Arachnothelphusa terrapes Ng, 1991

Ng, Peter K. L. & Ng, Paul Y. C., 2018, 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), ZooKeys 760, pp. 89-112 : 89

Arachnothelphusa terrapes Ng, 1991 Figure 5

Arachnothelphusa terrapes Ng, 1991: 8, figs 3-6; Ng et al. 2008: 69.

Material examined.

Holotype: male (17.6 × 13.3 mm) (ZRC 1992.7918), Danum Valley Field Centre, station 507, in dry stump on ridge, Lahad Datu, Sabah, Borneo, leg. H.K. Voris, 23 October 1990. Paratype: female (25.7 × 18.6 mm) (ZRC 1992.7919), Danum Valley, Lahad Datu, Sabah, Borneo, leg. S.C. Choy, 21 July 1989. Others: 1 male (30.8 × 20.5 mm), 1 female (30.1 × 20.5 mm, with 26 juvenile crabs) (ZRC 2017.1205), from water-filled tree buttress, ca. 35 cm above ground Danum Valley, Lahad Datu, Sabah, Borneo, Malaysia, 20 July 2017.

Comparative material.

Arachnothelphusa kadamaiana (Borradaile, 1900): 1 female (23.2 × 17.1 mm) (ZRC 2009.0094), Poring, Basin 1A, Sabah, Malaysia, Borneo, coll. R.F. Inger et al., 12 August 1992; 3 males (21.1 × 15.8 mm, 22.8 × 16.5 mm, 25.3 × 18.5 mm) (ZRC 2002.0097), Crocker Range, Sabah, 5°27'N 116°03'E, coll. I. Das, 24 April 2001. Arachnothelphusa aff. kadamaiana : 1 female (19.0 × 14.2 mm) (ZRC 2002.0098), Bako National Park, Sarawak, coll. I. Das and L. Grismer, 27 March 2001. Arachnothelphusa merarapensis Grinang, Pui & Ng, 2015: Holotype male (22.5 × 16.8 mm) (ZRC 2016.0297), water-filled tree-hole, ca. 100 cm above ground, steep dipterocarp forest, Merarap Hot Spring Resort, Lawas, northern Sarawak, Malaysia, Borneo, 4°22'25.4"N, 115°26'10.1"E, 485 m asl, coll. J. Grinang and Y.M. Pui, 31 October 2014.

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. 5 B–F). The dark purple colouration appears considerably darker when the animal is dry, explaining the original paler colour observation by Ng (1991).

Remarks.

Ng (1991) established Arachnothelphsua for several Bornean species previously classified as Thelphusula Bott, 1969, with Potamon (Potamon) melanippe De Man, 1899, as the type species. Currently, four other species are recognised: A. kadamaiana (Borradaile, 1900), A. rhadamanthysi (Ng & Goh, 1987), A. terrapes Ng, 1991, and A. merarapensis Grinang, Pui & Ng, 2015, all from northern Borneo. One species originally included by Ng (1991) in Arachnothelphusa , Parathelphusa (Liotelphusa) nobilii Colosi, 1920, was transferred to Stygothelphusa Ng, 1989, by Ng and Álvarez (2000) (see also Ng 2013; Ng and Grinang 2014).

Arachnothelphusa terrapes is easily distinguished from congeners by the deep U-shaped 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 ( Grinang et al. 2015: fig. 1A, B). Other congeners have the epibranchial tooth separated by a V-shaped notch or the margin is almost entire (De Man 1899: pl. 9; Ng and Goh 1987: pl. 3A; Ng 1991: fig. 1A; Grinang et al. 2015: fig. 6A).

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). Grinang et al. (2015) 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 Grinang et al. (2015) 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), Grinang et al. (2015), Ng et al. (2015), 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, and Scandarma 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.