Borneosa tenebrosa
Creators: name: Ng, Peter K L
name: Grinang, Jongkar
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Gecarcinucidae
Borneosa
Borneosa tenebrosa

description: Borneosa tenebrosa (Holthuis, 1979)
(Figs. 2, 3, 19A, 20A–L, 23A)
Sundathelphusa tenebrosa Holthuis, 1979: 39, pl. 7, text-fig. 8; Holthuis 1986: 594; Guinot 1988: 13; Guinot 1994: 171; Fransen et al. 1997: 136; Ng 2004: 329; Ng and Yeo 2007: 12; Grinang and Nyanti 2007b: 2 (part); Ng et al. 2008: 73; Cumberlidge et al. 2009: unpaginated appendix; Klaus et al. 2009: 514; McFarlane et al. 2011: 12, figs. 2 (unnumbered), 4; Ng 2015: 494, figs. 3, 7F–H.
© 2022 Academia Sinica, Taiwan
Material examined: Holotype male (23.5 × 19.1 mm) (NHM 1979.135), Camp 2, in 2 × 2 m pool, 10 cm deep, clear cold mountain stream with pebble and gravel bottom, hill dipterocarp forest, 4°03'N 114°53'E, 500 m asl, Gunong Mulu National Park, Sarawak, Malaysia, coll. J. Dring, 18 March 1978; paratype male (21.9 × 17.8 mm) (NHM 1979.136), same data as holotype; 1 paratype female (NNM D31972), Deer Cave, Gunong Mulu National stream near southwest entrance, under rock, Sarawak, Malaysia, coll. P. Chapman, 10 March 1978; 1 paratype female (NNM D31979), Deer Water Cave, Gunong Mulu National Park in stream about 100 m from entrance, Sarawak, Malaysia, coll. P. Chapman, 25 April 1978; 2 paratype females (NNM D31972), Clearwater Cave, Gunong Mulu National Park in main stream passage near river junction, coll. P. Chapman, 13 April 1978; 1 female (30.8 × 24.6 mm) (ZRC 1989.3420), Sungei Melinau, Merimau Gorge, Gunong Mulu National Park, Sarawak, Malaysia, coll. R.P. Lim & G. Liew, 16 April 1978; 1 juvenile (ZRC 1989.3682), Sungei Melinau, Merimau Gorge, Gunong Mulu National Park, Sarawak, Malaysia, coll. R.P. Lim, 17 April 1978; 1 male (16.1 × 13.0 mm) (ZRC 2013.1293), Mulu Summit trail at Camp 2, Gunong Mulu National Park, Sarawak, Malaysia, coll. A.D. Tranh, 4 November 2006.
Diagnosis: Adult carapace transversely subquadrate, width to length ratio 1.23–1.25 (Figs. 2A, B, 3E); striae on lateral parts of carapace distinct (Figs. 2B, 3E); branchial regions gently convex in frontal view (Fig. 2C); dorsal margin of frontal median triangle adjacent to but not contiguous with lateral margins, separated by a gap or connected by small flattened granules (Fig. 2C–F); exorbital tooth with outer margin almost entire (Figs. 2B, 3E); epibranchial tooth very low, just visible, separated from rest of margin by shallow low cleft (Figs. 2B, 3E); median lobe of posterior margin of epistome acutely triangular (Fig. 2C–F); ischium of third maxilliped subquadrate (Fig. 3A); fingers of adult male cheliped closing along entire cutting margins (Fig. 3D); P3 dactylus longer than P4 dactylus (Fig. 3G, H); male pleonal somite 6 rectangular, longer than broad (Fig. 3C); male sternopleonal cavity not prominently anterior, distance between tip of cavity and suture of thoracic sternites 2 and 3 subequal to length of thoracic sternite 2 (Fig. 3B); adult Gl subterminal segment relatively slender; terminal segment subcylindrical, distal half gently curved outwards, tapering gradually to truncate tip, 0.25–0.28 times length of subterminal segment, distal opening large, ovate (Fig. 20F, G, I–K); vulvae transversely subovate (Fig. 23A).
Description: Carapace transversely subquadrate, width to length ratio 1.23–1.25 (Figs. 2A, B, 3E); regions separated by distinct grooves; dorsal surface gently convex, glabrous, pitted, rugose, especially along margins and cristae, striae on lateral parts of carapace most distinct (Figs. 2B, 3E); frontal region with low granules and rugae especially nearer epigastric region, orbital regions finely pitted to smooth; cervical groove deep, broad, extending from lateral edge of postorbital cristae to deep H-shaped gastric groove; branchial regions gently convex, not distinctly inflated in frontal view (Figs. 2B, C, F, 3E); postorbital cristae relatively sharp, distinct, gradually becoming lower, less distinct towards cervical groove, with separate short, low rugose crista just before anterolateral margin; postorbital crista separated from epigastric crista by shallow oblique groove; epigastric cristae anterior of postorbital cristae, distinct, not sharp, prominently rugose, margins sloping anteriorly, separated medially by deep longitudinal groove which extends to just before mesogastric area (Figs. 2B, F, 3E). Frontal margin slightly sinuous to almost straight, slightly deflexed; frontal median triangle transversely narrow, median in position, edges of dorsal margin adjacent to but not contiguous with lateral margins, separated by a gap or connected by small flattened granules (Fig. 2C–F). Exorbital tooth low, broadly triangular, outer margin length ca. 4 times longer than inner margin, outer margin almost straight, lined with small granules, almost entire, curving to meet anterolateral margin, forming very small triangular notch (Figs. 2B, 3E). epibranchial tooth very low, just visible, separated from rest of margin by shallow low cleft (Figs. 2B, 3E); anterolateral margin gently convex, lined with small granules, adjacent surfaces covered with numerous rows of strong oblique striae; not clearly demarcated from posterolateral margin; posterolateral margin gently concave, adjacent surfaces lined with strong oblique striae, converging to almost straight or gently convex posterior carapace margin (Figs. 2B, 3E). Pterygostomial, suborbital, sub-hepatic and subbranchial regions distinctly rugose or lined with oblique granules (Fig. 2C–F). Posterior margin of epistome with median lobe acutely triangular, tip sharp; lateral margin with low, broadly triangular lobe, separated by deep fissures (Fig. 2C–F). Third maxilliped quadrate; ischium subquadrate, longer than broad, sulcus deep, oblique, closer to inner margin; merus squarish, lateral and distal margins cristate, medially depressed; exopod slender, reaching to about half length of merus, with distinct triangular projection on inner margin of distal part, flagellum long, extending across width of merus (Fig. 3A). Terminal segment of mandibular palp bilobed, inner lobe twice length of outer lobe, flap-like.
© 2022 Academia Sinica, Taiwan
Chelipeds subequal with one chela larger but not inflated; outer surfaces of articles rugose (Figs. 2A, 3D). Basis-ischium fused, with low tooth on anterior margin (Fig. 3B). Merus trigonal in cross-section, margins uneven, covered with striae and low granules; anterior margin with low subterminal tooth. Upper surfaces of carpus with broad, shallow longitudinal depression, inner angle with prominent sharp spine, base with low tubercle (Fig. 2A). Fingers of chela longer than palm; cutting edges with numerous sharp teeth and denticles, tips of fingers strongly curved; fingers of adult male cheliped closing along entire cutting margins (Fig. 3D).
© 2022 Academia Sinica, Taiwan
Ambulatory legs not elongated, P3 longest; outer surfaces gently rugose (Figs. 2A, 3F–I). Merus stout, anterior margin slightly crested, uneven, appears weakly serrated, dorsal subterminal angle just visible but without spine or tooth; propodus with 2 rows of downward pointing spines on posterior margin; dactylus almost straight or gently curved, margins with numerous well developed spines pointing obliquely distally; P3 dactylus longer than P4 dactylus (Fig. 3F–I).
Surface of thoracic sternum punctate; sternites 1 and 2 completely fused, forming triangular plate; separated from sternite 3 by gently concave suture; sternites 3 and 4 proportionately wide, completely fused except for lateral clefts and shallow median groove; sternopleonal cavity deep, almost reaching imaginary line connecting anterior edges of coxae of chelipeds, cavity not prominently anterior, distance between tip of cavity and suture of thoracic sternites 2 and 3 subequal to length of thoracic sternite 2 (Fig. 3B); pleonal locking mechanism composed of rounded tubercle on submedian part of sternite 5 (Fig. 19A).
© 2022 Academia Sinica, Taiwan
Pleon distinctly T-shaped; all somites and telson free; somite 1 longitudinally narrow, extending across width of posterior carapace margin, reaching bases of coxae of P5; somite 2 as wide as somite 1; somite 3 subrectangular, with convex lateral margins; somites 4 and 5 trapezoidal with gently concave to straight lateral margins; male pleonal somite 6 rectangular, longer than broad, distal and proximal margins subequal; telson triangular with concave lateral margins, rounded tip (Fig. 3B, C).
Adult Gl directed obliquely outwards; subterminal segment relatively slender, tapering gradually from broad base, sinuous on proximal part of outer margin, rest of margin gently concave; terminal segment clearly demarcated from subterminal segment but grooves shallow; terminal segment subcylindrical, dorsal and ventral surfaces with numerous long setae, distal half gently curved outwards, tapering gradually to truncate tip, 0.25–0.28 times length of subterminal segment, groove for G2 marginal or slightly dorsal in position, dorsal and ventral folds slightly overlapping, distal opening large, ovate (Fig. 20F, G, I–K). G2 longer than Gl, flagellum well developed, about one-third length of basal segment (Fig. 20H, L).
Female: Adult female chelipeds are relatively slenderer compared to those of males. The female pleon is ovate, has all the somites and telson free, and covers most of the thoracic sternal surface. The vulvae are large, transversely subovate, without a clear sternal cover with the posterior margin slightly rimmed, and are positioned on the submedian part or anterior third of sternite 6 (Fig. 23A).
Colour: Holthuis (1979: 42) described the colour as follows: “brown with orange-brown edges to the carapace… Dorsally dark brown with a slight purplish tinge. Venter pale brown, mouthparts dark and purple tinged.” This is similar to colour photographs of the species in McFarlane et al. (2011).
Remarks: The taxonomy of this species has been discussed at length by Ng (2015) and there is no need to elaborate further here.
Biology: The species was described from specimens collected in Deer Cave, Deer Water Cave and Clearwater Cave in Gunong Mulu National Park, the highest altitude being 500 m but they are clearly not troglobites (Holthuis 1979) (see also McFarlane et al. 2011). It has also been found in streams outside caves (ZRC 2013.1293). Although most of the specimens have been found in caves, they should be considered as facultative cavernicoles at best as they have none of the adaptations associated with troglobites (cf. Holthuis 1986; Guinot 1988 1994).
description: Published as part of Ng, Peter K L & Grinang, Jongkar, 2022, Establishment of A New Bornean Genus of Gecarcinucidae (Crustacea: Brachyura), with Descriptions of Five New Species., pp. 1-35 in Zoological Studies 61 (13) on pages 3-7, DOI: 10.6620/ZS.2022.61-13, http://zenodo.org/record/8074520