Geosesarma bunian, Ng & & Ahmad & Amirrudin B., 2025

Geosesarma bunian View in CoL , new species

( Figs. 1–5)

Material examined. Holotype: male (10.1 × 10.1 mm) ( ZRC 2024.0558 View Materials ), Gunung Jerai ( Kedah Peak), Kedah, Peninsular Malaysia, 1,150 m asl, 5°47′18.67″N, 100°26′5.28″E, coll. S. Khadijah-Ahmad, Noorhasanah binti Abdul Rahman, CT Lau & Mohd Mushahril Bin Abdul Shukor, 21 October 2023 GoogleMaps . Paratypes: 1 male (8.3 × 8.3 mm), 2 females (10.1 × 9.9 mm, 8.4 × 8.3 mm) ( ZRC 2024.0559 View Materials ), same data as holotype GoogleMaps .

Diagnosis. Carapace quadrate, as wide as long, adult width to length ratio ca. 1.0, lateral margins parallel ( Figs. 2, 4A, B); dorsal surface with regions just visible, anterior regions with small, low rounded granules on gastric regions ( Figs. 2C, D, 4A, B); front distinctly deflexed, 2 frontal lobes broad with almost straight margins in dorsal view; postfrontal, postorbital cristae sharp, distinct ( Figs. 2C, D, 3A, 4A, B); external orbital tooth (= first lateral tooth) triangular, directed obliquely laterally, outer margin gently convex to almost straight, tip extending just beyond lateral margin; second lateral tooth low, distinct, separated from rest of margin by cleft ( Figs. 2C, D, 4A, B). Merus of third maxilliped subovate, shorter than ischium; exopod slender with no trace of flagellum ( Fig. 5A). Outer surface of palm of adult male covered with small, low rounded granules and striae; inner surface granulated but without transverse ridge; dorsal margin of dactylus with cluster of 12–14 tubercles on proximal part and another 7 or 8 arranged in a longitudinal row towards tip of finger, reaching to three-quarters length of finger ( Fig. 3B–D). Ambulatory legs with long, slender merus, with low subdistal spine on dorsal margin, surfaces gently rugose ( Figs. 1A, B, 4C). Male pleon triangular; somite 6 wide with convex lateral margins; telson almost semicircular, slightly recessed into distal margin of somite 6 ( Fig. 3E, F). G1 relatively stouter; outer margin of subdistal part of subterminal segment (in ventral view) with distinct wide shelf-like angle ( Fig. 5B–E), distal chitinous part elongated, bent at angle of ca. 55° from longitudinal, distal part gently curved, tip spatuliform ( Fig. 5B–I). Vulvae on anterior half of sternite 6, raised, opens obliquely inwards, operculum visible, with large truncate vulvar process that arches over opening ( Fig. 4F).

Etymology. The name is derived from a Malay term used to describe the “hidden people” of the forest or Orang Bunian. They are often described as beautifully dressed forest denizens. The name, bunian , here used as a Latin noun in apposition, alludes to the beautiful colours of the new species as well as it escaping scientific attention until now.

Variation. The smaller male paratype of G. bunian (8.3 × 8.3 mm, ZRC 2024.0559) has relatively more slender chelae, and the cluster of tubercles on the proximo-dorsal part of the dactylus is not evident. The dorsal surface of the carapace of this smaller male is also relatively smoother and it does not show the marked two-colour tone ( Fig. 4A) of the holotype male and paratype females ( Figs. 2C, D, 4B). It agrees with the holotype, however, in all other characters.

Colour. Ambulatory legs and carapace with posterior half dark grey to almost black, with numerous small specks of blue and white; anterior half of carapace and chelipeds pale yellow, fingers white ( Fig. 1).

Remarks. Morphologically, G. bunian , new species, is closest to G. faustum but the G1 subterminal article is proportionately stouter and shorter ( Fig. 5B, C) (versus more slender and longer; Ng, 2017: fig. 12B, C); and the narrowed shelf-like distal part of the subterminal article before it meets the chitinous distal part is relatively wider ( Fig. 5B–E) (versus narrower; Ng, 2017: fig. 12B–E). In addition, the chitinous distal part of G. bunian ( Fig. 5B–E) also appears to be slightly shorter than that of G. faustum ( Ng, 2017: fig. 12B–E) because the tip is more rounded in shape. On the basis of the specimens on hand, the male pleonal somite 6 of G. bunian is also relatively transversely narrower ( Fig. 3F) than that of G. faustum ( Ng, 2017: fig. 11E).

The G1 of G. bunian is perhaps more like that of G. todaeng in that it is relatively stouter with the more prominent shelf-like structure on the distal part of the subterminal article; the chitinous distal part, however, is more strongly bent (55° versus 45° along the longitudinal axis) (cf. Fig. 5B–E, Ng et al., 2023: fig. 5A–E). Geosesarma todaeng can easily be distinguished from G. bunian in having a more inflated carapace with the gastric regions more swollen ( Ng et al., 2023: fig. 3A); the species is also known from lowland freshwater swamps in southern Thailand rather than the montane hill habitats G. bunian occurs in (see Ecology section).

Unlike adult male G. faustum and G. todaeng , in G. bunian , the proximal part of the dorsal margin of the dactylar finger has a dense aggregation of relatively larger rounded and/ or sharp tubercles numbering 12–14 with another seven or eight arranged in a longitudinal row towards the tip of the finger ( Fig. 3D); and when viewed laterally, nine or 10 can be clearly counted ( Fig. 3C). In G. faustum and G. todaeng , the proximal part has only three or four more prominent tubercles which are more acute in form and most can be counted laterally ( 7–9 in G. faustum and 7 or 8 in G. todaeng ; Ng, 2017: fig. 11D; Ng et al., 2023: fig. 3F, G).

In G. foxi View in CoL and G. serenei View in CoL , the vulvae open laterally inwards ( Ng, 2017: figs. 5F, 6H); this contrasts with to those of G. faustum View in CoL , G. todaeng View in CoL and G. bunian View in CoL that are directed obliquely inwards ( Fig. 4F; Ng, 2017: fig. 11H; Ng et al., 2023: fig. 4E).

There appears to be a colour difference between G. bunian View in CoL and G. faustum View in CoL ; the former having the anterior half of the carapace and chelipeds pale yellow ( Fig. 1); whilst the latter has these parts bright yellow to orange ( Ng, 2017: fig. 8).

Ecology. The type locality, Gunung Jerai or Kedah Peak , is an isolated patch of highlands in the northern state of Kedah in Peninsular Malaysia. Only one species of freshwater crab is known from this mountain, the gecarcinucid Phricotelphusa amnicola , described from the west slope at an altitude of 800 m asl ( Ng, 1994) .

Geosesarma bunian , new species, was collected in burrows on the slope alongside the jeep track towards the summit of Gunung Jerai, at an altitude of about 1,150 m asl at night. Some specimens were observed on a low shrub about 1.5 m from the ground. One of the specimens was found in a burrow beside one housing a mygalomorph spider. The species probably has arboreal habits similar to G. faustum and G. foxi as well (see Ng, 2017).