Salangathelphusa peractio, a new species of lowland freshwater crab from Pulau Langkawi, Peninsular Malaysia (Crustacea, Brachyura, Gecarcinucidae)

Abstract A new species of lowland freshwater crab of the family Gecarcinucidae, Salangathelphusa peractio, is described from Langkawi, an island off the northwestern coast of peninsular Malaysia. Salangathelphusa peractio sp. n. can be separated from S. brevicarinata (Hilgendorf, 1882) in having a proportionately broader external orbital tooth, a distinctly concave posterolateral margin, and the terminal segment of the male first gonopod is not distinctly bent laterally outwards; and from S. anophrys (Kemp, 1923) by its more quadrate carapace and the terminal segment of the male first gonopod possessing a relatively longer and less curved distal part. This is sixth wholly freshwater brachyuran species known from the island.

Salangathelphusa can easily be distinguished from all other southeast Asian gecarcinucids by possessing the following combination of characters: four teeth on its anterolateral margin (including the external orbital tooth); a dorsal carapace surface which is smooth with the postorbital cristae barely visible or absent; a male first gonopod which is very short and stout, with the terminal and subterminal segments clearly demarcated and a short terminal segment which has the basal part dilated and the distal part sharply tapering; and the male second gonopod has a long distal segment which is longer than half the length of the basal segment (Bott 1968(Bott , 1970Ng 1988Ng , 2004. Bott (1970: 108) synonymised Paratelphusa (Paratelphusa) anophrys Kemp, 1923, with S. brevicarinata without any comment. Ng et al. (2008: 71), however, listed S. anophrys (Kemp, 1923) as a valid species of Salangathelphusa in their synopsis of the world Brachyura but did not elaborate. The genus Salangathelphusa will need to be revised as there are clearly more than the two recognised species. The author (with Darren Yeo) have examined the types of Parathelphusa salangensis Ortmann, 1893, and S. anophrys Kemp, 1923 (the type of Parathelphusa brevicarinata Hilgendorf, 1882, is no longer extant), as well as material from various parts of southern Thailand and northern Malaysia. It is clear that what is now called "S. brevicarinata" is a species complex. More material is currently being consolidated to present a more complete revision in subsequent years.
In 2015, the author examined old collections collected from the northwestern peninsular Malaysian island of Langkawi that had been collected during an expedition there by the Malayan Nature Society in 2003 (see Ahmad and Lim 2006). Among the unidentified material were two specimens of Salangathelphusa. Subsequent collections in Langkawi obtained more specimens of the same species, which can easily be distinguished from S. brevicarinata and S. anophrys by its distinctive male first gonopod. The present note has been prepared to make the name of this new species from Langkawi available for other studies (including conservation) because the revision of Salangathelphusa is still some years in the future. This present discovery of another freshwater crab species from Langkawi is somewhat surprising because the fauna of the island has been well studied. Currently four primary freshwater crab species are known -one species of Potamidae: Stoliczia bella Ng & Ng, 1987), and three species of Gecarcinucidae: Sayamia sexpunctata (Lanchester, 1906), Phricotelphusa gracilipes Ng & Ng, 1987, and Siamthelphua improvisa (Lanchester, 1902) Ng 1987, 1989;Ng 1988Ng , 2004). In addition, there is an endemic species of wholly freshwater sesarmid, Geosesarma foxi (Kemp, 1918) in the highlands of Langkawi Island (Ng 2017).

Materials and methods
Specimens examined are deposited in the Zoological Reference Collection (ZRC) of the Lee Kong Chian Natural History Museum, National University of Singapore; and the Zoological Survey of India (ex Indian Museum), Calcutta, India. Measurements, in millimetres, are of the maximum carapace width and length, respectively; while the abbreviations G1 and G2 are used for the male first and second gonopods, respectively. The terminology used follows that in Ng (1988) and Davie et al. (2015).
Chelipeds subequal, outer surface of merus, carpus and palm rugose; palm of right chela slightly larger; fingers not gaping, longer than palm, tips gently hooked, cutting edges without molariform teeth; merus short, stout, surface rugose with distinct subdistal tubercle on dorsal margin; carpus with strong, obliquely directed subdistal spine on inner margin; merus with low subterminal spine (Figs 1, 2B).
G1 relatively short, stout; subterminal segment proportionately stout, gradually tapering towards distal half, outer margin gently sinuous; terminal segment less than half length of subterminal segment, outer margin convex, rounded tip directed upwards towards buccal cavity, inner basal part swollen, much wider than distal half, entire structure gently twisted towards sternal surface (Fig. 3A-E). G2 longer than G1; distal segment with long flagellum, ca. 0.6 times length of elongate basal segment (Fig. 3F).   Females. Adult females closely resemble adult males except that the chelae are relatively more slender. The adult female pleon is ovate and covers most of the thoracic sternum (Fig. 4D), with the vulvae on the anterior half of sternite 6 being large, ovate, and possessing a prominent operculum (Fig. 4E).
Variation. Smaller specimens (ca. 15 mm carapace width and below) have relatively more quadrate carapaces (width to length ratio 1.13-1.17), the merus of the ambulatory leg has a small dorsal subdistal spine and the male pleonal somite 6 is proportionately more trapezoidal in shape (Fig. 4A, B).
Colour. In life, Salangathelphusa peractio sp. n. is light brown to orange on all its dorsal surfaces; the dorsal surface of the carapace has large reddish-brown spots and markings at or near the cervical and gastro-cardiac grooves; and the chelipeds and am- Figure 6. Salangathelphusa anophrys (Kemp, 1923), male (26.9 × 20.7 mm) (ZRC 1989(ZRC .2011, Trang, Thailand. A overall habitus B dorsal view of carapace C male anterior thoracic sternum and pleon. bulatory legs have numerous small reddish-brown spots ( Fig. 5A-C, E). The fingers of the chela are dark orange and the ventral surfaces of the cephalothorax white (Fig. 5D).
Etymology. The species name is derived from the Latin word "peractio" which means "ending of a story". It alludes to the discovery of the present freshwater species, arguably the last one the author will describe from Langkawi, ending his 30-year history with the island. Gender feminine.
Remarks. Salangathelphusa peractio sp. n. can easily be separated from S. brevicarinata in that its external orbital tooth is proportionately broader (Fig. 1A, B) (external orbital tooth more acutely triangular in S. brevicarinata; cf. Bott 1970: pl. 20 fig. 33); the posterolateral margin is distinctly concave (Fig. 1A, B) (posterolateral margin gently concave to almost straight in S. brevicarinata; cf. Bott 1970: pl. 20 fig. 33); and the subterminal segment of the G1 is proportionately stouter with the distal half less slender  (Kemp, 1923), male (26.9 × 20.7 mm) (ZRC 1989(ZRC .2011, Trang, Thailand. A left G1 (ventral view) B left G1 (dorsal view) C left G2. Scale bars 1.0 mm and the terminal segment is not distinctly bent laterally outwards with the distal part directed upwards towards the buccal cavity (Fig. 3A, B, C, E) (G1 subterminal segment more slender along distal half with the terminal segment bent inwards and the distal part directed obliquely laterally in S. brevicarinata; cf. Bott 1970: pl. 30 fig. 78). From S. anophrys, S. peractio can be separated by its relatively more quadrate carapace (Fig. 1A, B) (carapace proportionately wider in S. anophrys; cf. Fig. 6A , B;Kemp 1923: pl. 4 fig. 10); and most significantly, the subterminal segment of the G1 is proportionately less stout and the terminal segment has the distal part relatively longer and less curved (Fig. 3A, B, C, E) (G1 subterminal segment stouter along entire length with the distal part of the terminal segment shorter and more strongly curved in S. anophrys; Fig. 7A, B).
Salangathelphusa peractio is known thus far only from southern streams at the base of Gunung Raya, the highest peak on Langkawi. Its distribution appears to be localised, being confined to shallow streams with fast flowing water, the substrate of the stream bed and banks being rocks of various sizes. The gecarcinucid Siamthelphusa improvisa was sometimes found together with Salangathelphusa peractio, but the for-mer species prefers areas with dense underwater vegetation and larger rocks. At Lubuk Semilang Park, Salangathelphusa peractio was found only in a small area a few hundred square metres, although there are several other similar areas with similar habitats which were not accessible. The park is not a protected area and is used by the public for all manner of recreational activities which partially pollute the area as well as causing substantial disturbance to the overall habitat. How these impacts affect the crabs is not known. Unfortunately, the species is not found in any fully protected site. The restricted distribution and potential habitat impacts means that the species should perhaps be regarded as vulnerable under current conservation guidelines (see Ng and Yeo 2007;Cumberlidge et al. 2009). Its precise status will need more surveys and studies on how habitat changes will affect its population.