Borneosa tenebrosa ( Holthuis, 1979 )

Ng, Peter K L & Grinang, Jongkar, 2022, Establishment of A New Bornean Genus of Gecarcinucidae (Crustacea: Brachyura), with Descriptions of Five New Species., Zoological Studies 61 (13), pp. 1-35 : 3-7

publication ID	https://doi.org/ 10.6620/ZS.2022.61-13
DOI	https://doi.org/10.5281/zenodo.8055960
persistent identifier	https://treatment.plazi.org/id/CF2E433D-3C30-FFA5-FC37-FCA33072FDD9
treatment provided by	Valdenar
scientific name	Borneosa tenebrosa ( Holthuis, 1979 )
status

Treatment

Borneosa tenebrosa ( Holthuis, 1979) View in CoL

( Figs. 2 View Fig , 3 View Fig , 19A View Fig , 20A–L View Fig , 23A View Fig )

Sundathelphusa tenebrosa Holthuis, 1979: 39 View in CoL , 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.

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; GoogleMaps paratype male (21.9 × 17.8 mm) ( NHM 1979.136 ), same data as holotype; GoogleMaps 1 paratype female ( NNM D31972 View Materials ), Deer Cave GoogleMaps , Gunong Mulu National stream near southwest entrance, under rock, Sarawak, Malaysia, coll. P. Chapman, 10 March 1978; 1 paratype female ( NNM D31979 View Materials ), Deer Water Cave GoogleMaps , Gunong Mulu National Park GoogleMaps in stream about 100 m from entrance, Sarawak, Malaysia, coll. P. Chapman, 25 April 1978; 2 paratype females ( NNM D31972 View Materials ), Clearwater Cave, Gunong Mulu National Park GoogleMaps in main stream passage near river junction, coll. P. Chapman, 13 April 1978; 1 female (30.8 × 24.6 mm) ( ZRC 1989.3420 View Materials ), Sungei Melinau , Merimau Gorge , Gunong Mulu National Park , Sarawak, Malaysia, coll. R.P. Lim & G. Liew, 16 April 1978; 1 juvenile ( ZRC 1989.3682 View Materials ), 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 View Materials ), 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 View Fig , 3E View Fig ); striae on lateral parts of carapace distinct ( Figs. 2B View Fig , 3E View Fig ); branchial regions gently convex in frontal view ( Fig. 2C View Fig ); 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 View Fig ); exorbital tooth with outer margin almost entire ( Figs. 2B View Fig , 3E View Fig ); epibranchial tooth very low, just visible, separated from rest of margin by shallow low cleft ( Figs. 2B View Fig , 3E View Fig ); median lobe of posterior margin of epistome acutely triangular ( Fig. 2C–F View Fig ); ischium of third maxilliped subquadrate ( Fig. 3A View Fig ); fingers of adult male cheliped closing along entire cutting margins ( Fig. 3D View Fig ); P3 dactylus longer than P4 dactylus ( Fig. 3G, H View Fig ); male pleonal somite 6 rectangular, longer than broad ( Fig. 3C View Fig ); 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 View Fig ); 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 View Fig ); vulvae transversely subovate ( Fig. 23A View Fig ).

Description: Carapace transversely subquadrate, width to length ratio 1.23–1.25 ( Figs. 2A, B View Fig , 3E View Fig ); 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 View Fig , 3E View Fig ); 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 View Fig , 3E View Fig ); 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 View Fig , 3E View Fig ). 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 View Fig ). 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 View Fig , 3E View Fig ). epibranchial tooth very low, just visible, separated from rest of margin by shallow low cleft ( Figs. 2B View Fig , 3E View Fig ); 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 View Fig , 3E View Fig ). Pterygostomial, suborbital, sub-hepatic and subbranchial regions distinctly rugose or lined with oblique granules ( Fig. 2C–F View Fig ). 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 View Fig ). 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 View Fig ). Terminal segment of mandibular palp bilobed, inner lobe twice length of outer lobe, flap-like.

Chelipeds subequal with one chela larger but not inflated; outer surfaces of articles rugose ( Figs. 2A View Fig , 3D View Fig ). Basis-ischium fused, with low tooth on anterior margin ( Fig. 3B View Fig ). 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 View Fig ). 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 View Fig ).

Ambulatory legs not elongated, P3 longest; outer surfaces gently rugose ( Figs. 2A View Fig , 3F–I View Fig ). 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 View Fig ).

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 View Fig ); pleonal locking mechanism composed of rounded tubercle on submedian part of sternite 5 ( Fig. 19A View Fig ).

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 View Fig ).

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 View Fig ). G2 longer than Gl, flagellum well developed, about one-third length of basal segment ( Fig. 20H, L View Fig ).

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 View Fig ).

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).