Arachnothelphusa rimba Ng 2021, n. sp.
Creators: name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2021
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Gecarcinucidae
Arachnothelphusa
Arachnothelphusa rimba

description: Arachnothelphusa rimba n. sp.   
(Figs. 1–6)  
 Material examined. Holotype: male (22.8 × 16.6 mm) (ZRC 2020.0366), station DZ-02; in tree holes, along upwards sloping forest trail behind base camp, at night, coll. K.O. Chan et al., 12 October 2019. Paratypes: 1 male (19.1 × 13.9 mm) (ZRC 2020.0086), station DZ-02; in tree holes, along upwards sloping forest trail behind base camp, at night, coll. K.O. Chan et al., 12 October 2019; 1 male (20.2 × 15.4 mm) (ZRC 2020.0087), station DZ-06, trail and stream behind base camp near boundary, in tree holes and on leaves, Sungei Mujok, at night, coll. K.O. Chan & I. Isa, 17 October 2019; 1 female (22.9 × 16.5 mm) (ZRC 2020.0088), station DZ-04, along sloping forest trail behind base camp, at night, coll. K.O. Chan, 15 October 2019. All locations from Lanjak-Entimau Wildlife Sanctuary, Sarawak, East Malaysia, Borneo.   
 Diagnosis. Carapace with dorsal surface gently surface punctate, rugose, finely granular; branchial regions gently inflated; epigastric and postorbital cristae distinct; cervical and H-shaped gastric grooves deep, not confluent; anterolateral margins distinctly convex, finely granulate but not serrate; antero- and posterolateral regions rugose; epibranchial tooth very low to not visible, barely separated from external orbital tooth; external orbital tooth acutely triangular, outer margin almost straight, subequal in length to inner margin (Figs. 1B, 2B–D). Basis-ischium of third maxilliped rectangular, long, length to width ratio 1.78; oblique median sulcus deep (Fig. 1D). Outer surfaces of chelipeds rugose; carpus rugose, with granules, inner angle with triangular tooth (Fig. 1A, G, H). Posterior margin of epistome wide, median love distinctly triangular (Fig. 1C). P2–P5 very long; merus unarmed, dorsal margin gently gently serrated; outer surface gently rugose; P5 merus slightly longer than length of carapace; length to width ratio of P4 and P5 merus ca. 5.27 and 5.20, respectively, for males, ca. 5.79 and 5.92, respectively, for female (Figs. 1A, 2A, 3). Male pleon distinctly T-shaped, lateral margins of somites 5 and 6 distinctly concave; somite 6 longer than telson; lateral margins of telson gently concave (Fig. 1E, F). G1 slender, sinuous, gently curving outwards; terminal segment relatively short, proximal part cylindrical, distal part suddenly tapering to long slender point, about 0.33 times length of subterminal segment (Fig. 4A–C). G2 with short distal segment, about 0.27 times length of basal segment (Fig. 4D). Vulvae large, occupying submedian part of sternite 6, without obvious sternal vulvar cover (Fig. 5B).   
 Variation. One of the paratype male specimens (20.2 × 15.4 mm, ZRC 2020.0087) has a relatively more rectangular carapace than the other specimens, with the branchial regions less prominently swollen laterally (Fig. 2D). In all other aspects, however, it agrees with the other types; its G1 structure being identical to the holotype. The left side of the anterolateral margin of the paratype female (22.9 × 16.5 mm, ZRC 2020.0088) is damaged, with a deep cleft, suggesting there had been an injury in the past (Fig. 2A, B).   
 Etymology. The species name is derived from the initiative that resulted in its discovery – Research for Intensified Management of Bio-Rich Areas of Sarawak (RIMBA) which was started by the Sarawak Forestry Corporation in 2015. The name is used as a noun in apposition.   
 Colour. In life, the carapace is brownish to brownish purple (Fig. 6). The chelae and proximal part of the fingers are purplish-brown, with the distal third of the fingers cream to pale-yellow (Fig. 6B, D–F, H). The ambulatory legs are purplish-brown, with the distal third of the dactylus pale yellow (Fig. 6). The ventral surfaces are yellowishwhite.   
 Remarks. The relatively flatter carapace allies A. rimba n. sp. with A. terrapes from Sabah. They can, however, easily be separated; A. rimba n. sp. has the anterolateral margin entire (Figs. 1B, 2B–D) (versus with deep U-shaped notch in A. terrapes; cf. Ng 1991: fig. 5); the palm and fingers of the adult male cheliped are proportionately shorter (Fig. 1G) (versus palm and fingers longer and more slender; cf. Ng & Ng 2018: fig. 5D); the G1 terminal segment is straight but with the distal part sharply tapering to an acute tip (Fig. 4A–C) (versus terminal segment gently curved upwards in A. terrapes; cf. Ng 1991: fig. 2D–G); and the G2 distal segment is relatively shorter (Fig. 4D) (versus distal segment longer in A. terrapes; cf. Ng 1991: fig. 6H). The G1 of A. rimba n. sp. superficially resembles that of A. melanippe but that of the latter is proportionately almost straight and is cone-shaped, gradually tapering to a sharp point (Ng 1991: fig. 2C–F); in A. rimba n. sp., the G1 terminal segment is straight but with the distal part sharply tapering to an acute tip (Fig. 4A–C). Unlike A. rimba n. sp. which has an entire anterolateral margin (Figs. 1B, 2B–D), that of A. melanippe has a distinct epibranchial tooth (Ng 1991: fig. 1).   
 The first report of crabs from the Lanjak-Entimau Wildlife Sanctuary was by Ng (1995b), who recorded two potamids, Ibanum aethes Ng, 1995, Isolapotamon stuebingi Ng, 1995; and one sesarmid Geosesarma katibas Ng, 1995. Isolapotamon stuebingi has since been synonymised under I. nimboni Ng, 1987 (Ng 2004; Ng & Grinang 2004). Ibanum aethes, Isolapotamon nimboni and Geosesarma katibas were collected in the survey of 2019, and addition, two other gecarcinucids, Perithelphusa borneensis (von Martens, 1868) and a new species of Sundathelphusa Bott, 1969, were also found. The latter will be treated later in a revision of the genus from Borneo.
description: Published as part of Ng, Peter K. L., 2021, On two new species of arboreal crabs from phytotelms in Sarawak, Borneo (Crustacea: Brachyura: Gecarcinucidae: Arachnothelphusa), pp. 407-418 in Zootaxa 5016 (3) on pages 409-413, DOI: 10.11646/zootaxa.5016.3.6, http://zenodo.org/record/5222295