Chinapotamon pusillum

Chinapotamon pusillum (Song, 1984) Figs 6, 7, 8, 10 Tiwaripotamon pusillum Song, 1984: 143–145, figs 1–9 Chinapotamon pusillum Dai & Naiyanetr, 1995: 63, fig. 10; Dai, 1999: 91–92, fig. 42; Ng et al. 2008: 161; Cumberlidge et al. 2009: table A 1. Chinapotamon cf. pusillum Le et al. 2024: 507–509, figs 4, 5, 10 B. Material examined. Holotype • 1 male (CAS CB-05188). Others • 1 male (15.9 × 14.2 mm) (IB - FC-CMx 01), a small stream flowing into the rice canal, in Song Mooc village, Dong Van Commune, Binh Lieu District, Quang Ninh Province, 21°33'11.5"N, 107°34'21.2"E, 22 March 2023, Do Van Tu leg. • 6 males (14.2 × 12.3 mm, 12.3 × 10.9 mm, 12.2 × 11.4 mm, 11.7 × 10.4 mm, 11.2 × 10.1 mm, 11.0 × 9.8 mm), 6 female (21.0 × 18.0 mm, 20.0 × 16.5 mm, 17.8 × 15.1 mm, 17.3 × 14.0 mm, 13.7 × 11.7 mm, 12.4 × 10.8 mm) (IB - FC-CMx 02), same data as above • 2 males (17.0 × 14.3 mm, 14.8 × 13.9 mm), 1 female (21.2 × 17.3 mm) (ZRC 2024.0313), same data as above. Diagnosis. Diminutive species (adult carapace width range between 14.0–21.0 mm). Carapace slightly wider than long, subquadrate, rugose, gently convex, region not distinct, indistinctly separated (Fig. 6 A). Epigastric and postfrontal cristae distinct (Fig. 6 A). External orbital and epibranchial teeth low; cleft between external orbital and epibranchial teeth deep (Fig. 6 A). Anterolateral margin of carapace slightly convex, cristate (Fig. 6 A). Exopod of third maxilliped reaching to 0.2 times length of merus with flagellum longer than width of merus (Fig. 6). Male major chela no gape when closed (Fig. 6 E). Ambulatory legs not elongated (Fig. 6 A). Sternites 3 and 4 without trace of groove demarcating suture (Fig. 7 A). Male telson relatively broad, lateral margins almost straight (Fig. 7 A). G 1 slender, sinuous, terminal article bent about 25 ° outward proximally, recurved distally, tip directed vertically (Fig. 8). Description of Vietnamese specimens. Carapace subquadrate, slightly wider than long, CW: CL ratio = 1.1 (median = 1.1); dorsal surface rugose, finely pitted, gently convex transversely, gently convex antero-posteriorly, regions poorly defined (Fig. 6 A). Frontal region rugose; lateral parts of anterolateral, hepatic and branchial regions rugose; protogastric, mesogastric, urogastric, cardiac and intestinal regions almost smooth except for pits; orbital regions slightly rugose; suborbital and pterygostomial regions rugose (Fig. 6 A, B). Epigastric cristae low, distinct, not sharp, separated by broad, shallow Y-shaped furrow, confluent with postorbital cristae; postorbital cristae low (Fig. 6 A). Cervical grooves shallow, indistinct; H-shaped median gastric groove shallow but distinct (Fig. 6 A). Frontal margin divided into two broad, low lobes, separated by broad, shallow concavity; margin of each lobe gently convex, confluent with supraorbital margin (Fig. 6 A). External orbital tooth low, demarcated from rest of anterolateral margin by small, shallow notch; epibranchial tooth low (Fig. 6 A). Anterolateral margins distinctly cristate, lined with small granules (Fig. 6 A). Posterolateral margin gently converging towards posterior carapace margin (Fig. 6 A). Orbits subovate; eye filling orbit; ocular peduncle long; cornea normal (Fig. 6 B). Supraorbital margin weakly concave, entire, lined with small, flattened granules (Fig. 6 B). Suborbital margin concave, complete, lined with small granules (Fig. 6 B). Posterior margin of epistome with distinct broad median triangle, each lateral margin with 2 lateral concavities (Fig. 6 C). Ischium of third maxilliped subrectangular, about 1.5 times longer than broad, smooth, with distinct median oblique groove; merus subquadrate, about 0.8 times as long as broad, surface smooth, anteroexternal angle broadly triangular, not expanded; exopodslender, reaching to 0.2 times length of merus, flagellum distinct, longer than width of merus (Fig. 6 D). Chelipeds slightly asymmetrical, relatively stout, no significant difference in asymmetry between males and females (Fig. 6 E). Anterior margin of basis-ischium almost smooth; inner-lower margins of merus granulated (Fig. 6 E). Outer surface of carpus gently rugose, inner distal angle with distinct sharp tooth, with 2 smaller teeth basally (Fig. 6 A). Outer surfaces of chelae smooth; chela palm in large males approximately 1.3 times as long as broad; major chela stouter, longer than minor chela (Fig. 6 E). Fingers of major chela, stout, curved, shorter than palm, outer surface lined with 2 rows of pits; cutting edges of both fingers with stout teeth, very small gape when closed, tips slightly cross (Fig. 6 E). Fingers of minor chela slender, fingers relative straight, almost no gap when closed (Fig. 6 E). Ambulatory legs not elongated; second pair longest, last pair shortest (Fig. 6 A). Outer surface of merus slightly rugose, dorsal margin weakly serrated, length to width ratio of fourth merus = 3.0; carpus slightly rugose, outer surface with submedian crista on first to third legs; propodus subrectangular; length to width ratio of fourth propodus = 1.5; dactylus gently curved, margins with short, sharp pectinate spines on both inner and outer margins (Fig. 6 A). Thoracic sternites relatively broad transversely (Fig. 7 A). 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. 7 B). Male pleonal locking tubercle at median part of sternite 5. Male pleon triangular; telson relatively broad with lateral margins almost straight, width to length ratio = 1.3; somite 6 broadly rectangular, width to length ratio = 2.4 (Fig. 7 A). G 1 slender, reaching to suture between sternites 4 and 5 with terminal and subterminal articles clearly demarcated; subterminal articles relatively slender, sinuous, proximal half sub-trapezoidal, wider than distal half, tapering gently distally, directed inward proximally but outward distally, 2.1 times as long as terminal article; terminal article slender, relatively gradually tapering, bent about 25 ° outward, tip not pointed, with groove for G 2 visible in ventral orientation (Fig. 8). G 2 slightly longer than G 1, distal article long, ratio of basal article to distal article = 2.1. Females. Female specimens similar to male in most non-sexual features. Female pleon ovate in mature individuals, covering thoracic sternites (Fig. 7 C). Vulvae set relatively far apart, small, subovate; directed ventral-mesially, positioned on proximal half of sternite 6 (Fig. 7 D). Colouration. Dorsal surfaces brown; ventral surface white; outer and inner surfaces of chelae light yellow (cf. Le et al. 2024: fig. 10 B). Distribution. This species is currently known from Dong Van Commune, Binh Lieu District, Quang Ninh Province, Vietnam and Fangcheng District, Guangxi Province, China. Notes on ecology. This species inhabits small streams and crevices with stagnant or slow-flowing water, where the banks are densely vegetated with shrubs and the substrate is rich in organic detritus (Fig. 9). Juveniles of Indochinamon spp. were also observed in the same habitat. Remarks. The discovery of Chinapotamon pusillum (Song, 1984), in northern Vietnam may not come as a surprise, especially considering the similar geography of the area, with China. The Vietnamese specimens were collected from Dong Van Commune, Binh Lieu District, Quang Ninh Province, Vietnam, an area directly bordering Fangcheng District, Guangxi Province, southern China, which is the type locality of C. pusillum (Fig. 15). The two areas are part of a contiguous mountain range known as Shiwan Mountain (Hu et al. 2023); it is very possible that the species is distributed throughout the range. Differences between C. pusillum (Song, 1984) and its closest congener, C. insulare, have been discussed in detail in Le et al. (2024). However, Le et al. (2024) treated specimens from Vietnam as C. cf. pusillum because they noted that they differed from the type description and figures in possessing more distinct anterolateral crista and a longer G 1 terminal article that reaches to beyond the suture between thoracic sternites 4 and 5. We obtained fresh photographs of the holotype of C. pusillum (Fig. 10) and we can now confirm that there are no obvious differences in the strength of the anterolateral cristae (Fig. 6 A). The difference noted in how far the G 1 terminal article reaches beyond the suture between thoracic sternites 4 and 5 (Fig. 7 B) can be affected by how much the male pleon is opened and flexed as well as the state of preservation of the specimens. Unfortunately, this is not apparent from the recent photographs of the holotype male. As such, we can only rely on the figure of this condition in Dai (1999: fig. 42.4) which may not have been drawn accurately. Nevertheless, on the basis of the available morphological characters and close geographical proximity, we are now confident they are actually conspecific with C. pusillum (Song, 1984) s. str. (Figs 6 – 8, 10, 15). Morphologically, C. pusillum (Song, 1984) and C. insulare Le, Dang & Do, 2024, are quite distinct compared to congeners. These two species have a carapace that is more subquadrate and G 1 terminal articles are directed more vertically, whereas in C. depressum (Dai, Song, Li & Liang, 1980), C. anglongense Dai & Naiyanetr, 1994, C. clarkei Ng, 2017, C. dashiwei Ng, 2017, C. glabrum (Dai, Song, Li & Liang, 1980), C. halangense sp. nov., C. longlinense Dai & Naiyanetr, 1994, C. maolanense Zou, Bai & Zhou, 2018 and C. xingrenense Dai & Naiyanetr, 1994, the carapace is more transversely subovate and the G 1 terminal article is distinctively bent outwards (Figs 1, 5, 6, 10; cf. Ng 2017: figs 2, 6, 8; Zou et al. 2018: fig. 7, table 2). For now, C. pusillum and C. insulare will be retained in the genus Chinapotamon until the genus can be revised.