Geosesarma foxi ( Kemp, 1918 )

Ng, Peter K. L., 2017, On the identities of the highland vampire crabs, Geosesarma foxi (Kemp, 1918) and G. serenei Ng, 1986, with description of a new phytotelmic species from Penang, Peninsular Malaysia (Crustacea: Decapoda: Brachyura: Sesarmidae), Raffles Bulletin of Zoology 65, pp. 226-242 : 227-232

Geosesarma foxi ( Kemp, 1918)

( Figs. 1–5 , 7A–H )

Sesarma foxi Kemp, 1918: 238 , figs. 3, 4. Sesarma (Geosesarma) foxi – Serène, 1968: 107. Geosesarma foxi – Serène & Soh, 1970: 407; Ng & Ng, 1987: 13;

Ng, 1988: 122, fig. 54; Ng, 2004: 333 (list); Ng & Yeo, 2007:

115 (list); Ng et al., 2008: 220 (list).

Material examined. 1 male (11.4 × 11.0 mm) ( ZRC 2016.0598 ), ca. mid-way Tangga Helang Seribu Kenangan hiking trail from base to just before summit of Gunung Raya , Langkawi Island, Peninsular Malaysia, coll. R.-M. Low, 16 July 2016 ; 3 males (11.5 × 11.4 mm, 9.6 × 9.7 mm, 7.8 × 7.7 mm), 3 females (11.8 × 11.9 mm, 7.5 × 7.3 mm, 6.8 × 6.7 mm) ( ZRC 2016.619 ), near start of Tangga Helang Seribu Kenangan hiking trail from below summit of Gunung Raya , Langkawi Island, Peninsular Malaysia, ca. 780 m asl, coll. J.-J. Li, 31 December 2016 ; 4 males (10.9 × 10.9 mm, 9.9 × 10.0 mm, 8.0 × 7.9 mm, 6.6 × 6.7 mm), 1 ovigerous female (11.7 × 11.8 mm), 3 females (10.8 × 10.9 mm, 8.6 × 8.4 mm, 8.2 × 8.1 mm) ( ZRC 2016.618 ), near start of Tangga Helang Seribu Kenangan hiking trail from below summit of Gunung Raya , Langkawi Island, Peninsular Malaysia, ca. 780 m asl, coll. P.Y.C. Ng, 12–13 January 2017 .

Diagnosis. Carapace quadrate, slightly wider than long or subequal, adult width to length ratio 1.00–1.04, lateral margins parallel ( Figs. 1A , 3A–C , 4 ); dorsal surface with regions just visible, anterior regions with small rounded granules on gastric regions ( Figs. 3A–C , 4 ); front distinctly deflexed, frontal lobes broad with gently convex margins in dorsal view; postfrontal, postorbital cristae sharp, distinct ( Figs. 3B–D , 4 ); external orbital tooth triangular, directed obliquely laterally, outer margin gently convex, tip not extending beyond lateral margin; second lateral tooth distinct, clearly separated from rest of margin by wide cleft ( Figs. 3B, C , 4 ). Merus of third maxilliped subovate, shorter than ischium; exopod slender with no trace of flagellum ( Fig. 7A ). Outer surface of palm of adult male covered with small rounded granules and striae; inner surface granulated but without transverse ridge; dorsal margin of dactylus with 7 or 8 low, non-chitinous tubercles on proximal half ( Figs. 1B , 3A , 4A, B , 5D ). Ambulatory legs with long, slender merus, with sharp subdistal spine on dorsal margin, surfaces gently rugose ( Figs. 1A , 3A , 4A, B ). Male pleon broadly triangular; somite 6 wide, with convex lateral margins; telson almost semicircular, not recessed into distal margin of somite 6 ( Fig. 5A–C ). G1 slender; outer margin of subdistal part of subterminal segment (in ventral view) with distinct shelf-like angle ( Fig. 7B–E ), distal chitinous part elongated, distal part gently curved, tip spatuliform ( Fig. 7B–G ).

Females. Females resemble males in all non-sexual characters, with the chelae equal, relatively slenderer and less inflated ( Fig. 4B ). The pleon is ovate, covering most of the thoracic sternum, with the telson broadly triangular with convex lateral margins and distinctly inserted into the distal margin of somite 6 ( Fig. 5E ). The vulvae on thoracic sternite 6 are large, positioned closer to suture with sternite 5, with an operculum and a large truncate vulvar process which arches over the opening ( Fig. 5F ). One female (11.7 × 11.8 mm, ZRC 2016.618) had several subovate eggs still under its pleon, with each measuring ca. 1.5 mm in diameter.

Variation. The carapace proportions vary slightly, with smaller males and females sometimes appearing slightly more trapezoidal in shape, with the lateral margins slightly diverging (e.g., Fig. 4B ). The second lateral tooth (after the external orbital tooth) is always distinct, although it may vary in size ( Figs. 3B, C , 4C, D ), and demarcated from the rest of the margin by a wide cleft. Smaller males (less than 7 mm carapace width) also have slender, equal chelae; and the G1, while resembling those of adults, are relatively straighter and poorly chitinised.

Colour. Kemp (1918: 240) described his fresh specimens of G. foxi as follows: “the carapace of the specimens is of a very dull reddish brown; the chelipedes are pale yellow suffused with pale red on the carpus and palm; the walking legs are deep brown with a fine mottling and dark chromatophores are thickly sprinkled on the abdomen.” The series of specimens on hand essentially confirm his observations. In live specimens, the anterior half of the carapace (from the H-shaped gastrocardiac groove) as well as the chelipeds are deep orange to almost red ( Fig. 2 ). The intensity of the colour varies only slightly. The posterior half of the carapace and ambulatory legs vary from light brown to purplish black, and covered with numerous small white or light blue spots ( Fig. 2 ). The frontal surfaces of the cephalothorax and thoracic sternum are deep orange ( Fig. 2B ), while the abdomen is orange with numerous small dark brown spots and mottling.

Taxonomic remarks. Kemp (1918: 240) described the species from two males measuring 9.8 × 9.8 mm and 9.7 × 9.8 mm, without specifying a holotype. Ng (1988: 122) designated the slightly larger male in the Zoological Survey of India in Calcutta as the lectotype of Sesarma foxi Kemp, 1918 . For comparisons with G. serenei and G. faustum n sp., see remarks for the latter species.

Sympatric taxa. Three other species of freshwater crabs occur on Gunung Raya. The semiterrestrial potamid Stoliczia bella Ng & Ng, 1987 , is found in or near permanent water bodies from near the base to about 700 m asl on Gunung Raya. At the base of Gunung Raya, the fully aquatic gecarcinucid Siamthelphusa improvisa ( Lanchester, 1902) is common in shallow streams with vegetation. The gecarcinucid Phricotelphusa gracilipes Ng & Ng, 1987 , is found in most of the streams and springs around the mountain, and even occurs in the forest floor and summit, sometimes far from permanent water sources. The author has also found it in piles of moist decaying leaves along the same trail where G. foxi was collected.

Figures

Fig. 1. Geosesarma foxi (Kemp, 1918), lectotype male (9.8 × 9.8 mm), Gunung Raya, Langkawi (after Kemp, 1918: figs. 3, 4). A, overall dorsal view; B, outer view of left chela.

Fig. 2. Geosesarma foxi (Kemp, 1918), colour in life, Gunung Raya, Langkawi. A, B, male (10.9 × 10.9 mm) (ZRC 2016.618); C, male (9.9 × 10.0 mm) (ZRC 2016.618); D, female (10.8 × 10.9 mm) (ZRC 2016.618); E, juvenile male (8.0 × 7.9 mm) (ZRC 2016.618); F, male (11.5 × 11.4 mm) (ZRC 2016.619); G, probably male (9.6 × 9.7 mm) (ZRC 2016.619). Photo credits: A–E, Paul Y.C. Ng; F, G, J.-J. Li.

Fig. 3. Geosesarma foxi (Kemp, 1918), male (11.4 × 11.0 mm) (ZRC 2016.0598), Gunung Raya, Langkawi. A, overall dorsal view; B, C, dorsal view of carapace (viewed from slightly different angles); D, frontal view of cephalothorax.

Fig. 4. Geosesarma foxi (Kemp, 1918). A, C, male (9.9 × 10.0 mm) (ZRC 2016.618); B, D, female (10.8 × 10.9 mm) (ZRC 2016.618). Both specimens from Gunung Raya, Langkawi. A, B, overall dorsal view; C, D, dorsal view of carapace.

Fig. 5. Geosesarma foxi (Kemp, 1918). A–D, male (11.4 × 11.0 mm) (ZRC 2016.0598); E, F, female (10.8 × 10.9 mm) (ZRC 2016.618). Both specimens from Gunung Raya, Langkawi. A, anterior thoracic sternum, telson and pleonal somites 4–6; B, anterior thoracic sternum, telson and pleonal somites 3–6; C, posterior thoracic sternum, telson and pleonal somites 2–6; D, outer view of right chela; E, female pleon; F, female sternopleonal cavity showing vulvae.

Fig. 7. A–H, Geosesarma foxi (Kemp, 1918), male (11.4 × 11.0 mm) (2016.0598), Gunung Raya, Langkawi; I–M, G. serenei Ng, 1986, holotype male (8.3 × 8.2 mm) (ZRC 1964.9.8.1), Bukit Larut, Perak; N–P, G. serenei Ng, 1986, young male (7.0 × 6.9 mm) (ZRC 2003.64), Bukit Larut. A, left third maxilliped (mostly denuded); B, I, N, left G1 (ventral view); C, J, left G1 (dorsal view); D, distal part of left G1 (ventral view); E, distal part of left G1 (dorsal view); F, K, O, distal part of left G1 (outer lateral view); G, L, P, distal part of left G1 (mesial view); H, M, left G2. Scales = 0.5 mm.