Terrapotamon longitarsus, Lheknim, Vachira & Ng, Peter K. L., 2016

Lheknim, Vachira & Ng, Peter K. L., 2016, A new species of long-legged terrestrial Terrapotamon Ng, 1986 (Crustacea: Brachyura: Potamidae) from limestone formations in Satun, southern Thailand, Zootaxa 4200 (1), pp. 143-152 : 144-151

Terrapotamon longitarsus n. sp.

( Figs. 1–5 )

Material examined. Holotype: male (47.4 × 38.6 mm) ( ZRC 2016.0161 ), found dead on limestone bedrock at Khao Raya Bung Sa, southernmost of Nakhon Si Thammarat mountain range, 30 m above mean sea level, Tambon Khuan Po, Muang Satun District , Satun Province, coll. V. Lheknim & P. Leelawathanagoon, 9 September 1999 . Paratypes: 1 male (40.8 × 32.5 mm) (PSUZC 20150726-01.01), limestone rock crevice, near locality of holotype, coll. R. Promdam & R. Engchoun, 26 July 2015; 1 male (28.4 × 23.8 mm) ( PSUZC 20150611-01.01 ), limestone rock crevice same locality as holotype, coll. R. Promdam & R. Engchoun, 11 June 2015 . Others: 1 male (35.0 × 29.5 mm), 1 female (23.8 × 20.2 mm) ( ZRC 2016.0162 ), Satun Province, coll. Y. Pun & S. Buatip, 20 May 2016 . All locations in southernmost part of Nakhon Si Thammarat mountain range, southern Thailand.

Comparative material. Terrapotamon abbotti ( Rathbun, 1898) : holotype male ( USNM 20641 ), Trang, southern Thailand, coll. W. L. Abbott ; 1 male, 1 female ( ZRC 1984.7717-7718 ), Surat Thani, ca. 8°25’N 99°58’E, coll. P. Naiyanetr, 16 September 1983 ; 3 males, 1 female ( ZRC 2012.683 ), Amphoe Khiri, Ratthanikhom, Bon Baeng Bap , southern Thailand, Surat Thani, coll. T. Sisuchat & D. Kuntisit, 16–19 July 1983 ; 4 males (36.7 × 30.0 – 46.6 × 35.8 mm), 3 females (33.6 × 27.8 – 40.0 × 32.7 mm) (PSUZC- 20021117 -02.01), wetlands along Huey Chan Daeng, Klong Chi sub-basin of Trang River Basin , in the vicinity of Roi Chan Pun Wang Ranger Station , 7.8875°N 99.325°E , 150 m above mean sea level, Wang Wiset District , Trang Province, coll. V. Lheknim & P. Leelawathanagoon, 17 November 2002 ; 2 males ( ZRC 1995.420 ), 1 male, 1 female ( ZRC 1995.421 ), 2 males (larger 46.6 × 37.5 mm) ( ZRC 1995.422 ), southern Thailand, coll. aquarium dealers, November 1993 ; 4 males ( ZRC 2001.771 ), 2 males ( ZRC 2001.772 ), 3 males ( ZRC 2001.773 ), 1 male ( ZRC 2012.1066 ), southern Thailand, coll. aquarium dealers, 31 October 2001 ; 2 males ( ZRC 2011.160 ), 1 male ( ZRC 2011.237 ), 1 female ( ZRC 2012.833 ), 1 male, 1 female ( ZRC 2012.1110 ), southern Thailand, coll. aquarium dealers, November 2001 . Terrapotamon palian Ng & Naiyanetr, 1998 : Holotype male (24.6 × 20.5 mm) ( ZRC 2012.685 ), Ton Tok Waterfall, Palian District , Trang Province, southern Thailand, coll. P. Naiyanetr, 21 May 1976 ; 2 paratype females (larger 24.9 × 20.5 mm) ( ZRC 2012.684 ), 5 other females ( CUMZ), same data as holotype ; 4 males (20.3 × 17.4 – 34.3 × 26.7 mm), 3 females (23.2 × 19.3 – 30.3 × 24.9 mm) (PSUZC-20020608-02.08), Ton Tok Waterfall, Klong Palian Basin , Ban Mai Dam , 7.28233°N 99.8845°E, 110 m above mean sea level, Palian District , Trang Province, coll. V. Lheknim & P. Leelawathanagoon, 8 June 2002 . Terrapotamon phaibuli Leelawathanagoon, Lheknim & Ng, 2010 : holotype: male (57.8 × 45.8 mm) ( PSUZC 20080528-02.01 ), fringing forest and rubber plantations at the upper reach of Klong Glay, Klong Glay Basin, Ban Pak Long , 200 above mean sea level, 8.79417°N 99.57767°E, Nopphitam District , Nakhon Si Thammarat Province, coll. V. Lheknim & P. Leelawathanagoon, 28 May 2008 ; paratypes: 1 male (27.3 × 22.8 mm), 1 female (44.5 × 35.2 mm), 6 juveniles (9.0 × 7.8 – 18.0 × 15.2 mm) (PSUZC- 20010930-06.10), fringing forest and rubber plantations at unnamed stream in the upper reach of Klong Glay, Klong Glay Basin, 8.732°N 99.65217°E, 260 m above mean sea level, Nopphitam District , Nakhon Si Thammarat Province, coll. V. Lheknim & P. Leelawathanagoon, 30 September 2001 ; 4 males (15.2 × 12.8 – 42.3 × 33.8 mm), 4 females (26.0 × 21.8 – 34.1 × 28.2 mm) (PSUZC-20020615-02.01), same locality as above, coll. V. Lheknim & P. Leelawathanagoon, 15 June 2002 ; 9 females (8.7 × 7.8–28.5 × 22.7 mm), 3 juveniles (PSUZC-20020423-03.06), same locality as above, coll. V. Lheknim & P. Leelawathanagoon, 23 April 2002 ; 8 males (20.4 × 17.4 – 56.9 × 46.4 mm), 1 female (25.8 × 21.7 mm) (PSUZC-20020615-01.02); 4 males (16.0 × 13.8 – 20.0 × 17.6 mm), 6 females (10.0 × 8.8 – 22.0 × 19.4 mm), 2 juveniles (PSUZC-20020615-01.03); 2 males (34.4 × 28.6 mm, 45.8 × 37.9 mm), 2 females (30.3 × 24.8 mm, 33.8 × 27.5 mm) ( ZRC), same locality as holotype, coll. V. Lheknim & P. Leelawathanagoon, 15 June 2002 ; 2 males (35.3 × 29.5 mm, 42.9 × 35.4 mm), 3 females (12.6 × 11.0 – 45.5 × 37.3 mm) (PSUZC-20080528-02.02), same locality as holotype, coll. V. Lheknim & P. Leelawathanagoon, 28 May 2008 ; 1 male (44.5 × 38.0 mm), 3 females (40.8 × 31.0–47.3 × 36.5 mm) (PSUZC-20020423-01.12), rubber plantations at upper reach of Klong Glay, Klong Glay Basin , Khao Nan National Pak Ranger Station, 8.80333°N 99.57367°E , 150 m above mean sea level, Nopphitam District , Nakhon Si Thammarat Province, coll. V. Lheknim & P. Leelawathanagoon, 23 April 2002 ; 12 juveniles (7.5 × 6.4 – 20.5 × 17.5 mm) (PSUZC-20080528-01.03), Huey Tong Yang, Ao Tong Yang, Sichon , Nakhon Si Thammarat, coll. V. Lheknim & P. Leelawathanagoon, 28 May 2008 . Non types: 1 juvenile male (PSUZC-20090809-03.06) along Huay Lek stream adjacent to dirt road at Ban Tab Nam Tao, Klong Glay Basin , 8.83107°N 99.6111°E , 151 m above mean sea level, Nopphitam District , Nakhon Si Thammarat Province, coll. V. Lheknim & P. Leelawathanagoon, 9 August 2009; 3 juvenile males (PSUZC-20090809-04.06) along Klong Pud stream adjacent to dirt road at Ban Pak Long, Klong Glay Basin, 8.79597°N 99.57705°E , 161 m above mean sea level, Nopphitam District , Nakhon Si Thammarat Province, coll. V. Lheknim & P. Leelawathanagoon, 9 August 2009 ; 2 males (15.5 × 14.0 mm, 40.0 × 33.5 mm) (PSUZC-20090930- 03.05), upper stream bank of Klong Khao Khae, Chong Kun Bed, Ban Khao Kae , Eastern River Basin of south Thailand, 9.20008°N 99.7291°E , 60 m above mean sea level, Don Sak District , Surat Thani Province, coll. V. Lheknim & P. Leelawathanagoon, 30 September 2009; 2 males (30.5 × 25.0 mm, 42.5 × 34.0 mm), 1 female (34.0 × 28.0 mm) (PSUZC- 20091002 -01.01) , 50 m from the stream bank, beneath rock in fruit orchards, Ban Khao Noi, Tapi River Basin, 8.88257°N, 99.56302°E , 386 m above mean sea level, Ban Na San District , Surat Thani Province, coll. V. Lheknim & P. Leelawathanagoon, 2 October 2009 .

Diagnosis. Carapace transversely ovate; dorsal surfaces mostly smooth, gently convex, regions poorly demarcated with grooves shallow ( Figs. 1 A, 3, 4A); H-shaped gastro-cardiac groove relatively shallow ( Figs. 1 A, 3, 4A); region adjacent to anterolateral margin with low, flattened granules; region adjacent to posterolateral region with low, short, oblique striae ( Figs. 1 A–C, 3, 4A, B); external orbital tooth acutely triangular; anterolateral margin entire, cristate, lined with granules, appears serrated; posterolateral margin strongly converging to convex posterior carapace margin ( Figs. 1 A–C, 3, 4A, B); epigastric cristae prominent, rugose, not sharp; postorbital cristae low rugose; epigastric cristae slightly anterior of postorbital cristae ( Figs. 1 A–C, 3, 4A, B); sub-orbital region with low flattened granules to almost smooth; pterygostomial region with outer part rugose to granular, inner part smoother; sub-branchial region covered with short rows of distinct few low granules ( Figs. 1 B, C, 4B); suborbital margin distinctly cristate ( Figs. 1 B, C, 4B); third maxillipeds with ischium subrectangular, long, exopod with short flagellum that is about width of merus ( Figs. 2 A, 4C); cheliped elongated in adult male ( Figs. 1 A, 2D); ambulatory legs very long, slender ( Figs. 1 A, 2C, 3); thoracic sternum relatively narrow ( Figs. 1 D, 2B, 4D); male pleon triangular, telson triangular with gently sinuous to almost straight lateral margin ( Figs. 1 D, 2B, 4E); G1 relatively stout, subterminal segment with outer subdistal part prominently concave, distal part dilated, swollen; terminal segment short, basal part dilated, rest of structure almost straight to slightly curving upwards; outer junction between terminal, subterminal segments appears bilobed ( Fig. 5 A–D, F–I); G2 longer than G1, distal segment about two-thirds length of basal segment ( Fig. 5 E).

Description. Carapace transversely ovate, wider than long; dorsal surfaces mostly smooth, gently convex, regions poorly demarcated with grooves shallow ( Figs. 1 A, 3, 4A); H-shaped gastro-cardiac groove relatively shallow, discernible (more prominent in smaller specimens, Figs. 1 A, 3, 4A). Frontal region gently rugose, covered by small flattened granules; epigastric, protogastric regions rugose; region adjacent to anterolateral margin with low, flattened granules; region adjacent to posterolateral region with low, short, oblique striae ( Figs. 1 A–C, 3, 4A, B). Frontal margin gently deflexed, confluent with supra-orbital margin; external orbital tooth acutely triangular, outer margin with small granules, slightly longer than inner margin, separated from rest of anterolateral margin by deep V-shaped notch; epibranchial tooth low but distinct; anterolateral margin entire, distinctly convex, cristate, lined with granules, appears serrated; posterolateral margin sinuous, strongly converging to convex posterior carapace margin ( Figs. 1 A–C, 3, 4A, B). Supraorbital margin sinuous, with low, flattened submedian granule ( Figs. 1 A–C, 3, 4B). Epigastric cristae prominent, rugose, not sharp, separated by deep narrow Y-shaped fissure; postorbital cristae low but distinct, granulated, rugose; epigastric cristae slightly anterior of postorbital cristae, separated from postorbital cristae by short oblique groove ( Figs. 1 A–C, 3, 4A, B). Orbits large; ocular peduncle stout, cornea large, well-developed ( Figs. 1 A–C, 3, 4B). Sub-orbital region with low flattened granules or almost smooth; pterygostomial region with outer part rugose to granular, inner part smoother; sub-branchial region covered with distinct low granules ( Figs. 1 B, C, 4B). Suborbital margin distinctly cristate, uneven or entire but not granulated (outer part of right margin in holotype dentate) ( Figs. 1 B, C, 4B). Epistome transversely rectangular, longitudinally relatively narrow; posterior margin cristate, lined with rounded granules, median part triangular, lateral parts gently concave ( Figs. 1 B, 4B). Mandibular palp simple, 3-segmented, terminal segment single-lobed.

Third maxillipeds relatively long longitudinally; ischium subrectangular, surface punctate, with deep median groove that extends almost entire length; merus subquadrate, anterolateral margin rounded, not auriculiform; exopod long, slender, with short flagellum that is about width of merus ( Figs. 2 A, 4C).

Chelipeds unequal in adult male, elongated in largest holotype male ( Figs. 1 A, 2D). Major chela basis-ischium with ventral margin lined with granules; merus long, ventral margin and adjacent areas with sharp granules, dorsal margin uneven; carpus with outer surface rugose, dorsal surface with distinct broad longitudinal groove, inner distal margin with long, sharp tooth and basal tubercle; chela long, relatively slender, outer surface, margins rugose; fingers gently curved, shorter than palm, with distinct large and small rounded teeth, forming narrow gape when closed ( Figs. 1 A, 2D, E). Minor chela similar to major chela except relatively smaller, more slender ( Fig. 1 A).

Ambulatory legs very long, slender; second leg longest; last leg shortest, surfaces of all segments gently rugose ( Figs. 1 A, 2C, 3). Merus laterally flattened, dorsal margin slightly cristate, uneven, may appear slightly serrated, without dorsal subterminal tooth; carpus long, with distinct low, longitudinal submedian ridge on outer surface; propodus with prominent median groove on outer surface; dactylus slender, gently curved, margin with sharp spines, scattered setae ( Figs. 1 A, 2C)

Thoracic sternum relatively narrow transversely, surface punctate; sternites 1, 2 completely fused, forming broad triangular plate with distinctly convex lateral margins, separated from sternite 3 by distinct gently concave (towards buccal cavity) suture; sternites 3, 4 fused, with shallow oblique depression demarcating sternites; sternopleonal cavity deep, reaching to imaginary line joining median points of coxae of chelipeds ( Figs. 1 D, 2B, 4D). Tubercle of pleonal locking mechanism relatively low, rounded, positioned on median part of somite 5, near edge of sternopleonal cavity.

Male pleon triangular; somite 1 curved, longitudinally narrow, reaching to bases of coxae of last ambulatory legs; somite 2 transversely subovate, lateral margin convex, reaching to bases of last ambulatory legs; somite 3 trapezoidal with gently convex lateral margin, entirely covering sternite 8 when closed; somites 4–6 trapezoidal with gently convex to sinuous lateral margin, somite 6 longitudinally longest; telson triangular with gently sinuous to almost straight lateral margin ( Figs. 1 D, 2B, 4E).

Male gonopore coxal. G1 relatively stout, appears gently sinuous; subterminal segment with outer subdistal part prominently concave, distal part dilated, swollen; terminal segment relatively short, basal part dilated, rest of structure almost straight to slightly curving upwards; outer junction between terminal, subterminal segments appears bilobed, more prominent in smaller specimens ( Fig. 5 A–D, F–I). G2 longer than G1; distal segment elongated, curved, about two-thirds length of basal segment ( Fig. 5 E).

Variation. The dorsal surface of the carapace appears smoother and more convex in the largest specimen (the holotype male), with the branchial regions relatively more inflated ( Fig. 1 A). In smaller specimens, the surfaces adjacent to the anterolateral margin in particular are more extensively and prominently granulated with the striae stronger, the postfrontal cristae are more granulated, the epigastric cristae are more distinctly rugose ( Fig. 3 ). The proportions of the ambulatory legs do not appear to vary substantially, although the chelipeds of the largest holotype male is relatively the longest ( Fig. 1 A). The G1 structure varies with age. The smallest male examined (28.4 × 23.8 mm, PSUZC 20150611-01.01) has the terminal segment relatively longer and still gently curving outwards ( Fig. 5 F, G) like in T. abbotti ; but as it gets larger (35.0 × 29.5 mm, ZRC 2016.0162), the terminal segment starts to gently curve upwards ( Fig. 5 H, I). As the specimens get larger, the degree of dilation on the distal part of the subterminal segment also increases, being most swollen in the largest male ( Fig. 5 A–D).

Etymology. The name, used as a noun in apposition, alludes to the long ambulatory legs of the species.

Remarks. The most obvious difference between T. longitarsus n. sp., and all congeners is its conspicuously longer ambulatory legs and adult male chelipeds ( Figs. 1 A, 2C, 3) (cf. Ng 1986: fig. 3a; Ng 1988: fig. 37A; Ng & Tan 1996: figs. 3, 6, 9; Ng & Naiyanetr 1998: fig. 1B, 2A; Leelawathanagoon et al. 2010: fig. 2A). In addition, the G1 terminal segment of T. abbotti and T. phaibuli is gently curved outwards (cf. Ng 1986: fig. 2a, b; Ng 1988: figs. 37D–F; Ng & Tan 1996: fig. 1; Ng 2004: fig. 8I; Leelawathanagoon et al. 2010: fig. 1F–I) whilst that of T. longitarsus n. sp., is almost straight to gently curving upwards ( Fig. 5 A–D, F–I). In this regard, the adult G1 terminal segment of T. longitarsus n. sp., more closely resembles that of T. palian (see Ng & Naiyanetr 1998: fig. 1F-I). In T. palian , however, the base of the terminal segment joins the swollen distal part of the subterminal segment smoothly, with the lateral margin of the junction evenly convex (Ng & Naiyanetr 1998: fig. 1F–I). In T. longitarsus n. sp., the outer margin of the junction between the G1 terminal and subterminal segments is not smooth and confluent, giving it a distinct bilobed appearance ( Fig. 5 A–D), which is apparent even in smaller male specimens ( Fig. 5 F–I).

The carapace of T. longitarsus n. sp., is more transversely ovate with the posterolateral margins more prominently converging towards the posterior carapace margin ( Figs. 1 A, 3, 4A) (carapace more rounded with the posterolateral margins more gently converging in congeners, cf. Ng 1988: fig. 37A; Ng & Tan 1996: figs. 3, 6, 9; Ng & Naiyanetr 1998: fig.1A; Leelawathanagoon et al. 2010: figs. 1A, 2A); and the third maxilliped of the holotype male, particularly the ischium, is proportionately more elongated ( Figs. 2 A, 4C) (distinctly shorter in congeners, cf. Ng 1988: fig. 37c; Ng & Naiyanetr 1998: fig. 1C; Leelawathanagoon et al. 2010: fig. 1C). The anterior thoracic sternum (sternites 1–4) of T. longitarsus n. sp., ( Figs. 1 D, 4D) is similar in form to that of T. phaibuli (cf. Leelawathanagoon et al. 2010: figs. 1D, 2C) but is relatively narrower transversely compared to T. abbotti and T. palian (cf. Ng 1986: 3b; Ng & Tan 1996: figs. 4, 7, 8; Ng & Naiyanetr 1998: fig. 1D).

The discovery of T. longitarsus n. sp., parallels the trend seen in many terrestrial and semiterrestrial crabs, in which one species in the genus, whose members typically have relatively short ambulatory legs, have evolved longer pereopods; presumably to be better and/or more agile climbers on rocks or trees (e.g., see Schubart & Ng 2009; Ng & Tay 2001).

Terrapotamon longitarsus n. sp., is the only species in the genus known from limestone formations in the Satun area. It appears to be a fully terrestrial species living among the crevices, rocks and caves in the forest karst landscape. The other Terrapotamon species are from more typical forest habitats, digging deep burrows in the soil (Ng & Tan 1996). The habits of T. longitarsus n. sp., are probably like those of the Japanese potamid Geothelphusa tenuimanus (Miyake & Minei, 1965) from Okinawa which lives in karst habitats (see Gima & Shokita 1980; Yoshigou 1999; Naruse et al. 2004). These karst potamids appear to share the same niche as the long-legged karst gecarcinids Discoplax longipes A. Milne-Edwards, 1867 , D. gracilipes Ng & Guinot, 2001 , and D. michalis Ng & Shih, 2015 from the western Pacific (see Ng & Guinot 2001; Ng & Shih 2015). These gecarcinids, although primarily associated with karst habitats, differ from potamids in that they still need to return to the sea to release their planktonic larvae. Potamids have large eggs and possess direct development, with the eggs hatching into juvenile crabs. The behaviour and habits of adult gecarcinids, however, are probably similar to terrestrial potamid crabs like Terrapotamon .

Figures

FIGURE 1. Terrapotamon longitarsus n. sp. Holotype male (47.4 × 38.6 mm) (ZRC 2016.0161). A, overall habitus; B, C, frontal views of cephalothorax; D, ventral view of cephalothorax showing third maxillipeds, anterior thoracic sternum and pleon.

FIGURE 2. Terrapotamon longitarsus n. sp. Holotype male (47.4 × 38.6 mm) (ZRC 2016.0161). A, right third maxilliped; B, ventral view of cephalothorax showing posterior thoracic sternum and pleon (right margin of somite 3 damaged); C, right ambulatory legs; D, dorsal view of right cheliped; E, outer view of right chela.

FIGURE 3. Terrapotamon longitarsus n. sp. Overall habitus. A, male (35.0 × 29.5 mm) (ZRC 2016.0162 a); B, paratype male (28.4 × 23.8 mm) (PSUZC 20150611 - 01.01); C, female (23.8 × 20.2 mm) (ZRC 2016.0162 b).

FIGURE 4. Terrapotamon longitarsus n. sp. Holotype male (47.4 × 38.6 mm) (ZRC 2016.0161). A, left side of carapace; B, frontal view of cephalothorax (setae omitted); C, right third maxilliped (setae omitted); D, thoracic sternites 1 – 4 and sternopleonal cavity; E, pleon (right margin of somite 3 drawn as entire). Scale bars: A, B, D, E = 10.0 mm; C = 5.0 mm.

FIGURE 5. Terrapotamon longitarsus n. sp. Gonopods. A – E, holotype male (47.4 × 38.6 mm) (ZRC 2016.0161); F, G, paratype male (28.4 × 23.8 mm) (PSUZC 20150611 - 01.01); H, I, male (35.0 × 29.5 mm) (ZRC 2016.0162 a). A, right G 1 (ventral view); B, distal part of right G 1 (ventral view); C, distal part of right G 1 (dorsomesial view); D, distal part of right G 1 (dorsal view); E, right G 2; F – I, left G 1 (ventral view) (setae not drawn). Scale bars: A, E, F – I = 2.0 mm; B – D = 1.0 mm.