Chinapotamon insulare sp. nov.

(Figs. 6–9, 10C)

Material examined. Holotype: 1 male (28.9 × 24.8 mm), IEBR-FC-CIx01, Vietnam, Quang Ninh Province, Van Don District, Bai Tu Long National Park, a small stream in Ba Mun Island, N 21°04’28.0” E 107°35’33.1”, coll. Do V.T. & Le H.A., 31 April 2018. Paratypes: 5 males (21.4 × 18.0 mm, 23.0 × 20.0 mm, 24.5 × 21.5 mm, 25.0 ×22.0 mm, 28.0 × 23.5 mm), 3 females (22.0 × 18.0 mm, 22.6 × 18.3 mm, 26.5 × 22.0 mm), IEBR-FC-CIx02, same data as holotype. Others: 2 males (23.9 × 20.5 mm, 22.3 × 18.3 mm), IEBR-FC-CIx03, Vietnam, Quang Ninh Province, Van Don District, Bai Tu Long National Park, a small stream in Ba Mun Island, N 21°04’28.0” E 107°35’33.1”, coll. Do V. T. & Le H.A., 5 June 2018 ; 1 female (17.9 × 15.2 mm), IEBR-FC-CIx04, Vietnam, Quang Ninh, Van Don, Bai Tu Long National Park, a small stream in Cai Lim Island, near Cai Lim Forest Ranger Station, N 21°06’30.6” E 107°34’42.8”, coll. Do V. T., 19 March 2023 ; 1 male (21.0 × 18.5 mm), IEBR-FC-CIx05, Vietnam, Quang Ninh, Van Don, Bai Tu Long National Park, Cao Lo stream in Ba Mun Island, N 21°05’00.3” E 107°35’59.0”, coll. Do V. T., 20 March 2023 ; 1 male (28.9 × 24.7 mm), IEBR-FC-CIx06, Vietnam, Quang Ninh, Co To District, a small stream in Co To Island, N 21°00’34.1” E 107°44’57.8”, coll. Phan Q. T., 22 March 2023 ; 1 male (19.5 × 16.0 mm), IEBR-FC-CIx07, Vietnam, Quang Ninh, Co To, a small stream in Thanh Lan Island, N 21°00’50.7” E 107°49’11.8”, coll. Phan Q. T., 23 March 2023.

Description. Carapace subquadrate, about 1.2 times broader than long; dorsal surface rugose, strongly convex, regions well defined (Figs. 6A, B). Frontal region rugose; lateral parts of anterolateral and branchial regions covered with small granules; mesogastric, urogastric, cardiac and intestinal regions covered with very small granules; orbital regions rugose; suborbital and pterygostomial regions, uneven, rugose covered by small granules (Fig. 6A, B). Epigastric cristae low, separated by broad, shallow Y-shaped furrow, separated from postorbital cristae by short, shallow groove; postorbital cristae low, confluent with epibranchial tooth, breaking up into rugae and granules before epibranchial tooth (Figs. 6A, B). Cervical grooves shallow, discernable; H-shaped median gastric groove shallow but distinct (Fig. 6A). Frontal margin divided into 2 broad, high lobes, separated by broad, deep concavity; margin of each lobe gently convex, confluent with supraorbital margin (Fig. 6A). External orbital tooth high, demarcated from rest of anterolateral margin by small, deep notch, lined with distinct granules; epibranchial tooth high, distinct (Figs. 6A, B).

Anterolateral margins convex, strongly cristate, lined with distinct granules (Fig. 6A). Posterolateral margin gently concave, strongly converging towards gently convex posterior carapace margin (Fig. 6A). Orbits subovate; eye fully filling orbit; ocular peduncle long; cornea normal (Figs. 6A, B). Supra- and infraorbital margin gently sinuous, lined with small but distinct granules (Fig. 6B). Suborbital margin concave, complete, lined with small granules (Fig. 6B). Posterior margin of epistome with distinct narrow median triangle, each lateral margin with 2 concavities (Fig. 6B).

Ischium of third maxilliped subrectangular, about 1.5 times longer than broad, with distinct median oblique groove; merus subquadrate, about 0.9 times as long as broad, surface smooth, anteroexternal angle broadly triangular, not expanded; exopod slender, reaching to one-fourth length of merus, flagellum distinct, longer than width of merus (Fig. 6C).

Male major chelipeds unequal, female major chelipeds more equal (Figs. 6B, D). Anterior margin of basisischium almost smooth; Inner margins of merus granulated (Figs. 6A, 7A). Outer surface of carpus rugose, inner distal angle with distinct sharp tooth, with 2 smaller teeth basally (Figs. 6A, 7A). Outer surfaces of chelae rugose; chela palm in large males approximately 1.4 times as long as broad; major chela stouter, longer than minor chela (Fig. 6D). Major chela with fingers stout, almost straight, shorter than palm, outer surface lined with 2 rows of pits; cutting edges of both fingers with stout teeth, almost no gap when fingers closed, not crossing (Fig. 6D). Minor chela with fingers slender, shorter than major chela, two fingertips relatively straight, almost no gap when closed (Fig. 6D).

Ambulatory legs not elongated; second pair longest, last pair shortest (Fig. 6A). Outer surface of merus slightly rugose, dorsal margin weakly serrated, length to width ratio of fourth merus 2.1; carpus slightly rugose, outer surface with submedian crista on first to third legs; propodus subrectangular, length to width ratio of fourth propodus 1.8; dactylus gently curved, margins with short, sharp corneous spines on both inner, outer and upper margins (Fig. 6A).

Thoracic sternum relatively narrow transversely (Figs. 7A). Sternites 1, 2 completely fused to form broadly triangular plate; separated from sternite 3 by complete suture; sternites 3, 4 completely fused, without suture; sternopleonal cavity reaching to imaginary line connecting bases of cheliped coxae (Fig. 7A). Male pleonal locking tubercle just behind median part of sternite 5 (Fig. 7A). Male pleon narrow triangular; telson relatively narrow, lateral margins concave, width to length ratio 1.4; proximal margin of telson slightly wider than the distal margin of somite 6; somite 6 subrectangular, width to length ratio 1.8 (Fig. 7A). Female pleon ovate, covering most of thoracic sternum (Fig. 8A). Vulvae subovate; low vulvar cover on extend oblique edge of vulva (Fig. 8B).

G1 slender, slightly sinuous, reaching to suture between sternites 4 and 5, with terminal and subterminal segments clearly demarcated; subterminal segment directed inward proximally but outward distally, 2.2 times as long as the terminal segment; terminal segment gradually tapering, bent at about 10°, gently curving inwards, tip subtruncate (Figs. 7B, 9A–D). G2 slightly shorter than G1, distal segment short, ratio of basal segment to distal segment 2.4 (Figs. 7B, 9E).

Live colouration. The dorsal surfaces are orangish- brown; this colour is brighter in the chelae; the ventral surfaces are white (Fig. 10C).

Etymology. The name alludes to the insular occurrence of the species: it has only been found on islands so far. Chinapotamon is neuter.

Habitat. The new species lives in small streams on the islands even in the stagnant pools of springs that do not have regular water with dead leaves, branches, humus, stones, pebbles, and coarse sand substrate (Fig. 11).

Distribution. The survey results from many locations in Vietnam (unpublished data) showed that the distribution range of this species is relatively narrow, found only on five islands of Quang Ninh province (Cai Lim, Ba Mun, Tra Ban, Co To, Thanh Lan) (Fig. 1).

Remarks. Chinapotamon insulare sp. nov. is easily separated from congeners by the combination of the following characters: (1) the carapace is small in size (17.9–28.9 mm); (2) carapace is subquadrate; (3) carapace dorsal surface is strongly convex and rugose; (4) epigastric and postorbital cristae are low; (5) epibranchial tooth is high, distinct, demarcated from rest of anterolateral margin by deep notch; (6) concavity between frontal margin lobes is deep; (7) there is almost no gap when male major chela closes; (8) last ambulatory merus is stout, 2.1 times as long as wide; (9) telson is relatively narrow; (10) G1 terminal segment is slender, bent at about 10° inwards; and (11) G2 is slightly shorter than G1.

Chinapotamon insulare sp. nov. looks most similar to C. pusillum in the shape of carapace and G1. However, the new species can be distinguished by (1) the carapace size is slightly larger (12.9–19.6 mm; Figs. 6A, B) (vs. 17.9–28.9 mm in C. pusillum; cf. Dai, 1999: pl. 4, fig. 4); (2) the carapace dorsal surface is strongly convex and rugose (Figs. 6A, B) (vs. the carapace dorsal surface is slightly convex and rugose; cf. Dai 1999: pl. 4, fig. 4); (3) the concavity between frontal margin lobes is deeply concave (Figs. 6A, B) (vs. the concavity between frontal margin lobes is shallow; cf. Dai 1999: pl. 4, fig. 4); (4) the epibranchial tooth is demarcated from rest of anterolateral margin by deep notch (Figs. 6A, B) (vs. the epibranchial tooth is demarcated from rest of anterolateral margin by shallow notch; cf. Dai 1999: pl. 4, fig. 4); (5) the last ambulatory merus is slender, length is 2.1 times longer than width (Figs. 6A, B) (vs. the last ambulatory merus is stouter, length is 1.6 times longer than width; cf. Dai 1999: pl. 4, fig. 4); (6) the G1 terminal bent at about 10° inwards (Figs. 7B, 9A–D) (vs. the G1 terminal bent at about 5° inwards; cf. Dai & Naiyanetr 1994: figs. 10.5, 10.6; Dai, 1999: figs. 42.5, 42.6); and (7) G2 basal segment is 2.4 times longer than distal segment (Fig. 8E) (vs. 2.0 times longer; cf. Dai & Naiyanetr 1994: fig. 10.7; Dai 1999: fig. 42.7).