both from the Gunung Mulu National Park, north Sarawak, the Malaysian part of Borneo and close to the southern border of Brunei: C. caeca Holthuis, 1979 and C. tipula Holthuis, 1979 . The area explored by Lips’ team ( Lips et al. 2010) in East Kalimantan, where the Lubang Gedung Cave ( Fig. 1B ) is located, is about 370 km southeast of the Gunung Mulu National Park. Like Cerberusa , Ngan gen. nov. has the exopod of the third maxillipeds without a flagellum. Both species of Cerberusa have a similar arrangement of the anterior part of the carapace, a similar front and probably a similar proepistome, a similar ornamentation of the carinate and non-incised anterolateral margin, and the posterolateral portion of carapace with several striae; the rounded vulvae are similar ( Fig. 6A ).

Differences between Ngan gen. nov. and Cerberusa concern the shape of the carapace, subquadrate in Ngan gen. nov. (trapezoidal in Cerberusa ), the postorbital and epigastric ridges marked in Ngan gen. nov. (inconspicuous, even absent or weak in Cerberusa ), the chelipeds with numerous spines and setae in Ngan gen. nov. (practically absent in Cerberusa ). In Ngan mayla sp. nov. the presence on the chelipeds of horny spines accompanied by setae and of simple setae without setules (spinelike setae), which are usually chemoreceptive ( Watling 1989; Garm & Watling 2013), is probably the sign of a high degree of troglomorphism (see Discussion).

Ibanum Ng, 1995 , an unmodified potamid genus from Borneo, not cavernicolous but probably semiterrestrial and known by two species, Ibanum aethes Ng, 1995 ( Ng 1995: figs. 4 – 6), the type species from Lanjak-Entimau Wildlife Sanctuary, Sarawak, and I. bicristatum (De Man, 1899) ( Ng 1995: figs. 1 – 3), from Kapuas, Kalimantan, is close phylogenetically to Ngan gen. nov. and Cerberusa (P.K.L. Ng, pers. comm.). The three genera share many similarities: carapace widest anteriorly; antero-lateral margin short, arcuate, carinate, granulated to gently serrated, without teeth or epibranchial incision, with the carina curving onto the carapace dorsal surface; front flat and rather straight; and posterolateral margins long, with transverse ridges. Such as Ngan gen. nov., Ibanum has well defined postorbital cristae. Ibanum distinguishes from both Ngan gen. nov. and Cerberusa by the presence of a flagellum on the exopod of the third maxilliped (P.K.L. Ng, pers. comm.). The vulvae of Ibanum ( Fig. 6D ) are smaller, oval, distinctly obliquely positioned on sternite 6, instead of rounded and central in Ngan mayla sp. nov. ( Fig. 6A ), Cerberusa caeca ( Fig. 6B ) and C. tipula ( Fig. 6C ).

Figures

FIGURE 1. A, Borneo Island and Kalimantan; B, Karst of Merabu, East Kalimantan, northernmost limestone massif of the Mangkalihat Peninsula; C, D, Lubang Gedung Cave, two views. Courtesy, J. and B. Lips.

FIGURE 3. Ngan mayla gen. et sp. nov., holotype female, 12.2 × 15.3 mm, MZB Cru 5710: A, frontal view; B, left cheliped; C, D, carapace and chelipeds; E, ventral view; F, ventral anterior view. Scale bars: A–E = 5 mm; F = 1 mm.

FIGURE 6. Vulvae: A, Ngan mayla gen. et sp. nov., holotype female, 12.2 × 15.3 mm, MZB Cru 5710; B, Cerberusa caeca Holthuis, 1979, female, 9.9 × 13.3 mm, ZRC 2013.1220; C, C. tipula Holthuis, 1979, female, 13.0 × 17.6 mm, ZRC 2013.1219; D, Ibanum aethes Ng, 1995, female, 10.8 × 14.7 mm, ZRC 2019.0141. B–D, courtesy P.K.L. Ng.