Dacuomon quynhi, Ng & Tri, 2023

Ng, Peter K. L. & Tri, Ngo Van, 2023, A new genus and three new species of freshwater crab (Crustacea: Brachyura: Potamidae) from central and northern Vietnam, Raffles Bulletin of Zoology 71, pp. 70-86 : 72-76

publication ID	https://doi.org/ 10.26107/RBZ-2023-0005
publication LSID	lsid:zoobank.org:pub:BA361B52-E36A-4C15-AB6B-FECA9CEF58AF
persistent identifier	https://treatment.plazi.org/id/C7C09257-0342-4950-BEC3-D38FCEEB6B53
taxon LSID	lsid:zoobank.org:act:C7C09257-0342-4950-BEC3-D38FCEEB6B53
treatment provided by	Felipe
scientific name	Dacuomon quynhi
status	sp. nov.

Treatment

Dacuomon quynhi , new species

( Figs. 1–4 View Fig View Fig View Fig View Fig )

Material examined. Holotype: male (27.7 × 23.8 mm) ( ZRC 2022.0047 View Materials ), intermittent small stream in Xuan Lam commune, Dong Xuan district, Phu Yên Province, central Vietnam, ca. 13°27′43.17″N 109°07′09.34″E, 160 m a.s.l., coll. V. T. Ngo, 13 January 2022 GoogleMaps . Paratypes: 2 males (23.9 × 20.4 mm, 22.9 × 19.5 mm) ( ZRC 2022.0048 View Materials ), same data as holotype GoogleMaps .

Comparative material. Kukrimon cucphuongense ( Đăng, 1975) , 6 males (largest 22.3 × 19.2 mm), 2 females (larger 18.3 × 15.6 mm), 3 juveniles ( ZRC 2022.0058 View Materials ), stream in Cuc Phuong National Park , about 16 km from main gate, Ninh Binh Province, northeastern Vietnam, coll. D.C.J. Yeo & H.H. Ng, 16 September 1997

Diagnosis. As for genus.

Description of male holotype. Carapace rounded, wider than long (ratio 1.16); dorsal surface gently convex in frontal view, glabrous, regions not prominently defined or inflated ( Figs. 1A, B View Fig , 2A–D View Fig ). Frontal regions with low granules, postorbital regions almost smooth; lateral surface of anterolateral and branchial regions covered with low granules, not prominent; mesogastric, urogastric, cardiac and intestinal regions almost smooth; suborbital, subhepatic and subbranchial regions with low granules and striae; pterygostomial regions with low granules ( Fig. 2B–D View Fig ). Epigastric cristae visible, rugose, not sharp, separated by shallow median Y-shaped furrow; epigastric cristae just anterior to postorbital cristae, separated from it by short furrow; postorbital crista distinct, sharp, raised, subparallel to frontal margin, outer edge lower, separated by small fissure from strong crista leading to anterolateral margin ( Fig. 2B, D View Fig ). Cervical grooves shallow, not reaching lateral margins, barely connected to H-shaped median gastric groove ( Fig. 2B View Fig ). Frontal margin gently sinuous in frontal and dorsal views ( Fig. 2B, D View Fig ). External orbital tooth distinct, triangular, outer margin more than twice length of inner margin, demarcated from rest of anterolateral margin by distinct V-shaped cleft; epibranchial tooth small, sharp ( Fig. 2B View Fig ). Anterolateral margins convex, cristate, lined with low granules, appearing entire ( Fig. 2B View Fig ). Posterolateral margin gently sinuous, converging towards convex posterior carapace margin ( Fig. 2B View Fig ). Orbits subovate; eye filling orbital space; eye peduncle relatively short, stout; cornea large, round, pigmented ( Fig. 2B–D View Fig ). Supraorbital margin slightly sinuous, entire ( Fig. 2B View Fig ). Suborbital margin concave, complete, lined with small, low granules; inner angle with low tooth ( Fig. 2C, D View Fig ). Antennae short, stretching across base of eyes; antennules short, folding transversely in rectangular fossa ( Fig. 2C–E View Fig ). Posterior margin of epistome with low, broadly triangular median structure, lateral margins gently concave ( Fig. 2C–E View Fig ).

Third maxillipeds covering most of buccal cavity when closed; covered with relatively dense long stiff simple setae which do not obscure surface; ischium subrectangular, with distinct oblique median groove, surface mostly smooth; merus subquadrate, surface almost smooth, margins cristate, anteroexternal angle rounded; exopod slender, reaching to about half length of merus, with elongate flagellum that reaches across width of merus ( Figs. 1B, D View Fig , 3A View Fig ).

Chelipeds slightly asymmetrical, right larger ( Fig. 2A, F View Fig ). Margins of basis-ischium and merus with numerous long setae. Anterior margin of basis-ischium lined with several small low tubercles and granules; margins of merus lined with prominent low tubercles, with larger tubercle on subdistal surface ( Fig. 3C View Fig ). Outer surface of carpus covered with low tubercles, inner distal angle with large sharp tooth and small sub-basal tubercle ( Fig. 2A View Fig ). Outer surface of chela covered with prominent tubercles and granules ( Fig. 2F View Fig ). Fingers of major chela long, gently curved, longer than palm, outer surface of both fingers lined with 2 rows of pits; cutting edges of both fingers with variously sized sharp teeth and denticles; dorsal margin of dactylus with low tubercles and granules ( Fig. 2F View Fig ). Fingers of minor chela similar to major chela in form but more slender ( Fig. 2F View Fig ).

Ambulatory legs relatively, slender; second pair longest, last pair shortest ( Figs. 1A View Fig , 2A View Fig ). Merus relatively short, outer surface gently rugose, dorsal margin uneven, subcristate, without subdistal spine or tooth; carpus rugose, dorsal margin cristate, outer surface with low submedian crista on first to third legs, that on fourth leg smooth; dorsal margin of propodus with crista, outer surface with low, submedian crista; dactylus relatively long, gently curved, quadrate in cross section, margins with short, sharp chitinous spines; margins of merus, carpus and propodus of first ambulatory leg with dense, long stiff simple setae which partially obscure surface; setae on other legs less dense and distinctly shorter ( Figs. 1B View Fig , 2A View Fig ).

Thoracic sternum, notably sternites 3 and 4, as long as broad, surface almost smooth ( Fig. 3C, D View Fig ). Sternites 1, 2 completely fused to form broadly triangular plate; separated from sternite 3 by distinct, gently concave suture (directed anteriorly); sternites 3, 4 completely fused without discernible groove or suture, only lateral notches visible ( Fig. 3C View Fig ). Sutures 4/5, 5/6, 6/7 and 7/8 medially interrupted; median longitudinal groove on sternites 7 and 8 ( Fig. 3D View Fig ). Penis coxal, emerging on condyle of coxa of fourth ambulatory leg. Sternopleonal cavity deep, anterior edge reaching imaginary line connecting median part of cheliped coxae ( Fig. 3C View Fig ). Sternite 8 not visible when pleon closed. Male press-button pleonal locking mechanism composed of pair of relatively low peg-like tubercles, positioned on posterior quarter of sternite 5 ( Fig. 3D View Fig ).

Pleon broadly triangular, all somites and telson free; telson triangular, as long as broad, lateral margins gently sinuous; somite 6 transversely trapezoidal, much wider than long, lateral margin gently convex; somites 3–5 trapezoidal, gradually decreasing in width, increasing in length anteriorly; somite 3 widest; somites 1 and 2 subrectangular, wide, reaching to bases of coxae of fourth ambulatory legs ( Fig. 3B, C View Fig ).

G1 relatively slender; subterminal segment widest basally, gradually tapering towards terminal segment, distal quarter neck-like, groove for G2 ventral; subterminal segment distinctly separated from terminal segment by grooves; terminal segment relatively short, subcylindrical, hookshaped, curved outwards, tapering to tip, dorsal flap low, not prominent ( Figs. 3E–H View Fig , 4A–C View Fig ). G2 elongate, longer than G1; basal segment longer than distal segment ( Figs. 3I View Fig , 4D View Fig ). Variation. No female specimens were available. The two smaller male paratypes agree with the holotype in most respects. The postorbital crista in these two specimens is almost contiguous with the crista connecting to the anterolateral margin, the structures appearing almost confluent. The small fissure separating the cristae is still visible but less pronounced.

Colour. In adult specimens, the dorsal surface of the carapace, chelae and ambulatory legs are purplish brown, with the fingers of the cheliped orange ( Fig. 1A, B View Fig ). Smaller specimens have dark grey carapaces with the chelae and ambulatory legs mottled dark and light brown ( Fig. 1C, B View Fig ) and the ventral surfaces are pale yellow to pale pink ( Fig. 1E View Fig ).

Etymology. The species is named after Nguyen Xuan Quynh from the Hanoi University of Science, who passed away in early 2022. He was the one who encouraged the first author to start studies on the aquatic fauna of Vietnam in the 1990s, and has been a good friend over the years. He will be sorely missed.

Remarks. As for genus.

Ecology. This is a semiterrestrial species, digging burrows under large rocks by the edge of the stream near a planted Acacia forest ( Fig. 1F View Fig ).