Stoliczia setoiyenica, new species

(Figs. 1–6)

Material examined. THAILAND; Klong Mumbang Basin, Western River Basin of south Thailand, Satun Province; Holotype, male (36.7 × 28.5 mm) (ZRC 2023.0411), Tam Pui Waterfall, Klong Mumbang Basin, Ban Wang Prachan, Khuan Don District, 6°45.165'N 100°9.505'E, 108 m amsl, coll. V. Lheknim & P. Leelawathanagoon, 11 November 2006.

Paratypes: 2 male (30.1 × 24.0, 22.6 × 17.9 mm) (ZRC 2023.0412), 1 male (25.0 × 18.2 mm), 1 female (21.1 × 16.1 mm) (PSUZC-20061111-01.03), upper reach of Huey Kum Sala, Klong Mum Bang Basin, Ban Wang Prachan, Khuan Don District, 6°42.406'N 100°10.473'E, 180 m amsl, coll. V. Lheknim & P. Leelawathanagoon, 11 November 2006 ; 1 female (27.5 × 21.5 mm) (ZRC 2023.0415, ex PSUZC-20061111-02.08), upper reach of Huey Kum Sala, Klong Mumbang Basin, Ban Wang Prachan, Khuan Don District, 6°42.239'N 100°10.450'E, 160 m amsl, coll. V. Lheknim & P. Leelawathanagoon, 11 November 2006 ; 1 female (18.7 × 15.0 mm) (ZRC 2023.0416, ex PSUZC-20061111-03.07), stream from Ton Din Cave, Klong Mumbang Basin, Ban Wang Prachan, Khuan Don District, 6°42.888'N 100°10.025'E, 120 m amsl, coll. V. Lheknim & P. Leelawathanagoon, 11 November 2006 ; 1 male (30.4 × 23.5) (ZRC 2023.0413, ex PSUZC-20061111-04.05), Tam Pui Waterfall, Klong Mumbang Basin, Ban Wang Prachan, Khuan Don District, 6°45.165'N 100°9.505'E, 108 m amsl, coll. V. Lheknim & P. Leelawathanagoon, 11 November 2006 ; 1 male (26.7 × 21.2 mm) (ZRC 2023.0414, ex PSUZC-20061111-05.07), lower reach of Tam Pui Waterfall, Klong Mumbang Basin, Ban Wang Prachan, Khuan Don District, 6°45.122'N 100°9.408'E, 105 m amsl, coll. V. Lheknim & P. Leelawathanagoon, 11 November 2006 . Others: 1 male (18.5 × 14.9 mm), 1 female (14.2 × 11.3 mm), 3 juveniles (10.6 × 9.0, 10.9 × 9.3, 5.3 × 4.5 mm) (ZRC 2023.0417), same data as holotype .

Comparative material. Stoliczia kedahensis Ng, 1992: holotype, male (39.0 × 29.0 mm) (ZRC 1989.3261), Sungai Tekai, ca. 30 milestone, on Naka to Nami road, Padang, Terap district, Kedah, coll. J. I. Furtado & M. N. A., 1 April 1967; paratype, 1 male (25.2 × 19.8 mm) (ZRC 1989.3262), same data as holotype. Stoliczia panhai Ng & Naiyanetr, in Ng, 1986: Holotype, male (30.3 × 23.5 mm) (ZRC 1990.11561), Ton Nga Chang waterfall, Amphoe Hat Yai , Songkhla Province, southwestern Thailand, coll. P. Naiyanetr; paratype, 1 male (32.7 × 25.0 mm) (ZRC 1990.11562), same data as holotype . Stoliczia ekavibhathai Ng & Naiyanetr, in Ng, 1986: Holotype, male (31.3 × 24.3 mm) (ZRC 1990.11563), Sai Khao waterfall, Amphoe Khok Pho , Pattani Province, southeastern Thailand, coll. P. Naiyanetr; paratype, 1 male (29.0 × 22.8 mm) (ZRC 1990.11564), same data as holotype .

Diagnosis. Carapace transversely subovate, flat, dorsal surface weakly setose, mostly smooth except for gently rugose anterolateral regions; epigastric cristae present as rugose swelling; postorbital cristae sharp, raised; external orbital tooth large, separated from epibranchial tooth and anterolateral margin; exopod of third maxilliped without trace of flagellum; ambulatory legs relatively stout, with numerous stiff setae; male pleon broadly triangular. G1 with broad subterminal segment, almost straight, outer distal margin with short but distinct concavity; terminal segment broadly conical, gently sinuous, weakly bent outwards with distinct dorsal fold on proximal half, tip sharp, gently hooked.

Description of holotype. Carapace transversely subovate, slightly broader than long, carapace width to length ratio: 1.28, relatively flat, mostly smooth; surfaces with scattered short, stiff setae (Fig. 1A, B). Frontal region covered in smooth granules; anterolateral regions gently rugose; posterolateral regions with low striae; mesogastric, urogastric, cardiac and intestinal regions almost smooth; orbital regions almost smooth; suborbital region almost smooth with few granules; pterygostomial region smooth, sub-hepatic and sub-branchial regions very weakly rugose (Fig. 1C). Epigastric cristae visible as gently rugose swelling, separated by inverted Y-shaped furrow, distinctly anterior to and confluent with postorbital cristae; postorbital cristae sharp, visibly raised, confluent with anterolateral margin (Fig. 1A, B). Cervical grooves shallow but distinct; H-shaped median gastric groove distinct (Fig. 1A, B). Frontal margin very weakly divided into 2 broad, very low lobes, very weakly separated by shallow concavity (Fig. 1A, B). External orbital tooth large, well-produced, outer margin longer than inner margin, demarcated from rest of anterolateral margin by postorbital cristae cleft, lined with very small, rounded granules (Fig. 1A, B). Epibranchial tooth not discernible, appearing as broad angle confluent with postorbital cristae (Fig. 1A, B). Anterolateral margins convex, subcristate, lined with small, rounded granules (Fig. 1B). Posterolateral margin converges posteriorly strongly before becoming subparallel towards posterior carapace margin (Fig. 1B). Orbits large, subovate; eye filling up most of orbital space; eye peduncle relatively short, stout; cornea large, pigmented (Fig. 1C). Supraorbital margin gently sinuous, entire (Fig. 1C). Suborbital margin concave, complete, lined with small, rounded granules (Fig. 1C). Epistome with distinct median triangle, apex rounded; lateral parts gently sinuous (Fig. 1D).

Third maxillipeds covering most of buccal cavity when closed, sparsely covered with short stiff setae; ischium sub-rectangular, with shallow but distinct median oblique groove; merus subquadrate, wider than long, anteroexternal angle rounded; exopod relatively slender, reaching to lower third of merus, without trace of flagellum or lobe (Fig. 3F).

Chelipeds slightly asymmetrical, right chela larger (Fig. 1A, E). Anterior margin of basis-ischium lined with small granules; margins of merus lined with small, rounded granules; outer surface of carpus gently rugose, inner margins distinctly granulated, distal tooth well produced with smaller basal one (Fig. 1A). Chelae relatively stout; outer surfaces gently rugose to smooth; fingers gently curved, as long as palm; outer surface lined with rows of pits; propodal finger with longitudinal groove lined with pits; cutting edges of both fingers with variously sized teeth and denticles (Fig. 1E).

Ambulatory legs not elongate, relatively stout, second pair longest, ca. 1.50 times carapace width, last pair shortest, covered with numerous short, stiff setae that do not obscure margins (Fig. 1A). Outer surface of merus slightly rugose, dorsal margin entire; carpus almost smooth, outer surface with low submedian crista; outer surface of propodus almost smooth with median groove; dactylus stout, gently curving, quadrate in cross section, margins with short, sharp spines, tips corneous (Fig. 1A).

Thoracic sternum relatively narrow transversely, surface almost smooth (Fig. 2). Sternites 1, 2 completely fused to form broadly triangular plate with straight lateral margins; separated from sternite 3 by distinct, gently sinuous suture; sternites 3, 4 completely fused without a trace of groove demarcating suture (Fig. 2A). Sternopleonal cavity reaching to imaginary line connecting median points of edges of cheliped coxae (Fig. 2D). Male pleon locking tubercle low, rounded, positioned on submedian part of sternite 5 (Fig. 2D).

Pleon broadly triangular, all somites and telson free; somites 3–6 trapezoidal, gradually decreasing in width; somite 6 wider than long, lateral margins gently convex (Fig. 2C). Somites 1 and 2 subrectangular, very wide, reaching to bases of coxae of fourth ambulatory legs, sternite 8 not visible when pleon closed (Fig. 2B).

G1 with subterminal segment relatively broad, stout, almost straight, distal lateral margin with distinct but short concavity; terminal segment very weakly bent outwards, tapering to gently hooked, sharp tip, dorsal fold distinct on proximal half (in dorsal view), outer lateral margin concave (Fig. 3A–D). G2 longer than G1, distal segment elongate, flagellum-like, about two-thirds length of basal segment (Fig. 3E).

Females. Female specimens are similar to males in most key non-sexual characters except the chelae are less asymmetrical although one chela is always larger in adults (Fig. 4A). The vulvae are relatively large, almost the width of mesial end of sternite 6, almost round, directly antero-mesially with a distinct membranous cover (Fig. 4C).

Life colouration. The dorsal surfaces and ambulatory legs of Stoliczia setoiyenica, new species, are a uniform dark olive colour (Fig. 6A). The dorsal surfaces of the cheliped merus and carpus are also a dark olive colour, but the fingers are darker, almost black (Fig. 6A). The tips of the chelipeds, joints of chela and ambulatory legs are a dull orange (Fig. 6A). The underside of the crab is pale olive, dirty yellow in colour with the underside of the chelipeds being pale reddish brown (Fig. 6B).

Remarks. On basis of external morphology, Stoliczia setoiyenica, new species, is most closely allied to two species from northern Peninsular Malaysia, viz., S. perlensis and S. kedahensis . Stoliczia setoiyenica, new species, can be easily distinguished from S. perlensis by the following differences: relatively broader frontal and postorbital region, epigastric and postorbital cristae set more posteriorly (Fig. 1A, B) (vs. narrower frontal and postorbital region, epigastric and postorbital cristae more anterior, set closer to frontal margin, cf. Ng 1992: pl. 2A); third maxilliped exopod without a trace of flagellum, only tufts of setae (Fig. 3F) (vs. third maxilliped exopod with short flagellum, cf. Ng 1992: fig. 3A); G1 subterminal segment proportionally broader, appearing stouter, with shorter, more pronounced distal outer margin concavity (Fig. 3A, B) (vs. G1 subterminal segment proportionally slenderer, with less pronounced distal outer margin concavity, cf. Ng 1992: fig. 3B, C); and G1 terminal segment broader, relatively shorter, about 0.22 times total G1 length, with more pronounced dorsal fold, tip sharp and gently hooked (Fig. 3C, D) (vs. G1 terminal segment more slender, relatively longer, about 0.27 times total G1 length, with less pronounced dorsal fold, tip rounded, not hooked, cf. Ng 1992: fig. 3D, E). With regards to Stoliczia kedahensis, S. setoiyenica, new species, can be distinguished from the former primarily by the following differences: carapace relatively less transverse, carapace width to length ratio ca. 1.25–1.31 (Fig. 1B) (vs. carapace distinctly transversely, carapace width to length ratio ca. 1.34, cf. Ng 1992: pl. 3A); carapace posterolateral margin converges less strongly posteriorly, gently concave, with a “waist-like” appearance (Figs. 1A, B, 4A, 5A) (vs. carapace posterolateral margin, straight, converges strongly posteriorly, cf. Ng 1992: pl. 3A); G1 subterminal segment distal outer margin concavity shorter (Fig. 3A, B) (vs. G1 subterminal segment distal outer margin concavity longer, cf. Ng 1992: fig. 4B, C); and G1 terminal segment gently curving outwards, tapering more strongly, broader proximally and more slender distally, with a more pronounced dorsal fold, tip sharp and gently hooked (Fig. 3C, D) (vs. G1 terminal segment gently curving inwards, tapering weakly, less broad proximally and broader distally, without any dorsal fold, tip blunt and not hooked, cf. Ng 1992: fig. 4D, E).

Two other species of Stoliczia can also be found in southern Thailand, namely S. panhai [type locality: Songkhla province] and S. ekavibhathai [type locality: Pattani province]. Stoliczia setoiyenica, new species, however, can be clearly differentiated from both species by the following differences: postorbital cristae gently sloping or subparallel to frontal margin, with a broader frontal and postorbital region (Fig. 1A) (vs. postorbital cristae parallel to frontal margin, with a very narrow frontal and postorbital region in S. ekavibhathai, cf. Ng 1992: pl. 6A); cervical grooves weakly developed (Fig. 1A, B) (vs. cervical grooves well developed in S. panhai, cf. Ng 1992: pl. 4A); exopod of third maxilliped without a trace of flagellum (Fig. 3F) (vs. exopod of third maxilliped with short flagellum in S. panhai, that in S. ekavibhathai, a short flap, cf. Ng 1992: figs. 5A, 7A); relatively longer ambulatory legs, second walking leg to carapace width ratio ca. 1.50 (vs. relatively shorter ambulatory legs, second walking leg to carapace width ratio ca. 1.37 in S. panhai); G1 subterminal segment proportionally broader, appearing more stout, with shorter distal outer margin concavity (Fig. 3A, B) (vs. G1 subterminal segment proportionally narrower, appearing more slender, with longer, weaker distal outer margin concavity in both S. panhai and S. ekavibhatha i cf. Ng 1992: figs. 5B, C, 7B, C); G1 terminal segment appearing broader, relatively shorter, about 0.22 times total length of G1, tip weakly hooked, sharp and with distinct dorsal fold (Fig. 3C, D) (vs. G1 terminal segment with no prominent dorsal fold present, slender and relatively longer, about 0.27–0.30 times length of G1, tip rounded in S. panhai, that in S. ekavibhathai slightly flared cf. Ng 1992: figs. 5D–F, 7D, E).

Etymology. The name of the new species is derived from local name previously used for Satun Province, “Negeri Setoi Mumbang Segara”, where all the types specimens were obtained. The name is used as a noun in apposition.

Notes on ecology. Stoliczia setoiyenica, new species, is so far, only recorded from the upper reaches of the Klong Mumbang Basin, a relatively small river basin, covering an area of approximately 406.8 km 2 (about 0.5% of the total southern Thailand river basin area), located at the southernmost part of Nakhon Sri Thammarath Range, draining into the Andaman Sea at Satun Province (Fig. 7). As with many potamids, Stoliczia setoiyenica, new species, is only found in the upper reaches of the watershed and the habitat is characterised by a clear moderately fast flowing stream with a sand and gravel substratum and large rocks, boulders scattered across the stream width (Fig. 8) (Ng 1988). There are no other potamids known from the Klong Mumbang Basin. Syntopic crustaceans include the gercarcinucid, Salangathelphusa anophrys (Kemp, 1923) and the abundant and widely distributed freshwater palaemonid prawn, Macrobrachium forcipatum Ng, 1995 . Another gecarcinucid, Siamthelphusa improvisa (Lanchester, 1901) was only found in the middle and lower areas of the basin. The Bumble bee snail, Anentome helena (von dem Busch, in Philippi, 1847) is distributed in low numbers in the upper and middle watershed, while the Malayan trumpet snails, Melanoides tuberculata (Müller, 1774) is very common, especially in the lower reaches of the basin. Syntopic fish species includes: Neolissochilus soroides (Duncker, 1904), Devario regina (Fowler, 1934), Barbodes rhombeus (Kottelat, 2000), Barbodes lateristriga (Valenciennes, in Cuvier & Valenciennes, 1842), Lepidocephalichthys berdmorei (Blyth, 1860), Homalopteroides smithi (Hora, 1932), Silurichthys schneideri Volz, 1904, Glyptothorax aff. major (Boulenger, 1894), Clarias leiacanthus Bleeker, 1851, Dermogenys aff. sumatrana (Bleeker, 1854), Monopterus albus (Zuiew, 1793), Betta pugnax (Cantor, 1849), Channa aff. limbata (Cuvier, in Cuvier & Valenciennes,1831) and Doryichthys martensii (Peters, 1868) .

Distribution. Stoliczia setoiyenica, new species, is so far only known from the upper reaches of Klong Mumbang Basin, Satun province, southern Thailand, near the border with Malaysia.