Description of two new species of the genus Heterochelamon Türkay & Dai, 1997 (Crustacea: Decapoda: Brachyura: Potamidae), from southern China

Systematics

Material examined. Holotype: male (25.8 × 23.1 mm) (NCU MCP 423601), Xinaobei Village (22°55′14.59″N 114°33′51.85″E, 47 m asl.), Baihua Town, Huidong County, Huizhou City, Guangdong Province, coll. Song-Bo Wang, Yi-Yang Xu and Jia-Ming Tian, 6th Jul. 2019. Paratypes: 3 males (30.0 × 25.8 mm, 26.2 × 23.1 mm, 21.3 × 18.6 mm) (NCU MCP 423602, TDRC 002005, TDRC 002006) and 2 females (26.1 × 23.5 mm, 20.6 × 18.5 mm) (NCU MCP 423603, TDRC 002007), same data as holotype. Others: 5 males (29.6 × 25.4 mm, 26.4 × 23.5 mm, 25.1 × 22.3 mm, 28.4 × 24.5 mm, 21.3 × 18.9 mm; NCU MCP 423606, NCU MCP 423607, NCU MCP 423608, NCU MCP 423609, NCU MCP 423610) and 2 females (21.1 × 19.1 mm, 14.5 × 12.5 mm; NCU MCP 423612, NCU MCP 423613), same data as holotype.

Comparative material. Heterochelamon yangshuoense Türkay & Dai, 1997: Holotype, male, IZCAS CB 05102, Yangshuo County, Guilin City, Guangxi Zhuang Autonomous Region, 8th May 1975. H. guangxiense Türkay & Dai, 1993: Holotype, male, IZCAS CB 01336, Guangxi Zhuang Autonomous Region, May 1974. H. purpureomanualis Türkay & Dai, 1997: not holotype, male, IZCAS CB, Yao Mountain, Xiuren Town, Lipu County, Guilin City, Guangxi Zhuang Autonomous Region, 1938. H. tessellatum Naruse, Zhu & Zhou, 2013: Holotype, male, NCU MCP 2012.0003, Dong Men Zhuang, Shuangluo Village, Sanli Town, Shanglin County, Guangxi Zhuang Autonomous Region, 14th Aug. 2006, Paratypes, 1 male, NCU MCP 2012.0004, 1 female, NCHUZOOL 13572, same data as holotype. H. castanea Naruse, Zhu & Zhou, 2013: Holotype: male, NCU MCP 2012.0001, Lituan Village, Baiming Town, Liujiang County, Liuzhou City, Guangxi Zhuang Autonomous Region, Aug. 2006, Paratype: 1 female, NCU MCP 2012. 0002, same data as holotype.

Diagnosis. Carapace subquadrate, dorsal surface covered with scattered minute pits, regions not clearly demarcated. External orbital angle triangular, sharp; postorbital cristae inconspicuous. Epibranchial tooth very sharp, distinct. Anterolateral margin cristate. Suborbital, pterygostomial regions smooth. Median lobe of epistome posterior margin protruded slightly. Third maxilliped exopod exceeding posterior margin of merus, with long slender flagellum. Chelipeds unequal, cutting edges of finger of larger cheliped with blunt teeth, with narrow gap when fingers closed. Ambulatory legs slender. Male telson triangular, lateral margins slightly concave. Female vulva located at middle of sternite 6. G1 terminal segment flat and medially bent, not tapered distally, apex rounded, reaching pleonal locking tubercle on mid length of sternite 5 but not exceeding sternites 4/5 suture.

Description. Carapace subquadrate, width about 1.1 times as length (n = 13), regions not clearly demarcated; dorsal surface convex longitudinally, smooth (Figs. 1A, 3A). External orbital angle triangular, sharp, separated from anterolateral margin by U-shaped notch; postorbital cristae inconspicuous, postfrontal lobe slightly convex, separated from each other; cervical groove indistinct; H-shaped gastric groove indiscernible (Figs. 1A and 3A). Epibranchial tooth very sharp, distinct; anterolateral margin cristate, straight, lined with 7–8 granules, shorter than posterolateral margin; posterolateral surface smooth (Figs. 1A, 3A). Frontal, supra-, infraorbital margins cristate, lined with indistinct granules; suborbital, pterygostomial regions smooth, with a few round granules (Fig. 1B). Epistome posterior margin distinctly cristate, median lobe low, triangular, lateral margins slightly sinuous (Fig. 1B).

Third maxilliped exopod exceeding posterior margin of merus, with long slender flagellum; merus about 1.2 times as broad as long, slightly depressed medially; ischium about 1.4 times as long as broad, with distinct median sulcus (Fig. 2B).

Male chelipeds unequal (Fig. 1A). Merus surface smooth without any pits; carpus with inner angle produced into strong spine, surface sparsely covered with pits (Fig. 1A). Cutting edges of larger cheliped fingers with row of large blunt teeth, with narrow gap when fingers closed (Fig. 2A). Ambulatory legs slender; third legs longest when stretched laterally, fourth ambulatory leg propodus about 1.6 times as long as broad, slightly shorter than dactylus, with scattered thorn-like setae (Fig. 2E).

Male thoracic sternites 2/3 demarcated by distinct suture, sternites 3/4 demarcated by shallow groove (Fig. 2C). Male sterno-pleonal cavity deep, reaching anteriorly to mid-length of thoracic sternite 4; median groove between sternites 7/8 long (Fig. 2E). Male pleon triangular, somites 3–6 gradually decreasing in width, increasing in length in males; somite 3 widest; somite 6 trapezoidal, margins slightly oblique (Fig. 2D). Telson triangular with rounded apex, lateral margin slightly concave (Fig. 2C). Female pleon broadly ovate (Fig. 3B); vulva small, ovate, located at middle of sternite 6, opening inwards, posterior margin slightly bulged (Fig. 3C).

G1 slender; terminal segment flat, bent medially, not tapered distally, distal part rounded; exceeding pleonal locking tubercle on mid length of sternite 5, not reaching suture between sternites 4/5 (Figs. 4A–4D, 2F). Distinct boundary between terminal segment and subterminal segment, latter length about 2.5 times as former, groove for G2 in ventral surface; G2 basal segment triangular, length about 2.1 times distal segment (Fig. 4).

Etymology. The species is named after the type locality, Huidong County, Huizhou City, Guangdong Province.

Distribution. The new species is known only from the type locality presently, Huidong County, Huizhou City, Guangdong Province, southern China.

Ecology. Specimens of the new species were collected at three sites along a river using fish cages or by hand (Figs. 5B, 5C). The water of the river is relatively clear, and the water flow is slow. The width of the river is approximately 1–3 m and the depth is approximately 0.2–0.3 m. The river bed is mainly sand and soil (Fig. 5E). There are many shrubs and low hills around the river, many households located dozens of meters away from the river and some vegetable gardens along the river (Fig. 5A). At one of the collection points, we also collected specimens of the gecarcinucid Somanniathelphusa sinensis H. Milne-Edwards, 1853 (Fig. 5D), which we believe coexists with the new species in the river.

Remarks. Compared to congeners, Heterochelamon huidongense n. sp. can easily be separated by its G1 not reaching sternites 4/5 suture in situ and terminal segment being medially bent (Figs. 2E and 4A, Table 1) (versus reaching sternites 4/5 suture and straight in congeners, except H. tessellatum which has G1 that also does not reach the sternites 4/5 suture but is strongly bent proximally), as well as a combination of characters: very sharp epibranchial tooth , triangular external orbital angle, slightly convex anterolateral margins (Fig. 1A), triangular male telson (Fig. 2C), and inward opening female vulva (Fig. 3C). For detailed differences between this new species and congeners, see Table 1.

Material examined. Holotype: 1 male (24.6 × 19.8 mm) (NCU MCP 342001), from Tongfu Village (24°17′10.89″N115°5′17.28″E, 267 m asl.), Toupai Town, Jinxiu Yao Autonomous County, Laibin City, Guangxi Zhuang Autonomous Region, coll. Xi-Jiao Wei, 8th Jul. 2011. Paratypes: 3 males (20.3 × 15.8 mm, 15.8 × 12.6 mm, 17.8 × 14.8 mm) (NCU MCP 342002, TDRC 002008, TDRC 002009) and 1 female (12.9 × 10.4 mm) (NCU MCP 342003), same data as holotype. Other: 1 male (14.1 × 11.4 mm) (NCU MCP 342006), same data as holotype.

Comparative material. See for Heterochelamon huidongense n. sp.

Diagnosis. Carapace trapezoidal, flat, surface covered with scattered minute pits. External orbital angle blunt, postorbital cristae, postfrontal lobe indiscernible. Epibranchial tooth blunt, distinct; anterolateral margin cristate, slightly convex. Suborbital, pterygostomial regions very smooth. Median lobe of epistome posterior margin distinctly protruding. Third maxilliped exopod with long flagellum. Chelipeds strongly unequal in adult males, surface smooth, lower cutting edge of major cheliped finger with very large, blunt teeth, with medium gap when fingers closed. Ambulatory legs slender. Thoracic sternites 2/3 demarcated by distinct deep suture, anterior part of sternite 3 convex. Male sterno-pleonal cavity deep, narrow. Male telson triangular, lateral margin straight. Female vulva reaching sternites 5/6 suture. Male G1 terminal segment rod-like, tapered distally, subdistal segment suddenly inwardly bent, reaching sternites 4/5 suture.

Description. Carapace trapezoidal, flat, width about 1.3 times as length (n = 6), regions not clearly demarcated; dorsal surface slightly convex, covered with inconspicuous pits (Figs. 6A, 8A). External orbital angle blunt, separated from anterolateral margin by shallow notch; postorbital cristae inconspicuous, postfrontal lobe indistinct; cervical groove indistinct; H-shaped gastric groove indiscernible (Figs. 6A, 8A). Epibranchial tooth blunt, distinct; anterolateral margin cristate, slightly convex, lined with several granules, shorter than posterolateral margin; posterolateral surface smooth (Figs. 6A, 8A). Frontal and orbital margins cristate, lined with indistinct granules, supra-, infraorbital margins glazed; suborbital, pterygostomial regions very smooth, not covered with granules (Fig. 6B). Epistome posterior margin distinctly cristate, median lobe triangular, distinctly protruding, lateral margins almost straight (Fig. 6B).

Third maxilliped exopod exceeding posterior margin of merus, with slender flagellum; merus about 1.2 times as broad as long, with median depression; ischium about 1.4 times as long as broad, longitudinal median sulcus distinct (Fig. 7B).

Chelipeds strongly unequal in adult males (Fig. 7A). Merus surface smooth without any pits; carpus inner angle forming stout spine, surface not covered with pits (Fig. 1A). Cutting edge of larger cheliped fixed finger with large rounded teeth, movable finger cutting edge lined with several small teeth, with medium gap when fingers closed (Fig. 7A). Ambulatory legs slender; third legs longest, the fourth ambulatory leg propodus about 2.0 times as long as broad, equal to dactylus in length (Figs. 6A, 7F).

Male thoracic sternites 2/3 demarcated by distinct deep suture, anterior part of sternite 3 convex, sternites 3/4 suture discernible as shallow groove, sterno-pleonal cavity reaching imaginary line joining posterior third of cheliped coxae (Fig. 7C). Male sterno-pleonal cavity deep, narrow; median longitudinal groove between sternites 7/8 medium-length (Fig. 7E). Male pleon triangular, third somite widest; somite 6 trapezoidal; somites 3–6 gradually decreasing in width (Fig. 7D). Telson triangular with rounded apex, lateral margin slightly convex (Fig. 7D). Female vulva very small, ovate, reaching sternites 5/6 suture, located at upper of sternite 6 (Fig. 8B).

Male G1 slender; terminal segment rod-like, almost straight, tapered distally; apex sharp, abruptly bent inwards subdistally; reaching beyond pleonal locking tubercle on mid length of sternite 5, to suture between sternites 4/5 (Figs. 7E, 9). Boundary between terminal segment and subterminal segment clear, latter length about 2.4 times as former, groove for G2 in ventral surface, G2 destroyed during dissection (Fig. 9).

Etymology. The species is named after the type locality, Jinxiu Yao Autonomous County, Laibin City, Guangxi Zhuang Autonomous Region.

Distribution. The new species is known only from the type locality presently, Jinxiu Yao Autonomous County, Laibin City, Guangxi Zhuang Autonomous Region, southern China.

Ecology. The collection site is a small river running behind Tongfu village. The river is mostly surrounded by dwarf mountains and many kinds of fruit trees. The river is approximately 2 m in width, the water depth is approximately 0.4 m, the riverbed mainly consists of sand and gravel and the water quality is good.

Remarks

Heterochelamon jinxiuense n. sp. can easily be differentiated from its congeners by its distally tapered G1 (Fig. 9, Table 1) (versus widening distally or not tapered distally in congeners), blunt external orbital angle (Fig. 6A, Table 1) (versus triangular or acutely triangular in congeners, except that of H. guangxiense which is also blunt), strongly unequal chelipeds in adult males and medium gap when male major chela fingers closed (Fig. 7A, Table 1) (versus strongly unequal in H. tessellatum, H. purpureomanualis and H. yangshuoense, but fingers with very broad and oblong gap when closed; other species unequal with narrow gap when closed). The detailed differences between this new species and congeners are also presented in Table 1.

The new species was collected in 2011, of which the material consisted of five males (one adult and four immature) and one female. The G2 of the only mature male had been destroyed by the time we examined the specimens. We were also unable to find this species in a subsequent collection trip. As of such, the G2 is not described here.

Phylogenetic analyses

We obtained the mitochondrial 16S rRNA gene molecular data from four species of Heterochelamon for this study. Other sequences used were downloaded from GenBank. To show where these two new species fit within the Asian freshwater crab phylogeny, 60 species of 47 genera in the family Potamidae Ortmann, 1896, were used (Table 2). The trees were constructed via the BI and ML methods and showed a high degree of consistency, and the topological structure was determined from BI with support values from both methods (Fig. 10). The phylogenetic results support the recognition of the two new Heterochelamon species, H. tessellatum and H. castanea cluster with the two new species to form an independent branch in the “China-East Asia Islands” clade (Shih, Yeo & Ng, 2009). It is possible that the two new species are in sister relationship. H. castanea is most closely related to the two new species in the two known congeneric species available for the molecular phylogenetic analysis.