Genus

Figs 2, 3, 4, 5, 6, 7

Type species.

Merapohra karsticola sp. nov. by monotypy.

Comparative material.

Erebusa calobates Yeo & Ng, 1999. Holotype. Laos • male; 20.8 × 17.8 mm; Khammouan Province, Tham Tê, near Ban Na; 11 Feb. 1998; L. Deharveng & A. Bedos leg.; ZRC 1998.1073. Paratypes. Laos • 2 males; 20.0 × 17.4 mm, 19.9 × 17.5 mm; 1 juvenile; same data as holotype; ZRC 1998.1074 – 1075 • 1 ovigerous female; 23.0 × 19.3 mm; Khammouan Province, Tham Houai Say, near Ban Khen; 26 Feb. 1998; F. Brehier leg.; ZRC 1998.1076.

Johora grallator Ng, 1988. Holotype. Peninsular Malaysia • female; 33.1 × 23.5 mm; Pahang, Pulau Tioman, Gunung Kajang, 792 m asl, Ulu Lalang, in insect trap baited with coconut, ca 2°51'23"N, 104°09'37"E; 18 Jun. 1962; J. A. Bullock leg.; ZRC 1989.2691. Non-type specimens. Peninsular Malaysia • 2 juvenile males; 11.2 × 8.8 mm, 11.9 × 9.0 mm; Pahang, Pulau Tioman, Gunung Kajang, under rock in dry cave; 27 Jun. 1996; T. H. T. Tan et al. leg.; ZRC 1996.1730 • 1 male; 32.7 × 23.1 mm; Pahang, Pulau Tioman, Gunung Kajang, ca 700 m asl, on trail; 8 Jul. 2024; J. Bartels leg.; ZRC 2024.0159.

Johora gua Yeo, 2001. Holotype. Peninsular Malaysia • male; 11.4 × 8.5 mm; Pulau Tioman, Gua Tengkuk Ayer, route to Gunung Kajang from Juara, ca 900 m asl; 1 Sep. 2000; H. H. Tan & T. M. Leong leg.; ZRC 2000.2236.

Johora punicea (Ng, 1985). Holotype. Peninsular Malaysia • male; 19.7 × 15.5 mm; Pahang, Pulau Tioman, under rocks, on slope adjacent to Sungei Besar, ca 300 m asl; 20 Jun. 1984; P. K. L. Ng leg.; ZRC 1984.6803. Paratypes. Peninsular Malaysia • 1 male; 21.6 × 16.0 mm; 2 females; same data as holotype; ZRC 1984.6804 – 6806. Non-type specimens. Peninsular Malaysia • 1 female; 19.9 × 15.6 mm; Pahang, Pulau Tioman, Paya; Jul. 2018; Honours Class leg.; ZRC 2022.0102 • 1 ovigerous female; Pahang, Kampung Paya; 19 Sep. 1995; P. K. L. Ng leg.; ZRC 2018.1130 • 1 male; Pahang, Pulau Tioman, Kampung Paya; Jul. 2005; H. H. Tan leg.; ZRC 2018.1113.

Demanietta sunthorni Lheknim, Leelawathanagoon & Ng, 2023. Holotype. Thailand • male; 36.6 × 27.7 mm; Ao Dan, south of Adang Island, unnamed creek, 6°30.732'N, 99°18.226'E, 35 m asl; 20 Oct. 2008; V. Lheknim & P. Leelawathanagoon leg.; ZRC 2022.0826.

Phaibulamon stilipes Ng, 1992. Holotype. Thailand • male; 28.5 × 22.7 mm; Kanchanaburi Province, Kwai Valley, Tham Nam Pah Khoan cave, from “ K 2 ” resurgence; 18 Aug. 1990; French Expedition Kwai 90 leg.; ZRC 1992.8325.

Terrapotamon abbotti (Rathbun, 1898). Non-type specimens. Thailand • 3 males; 39.2 × 32.3 mm, 29.5 × 24.3 mm, 25.7 × 20.8 mm; 1 female; 33.4 × 27.8 mm; Surat Thani Province, Bon Baeng Bap, Ratthanikhom, Amphoe Khiri; 16–19 Jul. 1983; T. Sisuchat & D. Kuntisit leg.; ZRC 2012.0683.

Terrapotamon longitarsus Lheknim & Ng, 2016. Holotype. Thailand • male; 47.4 × 38.6 mm; Satun Province, Muang Satun District, Tambon Khuan Po, found dead on limestone bedrock at Khao Raya Bung Sa, southernmost of Nakhon Si Thammarat mountain range, 30 m asl; 9 Sep. 1999; V. Lheknim & P. Leelawathanagoon leg.; ZRC 2016.0161.

Terrapotamon palian Ng & Naiyanetr, 1998. Holotype. Thailand • male; 24.6 × 20.5 mm; Trang Province, Palian District, Ton Tok Waterfall; 21 May 1976; P. Naiyanetr leg.; ZRC 2012.0685.

Thampramon tonvuthi Ng & Vidthayanon, 2013. Non-type specimens. Thailand • 1 male; 33.9 × 27.8 mm; Baan Chompu southwest of Thung Salaeng Luang National Park, Tham Phra Wangdaeng, cave entrance, 16.8379°N, 100.877°E; 8 Oct. 1997; C. Vidthayanon leg.; ZRC 2012.1126 • 1 female; 42.7 × 36.3 mm; same locality as male, in twilight zone of cave, ca 100 m from entrance, on wet rock wall, 4 m above stream above bat guano; 28 Aug. 2002; D. Smart leg.; ZRC 2012.1127.

Nemoron nomas Ng, 1996. Holotype. Vietnam • male; 23.7 × 20.3 mm; Quang Binh Province, Hang Toi, Phong Nha, station Viet 062, 35 m asl, in cave, about 300 m inside; 11 Jan. 1995; L. Deharveng, A. Bedos & Levet leg.; ZRC 1996.0094.

Tiwaripotamon edostilus Ng & Yeo, 2001. Holotype. Vietnam • male; 36.4 × 28.3 mm; Ha Long Bay, Cat Ba Island, cave at Gia Luang; 28 Sep. 1998; L. Deharveng leg.; ZRC 2000.0096. Paratype. Vietnam • male; 40.5 × 31.1 mm; same data as holotype; ZRC 2000.0097.

Diagnosis.

Carapace (Figs 2 A, 2 B, 5 A) transversely subovate, slightly wider than long, CW / CL ratio 1.30–1.40; dorsal surface gently convex, smooth, glabrous; branchial regions not prominently expanded laterally, not inflated in frontal view, lateral branchial regions not discernible in dorsal view; orbits oblique in frontal view; epigastric and postorbital cristae not clearly discernible, appearing as smooth margin with small granulations, epibranchial crista slightly anterior of postorbital crista, postorbital crista with lateral part breaking up into low striae and granules approaching but not joining anterolateral margin; postorbital, postfrontal and epibranchial regions with indistinct low granules, rugae and / or punctae (Fig. 2 D); epibranchial tooth poorly defined, weakly separated from external orbital tooth by shallow cleft; external orbital tooth well produced, narrowly triangular with concave lateral margins, outer margin ~ 1.77 × length of inner margin, just before suborbital margins; margins of posterior epistomal margin with low median lobe, lateral parts gently sloping downwards, not parallel with frontal margin (Fig. 2 E). Third maxilliped with subquadrate merus, ischium rectangular, length to width ratio 1.40, exopod relatively slender, outer margin straight, flagellum short, approx. half-length of merus (Fig. 4 F). Ambulatory legs and dactylus (Fig. 2 A) slender, elongate, margins and surfaces smooth. Male thoracic sternites 1 and 2 fused, relatively narrow, width-to-length ratio 2.87, sternites 3 and 4 fused, relatively narrow, but with shallow, visible grooves demarcating sutures; thoracic sternite 8 completely hidden when male pleon closed (Fig. 3 A, B); male pleon (Fig. 3 B, C) broadly triangular; male telson broadly triangular, lateral margins almost straight. Male sternopleonal cavity (Fig. 3 D) reaching imaginary line joining median point of cheliped coxae; press-button tubercle of male pleonal locking mechanism distinct, on posterior third of sternite 5. G 1 subterminal article (Fig. 4 A – D) relatively stout, straight; terminal article long, 0.6 × length of subterminal article, strongly curving outwards anteriorly before recurving posteriorly, gently tapering throughout to broad tip, with long, medial dorsal flap. Vulva (Fig. 5 C) conspicuously large, directed mesial-ventrally, occupying almost length of sternite 6, impinging upwards onto sternite 5, sternite 5 forming ledge-like overhang over sternite 6 and vulvae; intervulvar distance, measured between mesial margins of both vulvae, 0.42 × vulvar distance, measured between lateral margins of both vulvae.

Etymology.

The new genus is named after the town of Merapoh in Lipis district, Pahang state, Peninsular Malaysia; an area known for numerous limestone hills and caves. Gender feminine.

Remarks.

The external morphology of Merapohra gen. nov., is similar to 25 karst dwelling potamid crabs found in Southeast Asia: Cerberusa tipula Holthuis, 1979, C. caeca Holthuis, 1979 (both from Sarawak, Borneo, Malaysia); Erebusa calobates Yeo & Ng, 1999 (Khammouan Province, central Laos); Kanpotamon duangkhaei Ng & Naiyanetr, 1993 (Kanchanaburi Province, western Thailand); Nemoron nomas Ng, 1996 (Quang Binh Province, central Vietnam); Ngan mayla Guinot & Rodríguez Moreno, 2024 (East Kalimantan, Borneo, Indonesia); Phaibulamon stilipes Ng, 1992 (Kanchanaburi Province, western Thailand); Schubartamon serenum Guinot & Rodríguez Moreno, 2024 (Vientiane Province, central Laos); Terrapotamon longitarsus Lheknim & Ng, 2016, Te. thungwa Promdam, Yeesin & Ng, 2017 (both from Nakhon Si Thammarat, Satun, Surat Thani, Trang Provinces, southern Thailand); Thampramon tonvuthi Ng & Vidthayanon, 2013 (Phetchabun Province, central Thailand: one species); and 14 species of Tiwaripotamon Bott, 1970 (from northern Vietnam and Guangxi Province, southern China); especially with regards to their dolichopodous form which are likely adaptations for life in karstic habitats. Of these taxa, Erebusa Yeo & Ng, 1999, Kanpotamon Ng & Naiyanetr, 1993, Ngan Guinot & Rodríguez Moreno, 2024, Phaibulamon Ng, 1992, Schubartamon Guinot & Rodríguez Moreno, 2024, and Thampramon Ng & Vidthayanon, 2013, are monotypic genera, while Cerberusa Holthuis, 1979, only has two known species. Terrapotamon Ng, 1986, has five known species but only the two listed here have long ambulatory legs, the other possessing short legs and are not found near caves (see Ng 1988; Leelawathanagoon et al. 2010; Promdam et al. 2017).

Merapohra can be immediately distinguished from Cerberusa, Ngan, Nemoron Ng, 1996, and Phaibulamon by the presence of a distinct albeit short flagellum on the exopod of the third maxilliped, a character absent in the latter four genera (Fig. 4 F, Table 1; cf. Holthuis 1979; Ng 1992 c: fig. 2 A; Ng 1996: fig. 2 A; Guinot and Rodríguez Moreno 2024 a). Among the remaining taxa that possess a flagellum on the third maxilliped exopod, Erebusa and Schubartamon differ markedly from Merapohra in possessing very short eyestalks; in Merapohra, the longer eye stalks are distinctly longer, sporting well developed and corneas pigmented, suggesting a more epigean or trogophilic lifestyle (Fig. 2 B, C, Table 1; cf. Yeo and Ng 1999: fig. 1 E; Guinot and Rodríguez Moreno 2024 b: figs 3 B, 7 B, 8).

* Original description based on female only.

Kanpotamon duangkhaei, Terrapotamon longitarsus, Te. thungwa, Thampramon tonvuthi, and members of Tiwaripotamon all have short to very short flagella on the exopods of their third maxillipeds and large pigmented eyes on longer eye stalks, which are more similar to Merapohra. The two Terrapotamon species are the most dissimilar, distinguishable by its higher, more subquadrate carapace, that is more strongly covered in rugosities and granules particularly around branchial, sub-branchial, pterygostomial regions, and anterolateral margins (cf. Lheknim and Ng 2016: fig. 1 A; Promdam et al. 2017: fig. 2 A); and a more strongly defined epigastric, postorbital cristae, anterolateral margin, and epibranchial tooth (cf. Lheknim and Ng 2016: fig. 1 A, B, C; Promdam et al. 2017: fig. 2 A, B). In Merapohra, the carapace is relatively flatter, more transversely sub-ovate, that is very weakly rugose or completely smooth on both dorsal and ventral surfaces (Fig. 2 B, C), with a much more weakly defined epigastric, postorbital cristae, anterolateral margin, and epibranchial tooth (Fig. 2 B) (Table 1).

In comparison to the species of Kanpotamon, Thampramon, and Tiwaripotamon, the new genus Merapohra can be differentiated by the following morphological differences: 1) carapace is transversely sub-ovate, relatively smooth, except for few gentle rugosities and granules around branchial regions (Fig. 2 B) (vs carapace less transverse, more strongly rugose around branchial regions in Thampramon, while in Kanpotamon and Tiwaripotamon, it is completely smooth, cf. Ng and Naiyanetr 1993: fig. 21; Ng and Yeo 2001: figs 1 A, 3 A, 4 A; Ng and Vidthayanon 2013: fig. 2; Shih and Do 2014: figs 4 A, 6 A; Do et al. 2016: figs 3 A, 4 A, 5 A; Do et al. 2017: figs 2 A, 5 A; Ng and Ngo 2022: fig. 2 A; Ng 2024: fig. 1 A; Dang and Do 2025: fig. 2 A); 2) anterolateral margin sub-cristate, very weakly serrated (Fig. 2 B) (vs anterolateral margins strongly cristate, appearing raised and serrated in Kanpotamon and Thampramon, while in Tiwaripotamon, the anterolateral margins can appear smooth to serrated, cf. Ng and Naiyanetr 1993: fig. 21; Ng and Yeo 2001: figs 1 A, 3 A, 4 A; Ng and Vidthayanon 2013: fig. 2; Shih and Do 2014: figs 4 A, 6 A; Do et al. 2016: figs 3 A, 4 A, 5 A; Do et al. 2017: figs 2 A, 5 A; Ng and Ngo 2022: fig. 2 A; Ng 2024: fig. 1 A; Dang and Do 2025: fig. 2 A); 3) epibranchial tooth less well-defined, appearing as a nodule, confluent with external orbital tooth (Fig. 2 B) (vs epibranchial tooth large, triangular, clearly separated from external orbital tooth by deep cleft in Kanpotamon and Thampramon, that in Tiwaripotamon, varies in size, from broad angle to triangular, still clearly separated from external orbital tooth, cf. Ng and Naiyanetr 1993: fig. 21; Ng and Yeo 2001: figs 1 A, 3 A, 4 A; Ng and Vidthayanon 2013: fig. 2; Shih and Do 2014: figs 4 A, 6 A; Do et al. 2016: figs 3 A, 4 A, 5 A; Do et al. 2017: figs 2 A, 5 A; Ng and Ngo 2022: fig. 2 A; Ng 2024: fig. 1 A; Dang and Do 2025: fig. 2 A); 4) both epigastric and postorbital cristae poorly developed, appearing smooth except for few granules at lateral margins (Fig. 2 B) (vs epigastric and postorbital cristae well-developed, appearing sharp in Kanpotamon and Thampramon, that in Tiwaripotamon is even more poorly defined, completely smooth, cf. Ng and Naiyanetr 1993: fig. 21; Ng and Yeo 2001: figs 1 B, 3 B, 4 B; Ng and Vidthayanon 2013: fig. 3; Shih and Do 2014: figs 4 B, 7 B; Do et al. 2016: figs 3 B, 4 B, 5 B; Do et al. 2017: figs 2 B, 5 B; Ng and Ngo 2022: fig. 3 B; Ng 2024: fig. 2 F, G; Dang and Do 2025: fig. 2 A); and 5) epistomal medial lobe, well produced, narrowly triangular (Fig. 2 D) (vs epistomal median lobe lower, appearing as broad triangle in Thampramon and Tiwaripotamon, cf. Ng and Yeo 2001: figs 1 A, 3 A, 4 A; Ng and Vidthayanon 2013: fig. 2; Shih and Do 2014: figs 4 A, 6 A; Do et al. 2016: figs 3 A, 4 A, 5 A; Do et al. 2017: figs 2 A, 5 A; Ng and Ngo 2022: fig. 2 A; Ng 2024: fig. 1 A; Dang and Do 2025: fig. 2 B).

Perhaps the most significant feature of Merapohra gen. nov., is in its unique G 1 morphology, where the terminal article is relatively long, about 0.6 × length of subterminal article, strongly curving outwards proximally, and nearly recurved 180 ° distally, with a large semi-circular dorsal flap (Fig. 4 B, D). No other potamid crab possesses a G 1 with such a strongly recurved distal terminal article. This contrasts with other long-legged karst dwelling species in Southeast Asia, which typically possess a terminal article that is proportionally shorter, about 0.2–0.4 × length of subterminal article, which in Kanpotamon, Terrapotamon, and Thampramon, is not recurved, while in Tiwaripotamon, it can range from curving downwards to being gently recurved, either without a dorsal flap, or if present, is always small and low, cf. Ng and Naiyanetr 1993: fig. 55; Ng and Yeo 2001: figs 2 D – H, 5 D – K; Leelawathanagoon et al. 2010: fig. 1 F, H; Ng and Vidthayanon 2013: fig. 7 C, D; Shih and Do 2014: fig. 3 A – H; Do et al. 2016: fig. 2 A – F; Lheknim and Ng 2016: fig. 5; Do et al. 2017: fig. 4 A – C, 7 A – D; Promdam et al. 2017: fig. 4; Ng and Ngo 2022: fig. 5 A – D; Ng 2024: fig. 5 A – D, F – H; Dang and Do 2025: fig. 5 B, C, E – G).

The only other cavernicolous potamid crab species from Peninsular Malaysia is Johora gua Yeo, 2001, described from Gua Tengkuk Ayer, near the summit of Gunung Kajang, Pulau Tioman. Johora gua is an unusual species, for the genus, featuring troglomorphic features, including distinctly elongated legs and reduced cornea size (Yeo 2001), that is unlike most species of Johora, which typically have stouter ambulatory legs and regular sized cornea (Ng 1988, 2020). Nevertheless, Merapohra differs from Johora gua and all other species of Johora by its very poorly developed, rounded epigastric and postorbital cristae; nearly indiscernible epibranchial tooth; and a G 1 terminal article that is strongly recurved with a pronounced dorsal flap (Fig. 4 B, D). This contrasts with all Johora species that are characterised by their well-defined epigastric cristae, postorbital cristae, and epibranchial tooth; and a G 1 terminal article that ranges from being straight to curving downwards, never recurved, typically without dorsal flap, if ever present, is very low (cf. Ng 1988: figs 13–23; Yeo 2001: figs 1 A, 2 C – F; Leelawathanagoon et al. 2005: fig. 2; Ng 2020: figs 2, 6–12).

There are some potamids that have been collected in karstic caves in Southeast Asia that have relatively short ambulatory legs, but these species do not have any characters that suggest they are troglophiles or troglobites. For example, Isolapotamon bauense Ng, 1987 (Sarawak, Malaysia), Shanphusa ywarngan Ng & Whitten, 2017, and Demanietta burmanica Ng, 2018 (both Myanmar), were collected from caves but none have cave adapted features (Ng 1987, 2018 b; Ng and Tan 1998; Ng and Whitten 2017) and are now regarded as troglophiles at most. The holotype of Xestomon tacu Ng, 2021, was collected from a cave in southern Vietnam; and the species is overall pale orange in colour, but the corneas are fully pigmented, and specimens have also been collected in several epigeal habitats (Ng 2021).