Borneosa kapit sp. nov.

(Figs. 12–14, 19D, 22A–H, 23D) urn:lsid:zoobank.org:act: 432F792E-60B3-4AD8-9AF1- D00BD8D087E7

Material examined: Holotype: male (37.4 × 28.4 mm) (SMF 48750), Balui River, Rejang River, 2°44.616'N 114°4.565'E, Sarawak, Malaysia, coll. A. Leek, 11 November 1982 . Paratypes: 2 males (20.6 × 16.9 mm, 12.8 × 9.9 mm) (UNIMAS.C.00039), first order stream, downstream, logged forest Sungei Pap in Belaga, 2°38'08.1"N 114°26'16.6"E, 535 m asl, Sarawak, Malaysia, coll. J. Grinang et al., 10 September 2007; 3 males (23.5 × 19.1 mm, 19.1 × 16.6 mm, 13.7 × 11.2 mm), 1 female (22.1 × 18.1 mm) (ZRC 2021.0518), second order stream, upstream, logged forest Sungei Belepu in Belaga, 2°37'22.4"N 114°24'55.3"E, 497 m asl, Sarawak, Malaysia, coll. J. Grinang et al., 11 September 2007; 1 male (12.0 × 11.0 mm), 1 juvenile) (UNIMAS.C.00045), second order stream, midstream, logged forest Sungei Belepu in Belaga, 2°37'26.0"N 114°25'02.4"E, 450 m asl, Sarawak, Malaysia, coll. J. Grinang et al., 11 September 2007; 1 juvenile (UNIMAS.C.00051), first order stream, downstream, logged forest Sungei Sunan in Belaga, 2°39'30.8"N 114°11'11.0"E, 476 m asl, Sarawak, Malaysia, J coll. J. Grinang et al., 9 September 2007; 1 male (21.9 × 18.7 mm) (ZRC 2021.0519), 2 females (27.3 × 23.4 mm, 12.4 × 10.9 mm) (UNIMAS.C.00101), second order stream, logged over forest Sungei Simalajau, Kapit, Sarawak, Malaysia, bought from Teresang Market, Kapit, J. Grinang, 14 April 2016; 1 male (37.5 × 29.4 mm), 1 female (29.5 × 23.5 mm) (ZRC 2021.0690), first order stream, Nanga Benin, Pelagus, Kapit, Sarawak, Malaysia, coll. J. Grinang et al., 6 September 2016; 1 male (32.3 × 25.6 mm) (ZRC 2021.0520), first order stream, logged over forest Sungei Lebau, Pelagus, Kapit, Sarawak, Malaysia, coll. J. Grinang et al., 11 December 2015; 2 males (35.8 × 28.0 mm, 35.8 × 28.6 mm), 2 females (40.6 × 31.4 mm, 36.9 × 29.0 mm) (ZRC 2021.0521), 3 males (35.8 × 28.6 mm, 35.3 × 28.0 mm, 35.3 × 28.3 mm, 36.9 × 28.7 mm), 2 females (36.6 × 29.9 mm, 28.5 × 23.6 mm) (UNIMAS.C.00102), second order stream, Sungei Yong, Kapit, Sarawak, Malaysia, sent by M.B. Sadar, 8 February 2020.

Diagnosis: Adult carapace transversely subovate, width to length ratio 1.20–1.32 (Figs. 12A, B, 14); striae on lateral parts of carapace strong (Fig. 12B); branchial regions convex in frontal view (Fig. 12C, F); dorsal margin of frontal median triangle contiguous with lateral margins (Fig. 12C–E); exorbital tooth with outer margin almost entire (Fig. 12B); epibranchial tooth low to absent, separated from rest of margin by low cleft when present (Figs. 12B, 14); median lobe of posterior margin of epistome obtusely triangular (Fig. 12C–E); ischium of third maxilliped subrectangular (Fig. 13A); fingers of larger adult male cheliped with gape at base of when closed (Fig. 13E); P3 and P4 dactyli subequal in length (Fig. 13G, H); male pleonal somite 6 rectangular, longer than broad (Fig. 13C); male sternopleonal cavity not prominently anterior, distance between tip of cavity and suture of thoracic sternites 2 and 3 longer than length of thoracic sternite 2 (Fig. 13B); adult Gl subterminal segment relatively stout; terminal segment subconical, distal half slightly curved outwards, tapering gradually to rounded tip, 0.30–0.33 times length of subterminal segment, distal opening relatively small, round (Fig. 22A–C, E–G); vulvae almost round (Fig. 23E).

Colour: In live, carapace and pereopods orange to light brown, darker on centre of carapace; with ventral surface pale yellow (Fig. 25C).

Etymology: The name is derived from the town of Kapit in the upper part of the Rejang River, where the species occurs. The name is used as a noun in apposition.

Remarks: Borneosa kapit sp. nov. and B. sarawakensis sp. nov. bear a close resemblance, but can easily be separated by several characters. Adult B. kapit have a carapace which is usually wider in proportions (width to length ratio 1.20–1.32, Fig. 12B) (versus more quadrate in B. sarawakensis, with the width to length ratio 1.18–1.26; Fig. 15A); the epibranchial tooth is small but still usually visible, and even when low, is with rare exceptions, demarcated from the anterolateral margin by a shallow cleft (Figs. 12B, 14) (versus epibranchial tooth not visible with the anterolateral margin appearing entire in B. sarawakensis; Fig. 15B); the branchial regions in adult males are relatively higher and appear more inflated in the frontal view (Fig. 12C) (versus with regions relatively lower and less convex in B. sarawakensis; Fig. 15C); the G1 terminal segment has the median part less distinctly bent, with the distal part more elongate with a more tapering tip and the opening proportionately smaller (Fig. 22A– C, E–G) (versus median part distinctly bent with the distal part ending in a rounded tip with a wide opening in B. sarawakensis; Fig. 22I–K). Significantly, while both species are from the Rejang drainage, B. kapit is from the headwaters of the river to the eastern part of Sarawak while B. sarawakensis is on the highlands on the western part of the drainage.

The good series of specimens of B. kapit shows that the degree of variation at the species level is not substantial. The carapace of smaller specimens is proportionately less wide, appearing more subquadrate and the convexity of the branchial regions also less pronounced. The size of the epibranchial tooth varies slightly, but it is always distinct (Fig. 14). As for the G1, in smaller specimens, the terminal segment is slightly slenderer with a slightly larger distal opening (Fig. 22A–C) compared to the condition in larger adults (Fig. 22E–G).

Biology: The known habitat of the species are small streams flowing through secondary forests or logged over forest, under a partly shaded or complete canopy. The waters are clear, fast flowing, and cold, with the substrates mainly composed of pebbles and cobblestones. The species is occasionally sold at the Teresang Market in Kapit which had been collected from Sungei Similajau (UNIMAS.C.00101), a stream which feeds the main Rejang river.