Dacuomon, 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 : 70-71

Dacuomon , new genus

Type species. Dacuomon quynhi , new species, by monotypy.

Diagnosis. Carapace rounded, wider than long; epigastric cristae rugose, low, not sharp, just anterior to sharp postorbital cristae; postorbital cristae not reaching anterolateral margin; anterolateral margin cristate, lined with low granules; median part of posterior margin of epistome with low, broadly triangular structure, lateral parts gently concave; third maxilliped with slender exopod with long flagellum, outer surface with numerous long simple setae; male pleon broadly triangular, somites 1 and 2 relatively wide; suture between male thoracic sternites 2 and 3 deep, sternites 3 and 4 relatively wide, sternopleonal cavity reaching to imaginary line connecting median part of coxae of chelipeds; pressbutton tubercle of male sternopleonal locking mechanism low, on posterior quarter of sternite 5; male thoracic sternite 8 not visible when pleon closed; outer surface of male chela with prominent low tubercles and granules; ambulatory legs relatively long, dactylus long, merus, carpus and propodus of first leg with especially dense long setae; G1 relatively slender; subterminal segment gradually tapering towards terminal segment, groove for G2 ventral; subterminal segment distinctly separated from terminal segment by grooves; terminal segment relatively short, subcylindrical, hook-shaped, curved outwards, tapering to tip, dorsal flap low; G2 elongate, longer than G1; basal segment longer than distal segment.

Etymology. The name Dacuomon is derived from the Vietnamese word for pebble or round stone (đá cuội), in arbitrary combination with the name Potamon . The gender of the genus is neuter.

Remarks. The strongly setose third maxillipeds and ambulatory legs (especially the first) ( Figs. 1B, D , 2A, D , 3A ) suggest Dacuomon , new genus, may be related to Pilosamon Ng, 1996b (type species Potamon (Potamon) laosensis Rathbun, 1904 ), Setosamon Yeo & Ng, 2007 (type species Potamon ubon Ng & Naiyanetr, 1993 ) and to a lesser degree, Kukrimon Yeo & Ng, 2007 (type species Potamiscus cucphuongensis Đăng, 1975 ). Dacuomon most closely resembles Pilosamon in the shape and structure of the carapace, and the postorbital cristae not confluent with the epibranchial tooth, although its anterolateral region is smoother ( Fig. 2A, B ) (cf. Yeo & Naiyanetr, 2010: fig. 1A, B). More significantly, Dacuomon differs in having a proportionately shorter space between the edge of the male sternopleonal cavity and the suture separating thoracic sternites 2 and 3 ( Fig. 3C ) (versus space distinctly longer in Pilosamon Yeo & Naiyanetr, 2010 : fig. 1E, F); the median part of the posterior margin of the epistome has a low, broadly triangular lobe ( Fig. 2C–E ) (versus median part acutely triangular in Pilosamon Yeo & Naiyanetr, 2010 : fig. 1C, D); the male pleon is broadly triangular ( Fig. 3B ) (versus male pleon more narrow and less wide in Pilosamon Ng, 1996b : fig. 2F; Yeo & Naiyanetr, 2010: fig. 1E, F); and the G1 terminal segment only has a low dorsal flap ( Figs. 3E–H , 4A–C ) (versus dorsal flap on G1 terminal segment high in Pilosamon Ng, 1996b : fig. 2B–E; Yeo & Naiyanetr, 2010: fig. 2). Pilosamon is known reliably so far only from Laos and northern Thailand ( Ng, 1996b; Yeo & Naiyanetr, 2010). Setosamon from Thailand has a very different G1 structure, with the terminal segment subtubular in form and the dorsal flap prominently swollen (cf. Ng & Naiyanetr, 1993: figs. 40B–E, 41B–E).

In external appearance, Dacuomon is probably most closely related to Kukrimon , the type species of which occurs in the adjacent of Ninh Binh Province, although their G1 structures are very diferent. While the third maxillipeds of Kukrimon are setose, the setae are neither as dense nor as long, and the first ambulatory leg is not as prominently setose compared to that of Dacuomon . In contrast to the simple tapering form of the G1 terminal segment of Dacuomon ( Figs. 3E–H , 4A–C ), that of Kukrimon is distinctly wider and resembles a broad kukri knife ( Yeo & Ng, 2007: fig. 8D, E). In Dacuomon , the postorbital crista is sharp and prominent ( Fig. 2B ) (versus low and indistinct in Kukrimon ; Fig. 5B ); the antero- and posterolateral regions are much smoother ( Fig. 2B ) (versus with distinct striae and granules in Kukrimon ; Fig. 5B ); the median part of the posterior margin of the epistome has a low, broadly triangular lobe ( Fig. 2C–E ) (versus median part more acutely triangular in Kukrimon ; Fig. 5C, D ); the outer surface of the chela is covered with prominent granules ( Fig. 2F ) (versus surface almost smooth in Kukrimon ; Fig. 5G ); and the male pleon is relatively wider especially at somite 6, with somite 1 wide and subrectangular in shape ( Fig. 3B, C ) (versus male pleon less wide, with somite 1 narrow; Fig. 5E, F ).

The setose carapace of Barbamon Shi, Pan & Sun, 2022 from western Yunnan in China looks superficially like that of Dacuomon . The two genera, however, are otherwise very different, with Dacuomon having the posterior margin of the epistome with the median lobe low and broadly triangular in shape ( Fig. 2C–E ) (versus median lobe prominent and more acutely triangular in Barbamon ; Shi et al., 2022: fig. 2C); the exopod of the third maxilliped has a well-developed flagellum ( Fig. 3A ) (versus flagellum short in Barbamon ; Shi et al., 2022: fig. 4A); the outer surface of chelae are covered with granules ( Fig. 3F ) (versus smooth in Barbamon ; Shi et al., 2022: fig. 2D); ambulatory legs are proportionately longer ( Fig. 2A ) (versus legs distinctly shorter in Barbamon ; Shi et al., 2022: fig. 2A, B); the space between the tip of the male sternopleonal cavity and suture between thoracic sternites 2 and 3 is relatively longer ( Fig. 3C ) (versus shorter in Barbamon ; Shi et al., 2022: fig. 2B); the male pleon is widely triangular in shape ( Fig. 3B, C ) (versus narrower and more quadrate in shape in Barbamon ; Shi et al., 2022: fig. 2B); and the G1 terminal segment is proportionately shorter and gradually tapering toward the tip ( Figs. 3E–H , 4A–C ) (versus terminal segment proportionately longer and blade-like in appearance in Barbamon ; Shi et al., 2022: figs. 4B, C, 5A, B).

Mitra et al. (2021) described Abormon (type species Abormon praecalvum Mitra, Pati & Ng, 2021 ) from northern India based on two species, one of which is very setose ( A. capillosum Mitra, Pati & Ng, 2021 ). Compared to Abormon , Dacuomon differs in having a more prominent median lobe of the posterior margin of the epistome, markedly in lacking a flagellum on the exopod of the third maxilliped, relatively shorter ambulatory legs, the space between the tip of the male sternopleonal cavity and suture between thoracic sternites 2 and 3 is relatively narrower, a less wide male pleon, and the G1 is distinctly shorter (cf. Mitra et al., 2021: figs. 5A, B, 6B, D, E).

Figures

Fig. 1. Dacuomon quynhi, new genus, new species, colour in life. A, B, holotype male (27.7 × 23.8 mm) (ZRC 2022.0047), Phu Yên Province, Vietnam; C–E, paratype male (23.9 × 20.4 mm) (ZRC 2022.0048), Phu Yên Province, Vietnam; F, type locality, Xuan Lam commune, Phu Yên Province, Vietnam.

Fig. 2. Dacuomon quynhi, new genus, new species, holotype male (27.7 × 23.8 mm) (ZRC 2022.0047), Phu Yên Province, Vietnam. A, overall dorsal view; B, dorsal view of carapace; C, frontal view of cephalothorax; D, subfrontal view of cephalothorax showing setose third maxillipeds; E, frontal view showing antennules, antennae and epistome; F, outer view of chelae.

Fig. 3. Dacuomon quynhi, new genus, new species, holotype male (27.7 × 23.8 mm) (ZRC 2022.0047), Phu Yên Province, Vietnam. A, left third maxilliped; B, ventral view of cephalothorax and pleon; B, male pleon; D, sternopleonal cavity showing G1 in situ; E, left G1 (ventral view); F, left G1 (dorsal view); G, terminal segment of left G1 (ventral view); H, terminal segment of left G1 (dorsal view); I, left G2.

Fig. 4. Dacuomon quynhi, new genus, new species, holotype male (27.7 × 23.8 mm) (ZRC 2022.0047), Phu Yên Province, Vietnam. A, left G1 (ventral view); B, left G1 (dorsal view); C, terminal segment of left G1 (ventral view); D, terminal segment of left G1 (dorsal view); E, left G2. Scales: A, D = 1.0 mm B, C = 0.5 mm.

Fig. 5. Kukrimon cucphuongensis (Đăng, 1975), male (22.3 × 19.2 mm) (ZRC 2022.0058), Ninh Binh Province, Vietnam. A, overall dorsal view; B, dorsal view of carapace; C, frontal view of cephalothorax; D, antennules, antennae and epistome; E, ventral view of cephalothorax and pleon; F, sternopleonal cavity showing G1 in situ; G, outer view of chelae; F, pleon.