Arachnothelphusa rimba, Ng, 2021

Ng, Peter K. L., 2021, On two new species of arboreal crabs from phytotelms in Sarawak, Borneo (Crustacea: Brachyura: Gecarcinucidae: Arachnothelphusa), Zootaxa 5016 (3), pp. 407-418 : 409-413

publication ID	https://doi.org/ 10.11646/zootaxa.5016.3.6
publication LSID	lsid:zoobank.org:pub:369BA2BD-7673-44EC-9D54-D846F112E4A4
DOI	https://doi.org/10.5281/zenodo.5222350
persistent identifier	https://treatment.plazi.org/id/A86C190B-FE7C-FFCB-FF0F-22B5E266FC3C
treatment provided by	Plazi
scientific name	Arachnothelphusa rimba
status	sp. nov.

Treatment

Arachnothelphusa rimba View in CoL n. sp.

( Figs. 1–6 View FIG View FIG View FIG View FIG View FIG View FIG )

Material examined. Holotype: male (22.8 × 16.6 mm) ( ZRC 2020.0366 View Materials ), 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 View Materials ), 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 View Materials ), 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 View Materials ), 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 View FIG , 2B–D View FIG ). Basis-ischium of third maxilliped rectangular, long, length to width ratio 1.78; oblique median sulcus deep ( Fig. 1D View FIG ). Outer surfaces of chelipeds rugose; carpus rugose, with granules, inner angle with triangular tooth ( Fig. 1A, G, H View FIG ). Posterior margin of epistome wide, median love distinctly triangular ( Fig. 1C View FIG ). 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 View FIG , 2A View FIG , 3 View FIG ). 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 View FIG ). 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 View FIG ). G2 with short distal segment, about 0.27 times length of basal segment ( Fig. 4D View FIG ). Vulvae large, occupying submedian part of sternite 6, without obvious sternal vulvar cover ( Fig. 5B View FIG ).

Variation. One of the paratype male specimens (20.2 × 15.4 mm, ZRC 2020.0087 View Materials ) has a relatively more rectangular carapace than the other specimens, with the branchial regions less prominently swollen laterally ( Fig. 2D View FIG ). 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 View Materials ) is damaged, with a deep cleft, suggesting there had been an injury in the past ( Fig. 2A, B View FIG ) .

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 View FIG ). 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 View FIG ). The ambulatory legs are purplish-brown, with the distal third of the dactylus pale yellow ( Fig. 6 View FIG ). 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 View FIG , 2B–D View FIG ) (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 View FIG ) (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 View FIG ) (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 View FIG ) (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 View FIG ). Unlike A. rimba n. sp. which has an entire anterolateral margin ( Figs. 1B View FIG , 2B–D View FIG ), 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.