Borneosa aspera (Ng & Stuebing, 1989)

(Figs. 1B, D, 4, 5, 19B, 20M–P, 23B)

Sundathelphusa aspera Ng & Stuebing, 1989: 14, figs. 1, 2; Ng and Yeo 2007: 12; Ng et al. 2008: 72; Cumberlidge et al. 2009: unpaginated appendix; Ng 2015: 497, figs. 4–6, 7I–K.

Material examined: Holotype male (42.6 × 32.8 mm) (ZRC 1989.2143), clear, swift forest stream, Sungei Rabergan, tributary of Sungei Muaya, near Mendalong, 4°57'N 115°43'E, 750 m asl, Sipitang District, Sabah, Malaysia. Others: 1 male (32.3 × 25.4 mm), 1 female (44.1 × 33.1 mm) (ZRC 2014.845), Temburong, Amo, Batu Apoi Forest Reserve, Ulu Temburong National Park, Brunei, coll. Jangorun anak Eri, 7 August 2014; 1 female (44.5 × 33.7 mm) (ZRC 2013.746), Bukit Pagon, Temburong District, Brunei, 900 m asl, coll. S. Salam, 6 July 2012; 2 females (29.3 × 23.0 mm, 19.7 × 15.6 mm) (ZRC 2012.1225), in primary forest stream, in area near Sarawak border, Brunei, coll. H. Mohammod, October 2012.

Diagnosis: Adult carapace transversely ovate, width to length ratio 1.27–1.30 (Figs. 4A, B, 5F); striae on lateral parts of carapace strong (Figs. 4B, 5F); branchial regions gently convex in frontal view (Fig. 4C, F); dorsal margin of frontal median triangle contiguous with lateral margins (Fig. 4C–E); exorbital tooth with outer margin almost entire (Figs. 4B, 5F); epibranchial tooth low, separated from rest of margin by shallow low cleft (Figs. 4B, 5F); median lobe of posterior margin of epistome obtusely triangular (Fig. 4C–E); ischium of third maxilliped subquadrate (Fig. 5A); fingers of adult male cheliped closing along entire cutting margins (Fig. 5D, E); P3 dactylus longer than P4 dactylus (Fig. 5H, I); male pleonal somite 6 rectangular, longer than broad (Fig. 5C); 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. 5B); adult Gl subterminal segment very slender; terminal segment cylindrical, distal half almost straight, tip wide, truncate, 0.35–0.37 times length of subterminal segment, distal opening very wide, ovate (Fig. 20M–O); vulvae transversely subovate (Fig. 23B).

Colour: In life, the carapace and pereopods of the species are dark to dull brown, with the ventral and frontal surfaces light brown to yellow and pale yellow; the pleon being reddish-brown (Fig. 24A–G).

Remarks: Described on the basis of one large male by Ng and Stuebing (1989) from northern Sabah, the species was subsequently recorded and redescribed at length on the basis of a larger series of specimens from Brunei (Ng 2015). Borneosa aspera is the most distinctive member of the genus as it reaches the largest sizes (a female, 44.5 × 33.7 mm, ZRC 2013.746) and males also have proportionately, the longest G1 terminal segment (Fig. 20M–O).

Biology: All the specimens were collected from primary highland rainforest streams.