The freshwater crabs of Danum Valley Conservation Area in Sabah, East Malaysia, with a description of a new species of Thelphusula Bott, 1969 (Crustacea, Brachyura, Gecarcinucidae, Potamidae, Sesarmidae)

Abstract Seven species of freshwater crabs from three families are recorded from and around the Danum Valley Conservation Area in Sabah, Malaysian Borneo: Thelphusula capillodigitus sp. n., Thelphusula dicerophilus Ng & Stuebing, 1990, Arachnothelphusa terrapes Ng, 1991, Terrathelphusa secula Ng & Tan, 2015, Parathelphusa valida Ng & Goh, 1987 (new record) (Gecarcinucidae); Isolapotamon ingeri Ng & Tan, 1998 (Potamidae); and Geosesarma danumense Ng, 2002 (Sesarmidae). The new species of Thelphusula Bott, 1979, can be distinguished from all congeners by a unique combination of morphological features, most notably the presence of dense patches of short setae on the fingers of the adult male chelipeds, as well as the structure of the male first gonopod. Arachnothelphusa terrapes is confirmed to be a phytotelm species. A key to all species in the conservation area is provided.

Here we review and add to the freshwater crab fauna of the Danum Valley Conservation Area. Specimens of a recently collected Thelphusula Bott, 1969, from Danum Valley proved to be a new species. While superficially resembling T. hulu Tan & Ng, 1997, from the Maliau Basin in Sabah in morphology and habits, it can easily be distinguished from this and all congeners by its setose male cheliped dactyli, as well as a number of other carapace features. It is here described as T. capillodigitus sp. n. In addition, Parathelphusa valida Ng & Goh, 1987, is added to the fauna for the area. Observations on the ecology of Arachnothelphusa terrapes Ng, 1991, originally described from Danum Valley, are also provided, showing that it is only the second confirmed tree-hole crab in South East Asia. A key to the seven species of Gecarcinucidae, Potamidae, and Sesarmidae in the Danum Valley Conservation Area is provided.

Materials and methods
The terminology used follows that in Ng (1988) and Davie et al. (2015). Measurements provided, in millimetres, are of the maximum carapace width and length, respectively. The abbreviations G1 and G2 are used for the first and second male gonopods, respectively. Specimens examined are deposited in the Zoological Reference Collection (ZRC) of the Lee Kong Chian Natural History Museum, National University of Singapore.
Description of male holotype. Carapace broader than long, not raised; dorsal surface gently convex, regions clearly demarcated, covered with very short setae which does not obscure surface; frontal margin almost straight, without distinct median concavity, not deflexed, approx. a third carapace width; frontal median triangle absent (Figs 2A-C, 3E); anterolateral margin not clearly separated from posterolateral margin; external orbital tooth low, broadly triangular; epibranchial tooth low but distinct, separated from external orbital tooth by shallow cleft; postfrontal surface slightly rugose; postorbital region surface; epigastric regions raised, rugose, not cristate, divided into 2 parts by narrow, deep median groove; postorbital cristae low but distinct, sharp, not confluent with epigastric cristae, not reaching anterolateral margin; cervical grooves and H-shaped gastric depression deep; gastric regions with prominent transverse striae; antero-and posterolateral regions with strong oblique striae; posterolateral margins concave, gently converging towards posterior carapace margin; posterior carapace margin straight ( Fig. 2A, B); pterygostomial, suborbital, sub-branchial, and subhepatic  Mandibular palp 2-segmented, terminal one distinctly bilobed. Third maxilliped covering majority of buccal cavity when closed; ischium subrectangular, distinctly longer than broad, with shallow submedian groove; merus quadrate, slightly broader than long; exopod long, slender, reaching median part of merus, flagellum long, exceeding width of merus (Fig. 2D).
Chelipeds asymmetrical, right larger; surface of merus slightly rugose, relatively long, trigonal in cross section, margins without teeth or spines; carpus surface distinctly rugose, subovate, inner distal angle with sharp spine with basal tubercle; palm relatively stout, longer than broad, outer surface slightly rugose to almost smooth; fingers subequal in length to palm, dactylus marginally longer than pollex, curving inwards, cutting margin of fingers lined with numerous denticles, fingers pitted (Figs 2A, 3A-D); dorsal and lateral surfaces of most of dactylus covered with dense short setae; lateral surface of pollex with mat of short, relatively less dense setae; tips of fingers strongly curved, corneous, glabrous ( Fig. 3B-D).
Ambulatory legs not prominently elongate, third pair longest, fourth leg shortest; segments laterally flattened laterally, surfaces mildly rugose; dorsal margin of merus gently serrated, no visible subdistal tooth; carpus of first to third legs with low median ridge, absent on carpus of fourth leg; margins of propodus and dactylus lined with numerous short spines (Figs 2A, 3F-I).
Variation. Unlike the male holotype (the largest specimen), the degree and extent of the setation on the fingers of the chelae of the two smaller paratype males are the same in both chelipeds. Male specimens less than 15 mm in carapace width do not have the setae on the fingers of the chelae. The outer surface of the chela in smaller specimens is also relatively more rugose compared to larger ones.
Etymology. The name is derived from the Latin capillus for hair and digitus for finger. The name is used as a noun in apposition.
Colour. In life, the carapace is mostly dark reddish brown; the sub-branchial regions, third maxillipeds, pleon and thoracic sternum is pale yellow; the ambulatory legs dark brown, faintly marmorated, with exception of pale yellow, faintly spotted merus; and the chelipeds are yellowish orange, with the inner surfaces paler and the setose patches on the surface of the male fingers light brown (Fig. 1).
Remarks. Thelphusula capillodigitus sp. n. can easily be distinguished from all congeners by the adult male possessing dense setae on the dorsal surfaces of the fingers of the chelipeds ( Fig. 3B-D), a character also absent in genera allied to Thelphusula: Adeleana Bott, 1969, Balssiathelphusa Bott, 1969, Stygothelphusa Ng, 1989, Arachnothelphusa Ng, 1991, and Coccusa Tan & Ng, 1998(cf. Bott 1969, 1970Ng 1989bNg , 1991Tan and Ng 1998;Ng and Guinot 2014). The absence of a clearly discernible frontal median triangle is a character T. capillodigitus shares with T. pueh, T. cristicervix and T. styx, but it can be distinguished from them by the presence of setose patches on the fingers of the adult male chelipeds ( Fig. 3B-D) as well as a G1 which is only slightly curved with a relatively shorter terminal segment that is approx. a third the length of the subterminal segment ( Fig. 4A-C). In contrast, the G1s in T. pueh and T. cristicervix possess a prominently curved terminal segment which is proportionately longer, being approx. half the length of the subterminal segment (cf. Ng and Grinang 2014: fig 3). In T. styx, the G1 has a relatively broader subterminal segment with the terminal segment distinctly upturned (cf. Ng 1989a: figs 2E, F). Thelphusula capillodigitus can further be distinguished from T. styx by its relatively more shallow cervical grooves which end at the H-shaped median depression with a level and straight frontal margin ( Fig. 2B) (versus with deeper and distinctly longer cervical grooves that extend to the posterolateral region of the carapace, and the frontal margin deflexed in T. styx; cf. Ng 1989a: fig.1). The carapace of T. capillodigitus is gently convex (Fig. 2B, C) whereas in both T. pueh and T. cristicervix, the carapaces are distinctly inflated (cf. Ng and Grinang 2014: figs 1C, 2C). In addition, T. pueh, T. cristicervix, and T. styx are only known from Sarawak.
In the general form of the carapace (not raised and relatively low) and relatively shorter ambulatory legs, T. capillodigitus most closely resembles T. sabana from Lahad Datu and T. hulu from the Maliau Basin, both in Sabah. Other than in the setose adult male cheliped fingers, T. capillodigitus can also be distinguished by the gastric regions prominently lined with transverse striae ( Thelphusula capillodigitus was collected in a clear flowing shaded jungle stream with an average temperature range of 26-28 degrees Celsius and near neutral pH. All specimens were collected during the day, under rocks, and appear to be mostly aquatic in habits, although one specimen was collected from a pitfall trap (ZRC 2009.0080). Parathelphusa valida was also present in the same stream in larger numbers. The presence of a second species of Thelphusula in Danum Valley is not surprising, considering that T. capillodigitus has more aquatic habits than T. dicerophilus (see next species). Figure 6A, B Thelphusula dicerophilus Ng & Stuebing, 1990: 46, fig. 1 Colour. In life, the carapace is reddish brown with the ambulatory legs lighter in colour; the chelipeds are orangish red with the fingers pale-yellow (Fig. 6A, B).

Thelphusula dicerophilus Ng & Stuebing, 1990
Remarks. The present series of specimens do not change the original description of this species in any way. The species does grow substantially larger than the type series, with the largest specimen here, a female measuring 26.1 × 21.3 mm (ZRC 2017.1047).
The available collection data indicates T. dicerophilus is a semiterrestrial nocturnal species and forages on the forest floor, usually in wet, swampy areas, digging burrows in the soft substrate; they were often caught in pitfall traps set near these areas (see also Ng and Stuebing 1990). This contrasts with the more aquatic habits of its congener in Danum Valley, T. capillodigitus sp. n. Ng, 1991 Type species. Potamon (Potamon) melanippe De Man, 1899, by original designation. Figure 5 Arachnothelphusa terrapes Ng, 1991: 8, figs 3-6;Ng et al. 2008 Colour. The live coloration of this species observed in the recent pair of specimens is a uniform dark purple colour on the dorsal surface of the carapace, ambulatory legs and chelipeds, with a pale purple to dull white on the thoracic sternum, pleon and distal portions of the ambulatory legs and cheliped fingers (Fig. 5B-F). The dark purple colouration appears considerably darker when the animal is dry, explaining the original paler colour observation by Ng (1991).
Arachnothelphusa terrapes is easily distinguished from congeners by the deep Ushaped sinus separating the truncate external orbital tooth from the epibranchial tooth (Ng 1991: fig. 3). Arachnothelphusa merarapensis has a superficially similar anterolateral margin except that the two teeth are separated by an obtusely triangular broad cleft instead   fig. 1A  The biology of species of Arachnothelphsua is not well known. All known species are represented by only very few specimens (Ng 1991) and there is often no accompanying ecological data. Arachnothelphsua melanippe and A. kadaimana were both described without any indication of their biology (De Man 1899; Borradaile 1900).  reported on a female specimen from Poring in Sabah but there was no information on where it was found. In the ZRC there are two lots of A. kadaimana (ZRC 2009.0094, ZRC 2002.0097) from Sabah, also without specific habitat data. A female specimen of Arachnothelphusa, close to but not conspecific with A. kadaimana (ZRC 2002.0098) from Bako National Park in Sarawak was collected from a tree trunk (I. Das, per. comm.). Arachnothelphsua terrapes was found low on shrubs (Ng 1991) while A. rhadamanthysi was collected on a stalagmite wall inside a cave (Ng and Goh 1987). The most detailed account so far was that by  for A. merarapensis from Merarap Hot Springs in Sarawak, who obtained the species from low tree holes approx. 150 cm from the ground. It is not known if the crabs live in phytotelms higher up on the forest trees. Arachnothelphsua rhadamanthysi has since been photographed by naturalists in the forested area outside Gomantong caves where it was first found, suggesting it is only a facultative cave dweller (Christensen 2015).
Arachnothelphusa terrapes was described from a pair of specimens, the first, a female collected in 1989 which moulted shortly after capture and died, leaving both the animal and exuvium in poor condition. The male holotype was collected a year later from a dry tree stump, with the live coloration being a deep reddish brown on dorsal surfaces, chelipeds and legs (Ng 1991: 11). In view of the present observations of this species as a tree hole specialist, it is likely the holotype male was only taking temporary refuge in the tree stump when it was found.
Two individuals of A. terrapes were observed at 0030 hours in Danum Valley, less than 50 m apart. The first, a large adult male was observed at the edge of a water-filled hole on a tree buttress, roughly 35 cm above the ground (Fig. 5A). A second, a female carrying newly hatched young under its pleon, was found inside a water filled tree hole approx. 150 cm above ground (Fig. 5F). This species is nocturnal and highly sensitive to light, swiftly retreating into their holes when disturbed. Additional observations by other naturalists who have photographed this species in Danum Valley suggest it is always found on trees and never on the forest floor itself (unpublished data). It is clear that A. terrapes is a true phytotelm species and predominantly arboreal in nature, living exclusively in tree-holes; although they will move in search of other tree holes if the one they are residing begins to dry up or when searching for a mate. All specimens have been observed on the lower parts of trees and it is not known if they climb much higher up. All specimens recorded so far have been solitary. The present observation of an adult female carrying young (Fig. 5F) is notable and confirms the species breeds in the phytotelm.
The biology of obligate arboreal crabs has been discussed at length by Sivasothi et al. (1993), Sivasothi (2000), Cumberlidge et al. (2005), Fratini et al. (2005), , , Wehrtmann et al. (2016) and Kumar et al. (2017). While most are primary freshwater crabs (sensu Yeo et al. 2014) of the families Potamidae and Gecarcinucidae, members of two South East and East Asian sesarmid genera, Geosesarma De Man, 1892, andScandarma Schubart, Liu &Cuesta, 2003, are primarily arboreal in habits (see Schubart et al. 2003;Naruse and Ng 2007;Ng 2017). There are of course some species of freshwater crabs that occasionally climb trees and use phytotelms but can also be found on the forest floor or nearby streams, and thus are not obligate arboreal species. In Asia, Sundathelphusa celer (Ng, 1991) (Gecarcinucidae), from the Philippines was collected in a tree hollow above ground, but it is not certain if it is a wholly arboreal species (Ng 2010). Perbrinckia scansor (Ng, 1995) from Sri Lanka has also been noted to have arboreal tendencies but is clearly a terrestrial species that occasionally climbs trees (Ng 1995: 183;Ng and Tay 2001: 148-149). In Hainan, China, some species of Neotiwaripotamon Dai & Naiyanetr, 1994, are known to be primarily arboreal (unpublished data;Shih 2008). In India, the only known true arboreal phytotelm species is the recently described Kani maranjandu Kumar, Raj & Ng, 2017. Genus Terrathelphusa  Type species. Geothelphusa kuhli De Man, 1883, by original designation. Figure 6E Terrathelphusa secula   Colour. The freshly dead type specimen was described as dark brown overall (Ng and Tan 2015: 448).

Terrathelphusa secula Ng & Tan, 2015
Remarks. The species was described from just outside the Danum Valley Conservation Area by  from a recently dead specimen. Terrathelphusa species are difficult to collect due to their secretive terrestrial habits and tendency to dig deep burrows, coming out only at night and during the wet season .
Terrathelphusa was established by Ng (1989c) for a group of terrestrial species from Java and Borneo (type species Geothelphusa kuhli De Man, 1883, from Java) and now contains 11 species (Ng et al. 2008;; although the genus is probably not monophyletic (unpublished data). Terrathelphusa secula is unusual among congeners in possessing a very ovate carapace and a G1 that is elongate with a long and strongly curved terminal segment .

Parathelphusa valida Ng & Goh, 1987
Remarks. The recently collected specimens agree well with the published descriptions and figures of the species, originally described from Gomantong, Bettontan and Lahad Datu in Sabah (Ng and Goh 1987). The species has a wide range in eastern Sabah (see also Ng 1995;Tan and Ng 1997).
Parathelphusa valida occurred syntopically with T. capillodigitus sp. n. and was present in a variety of habitats including jungle streams, swampy areas and on the forest floor. It has not been previously formally recorded from Danum Valley, which is surprising, considering it is by far the most common species there and there are many specimens in the museum dating back to the 1980s.

Isolapotamon ingeri Ng & Tan, 1998
Remarks. Isolapotamon ingeri belongs to the same group of species as I. kinabauense (Rathbun, 1904) andI. anomalum (Chace, 1938) (both from the Mount Kinabalu area in northern Sabah), with the distal part of the terminal segment of the G1 expanded and flap-like . Like these species, I. ingeri usually occurs in large streams and rivers with large rocks and fast flowing water.

Family Sesarmidae Dana, 1851
Genus Geosesarma De Man, 1892 Type species. Sesarma (Geosesarma) nodulifera De Man, 1892, by subsequent designation (Serène and Soh 1970). Figure 7A-C Geosesarma danumense Ng, 2002:  Colour. Geosesarma danumense has a dark yellow to orange carapace, purple ambulatory legs with scattered white specks, orange merus and carpus of the chelipeds, with the palm and fingers white (Fig. 7A-C). The eggs of the recently collected ovigerous female were observed to be a bright reddish orange in colour and large in size, indicating that the development is probably direct (see Soh 1969;Ng and Tan 1995). The live coloration is very similar to the related G. sabanum Ng, 1992 from Tawau (Fig. 7D). Remarks. Geosesarma danumense and G. sabanum are morphologically close, although the external orbital tooth of the latter species is proportionately more slender, the frontal margin less truncate, the ambulatory meri proportionately shorter, and most significantly the corneous distal part of the G1 is proportionately longer (see Ng 2002).

Geosesarma danumense Ng, 2002
The holotype of G. danumense was obtained from a pitfall trap while the recent large ovigerous female (ZRC 2017.1273) was collected from under a rotting log. Specimens have also been observed climbing small shrubs. In this respect, it probably has similar habits to G. sabanum from Tawau which has been observed by the second author to hide between the leaves of Pandanus sp. during the day, emerging only at night to forage on low lying vegetation and occasionally amongst leaf litter (unpublished data). The terrestrial habits of G. danumense and G. sabanum probably parallel those known for species in Peninsular Malaysia and Indonesian Kalimantan (Ng 2015(Ng , 2017. Manuel-Santos et al. (2016: 336) commented that the three species of Geosesarma on Palawan Island in the Philippines, G. lawrencei Manuel-Santos & Yeo, 2007, G. batak Manuel-Santos, Ng & Freitag, 2016, and G. tagbanuana Manuel-Santos, Ng & Freitag, 2016, are morphologically very similar to the Sabahan G. danumense and G. sabanum, notably in their relative large adult size and long slender ambulatory legs. Their G1 structures, however, are very different, with those of the latter two species proportionately much shorter and stouter (cf. Ng 1992Ng , 2002Manuel-Santos and Yeo 2007;Manuel-Santos et al. 2016).