Laevimon, Yeo, Darren C. J. & Ng, Peter K. L., 2005

Yeo, Darren C. J. & Ng, Peter K. L., 2005, On a new genus and species of freshwater crab from Vietnam, with comments on the nomenclatural status of Orientalia Dang, 1975 (Crustacea: Brachyura: Potamidae: Potamiscinae), Zootaxa 917, pp. 1-15 : 2-11

publication ID	https://doi.org/ 10.5281/zenodo.171066
DOI	https://doi.org/10.5281/zenodo.5694177
persistent identifier	https://treatment.plazi.org/id/03E787AA-FFAA-084D-FE9A-F909542A4A73
treatment provided by	Plazi
scientific name	Laevimon
status	gen. nov.

Treatment

Laevimon View in CoL , new genus

Type species. Laevimon kottelati , new species, by present designation.

Diagnosis. Carapace distinctly transverse, high; dorsal surface strongly convex longitudinally and transversely; smooth; grooves weak to indistinct; regions poorly defined; epigastric cristae very weak, indistinct, confluent with postorbital cristae; postorbital cristae indistinct, rounded; antennular fossae slitlike; external orbital angle acutely triangular, separated from epibranchial tooth by distinct, rounded cleft; epibranchial tooth distinct, very low, rounded; anterolateral margin distinctly convex, entire, very weakly cristate; median tooth of posterior margin of epistome low, lateral and outer parts forming continuous sinuous margin, with lateral part convex and outer part gently concave. Ischium of third maxilliped broadly rectangular; exopod reaching beyond upper edge of ischium, with well developed flagellum subequal to or exceeding width of merus. Chelipeds distinctly unequal. Ambulatory legs moderately long, with long, slender dactyli. Suture between thoracic sternites 3 and 4 demarcated by strong ridge; abdominal cavity reaching imaginary line joining median points of coxae of chelipeds. Male abdomen narrowly triangular. G1 terminal segment about 0.3 times length of subterminal segment, subcylindrical, straight, tip truncate with inner distal angle produced as sharp, narrow projection, proximal part with narrow, humplike, dorsal flap; subterminal segment relatively slender. G2 with distal segment longer than half of basal segment.

Etymology. The genus name is derived from laevis, Latin for smooth or polished, in arbitrary combination with the genus name, Potamon , in allusion to the carapace form of the genus. The gender is neuter.

Remarks. Laevimon , new genus, is morphologically closest to Hainanpotamon Dai, 1995 , owing to its high, swollen and smooth carapace; long exopod flagellum of third maxilliped; long, slender ambulatory dactyli; and slender G1 with a short, straight terminal segment and narrow, proximal dorsal flap. Laevimon , however, can be immediately distinguished from Hainanpotamon by the following characters: i) carapace proportionately more strongly convex and raised laterally (versus carapace relatively less strongly convex); ii) postorbital cristae indistinct (versus postorbital cristae distinct); iii) epibranchial tooth very low, rounded (versus epibranchial tooth relatively more developed, triangular); iv) anterolateral margin entire, very weakly cristate (versus anterolateral margin weakly serrated, relatively more distinctly cristate); v) posterior margin of epistome with the lateral and outer parts forming a continuous sinuous margin, with the convex lateral part confluent with the gently concave outer part (versus posterior margin of the epistome with the lateral parts straight and distinct from gently concave outer parts); vi) suture between thoracic sternites 3 and 4 demarcated by distinct, strong ridge (versus suture between thoracic sternites 3 and 4 not discernible); and vii) G1 terminal segment subcylindrical, with tip truncate (versus G1 terminal segment conical, with tip tapered) ( Figs. 2–6 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 ; Dang 1967: fig. 6, 1980: fig. 242; Dang & Tran 1992: figs. 1, 2; see also Table 1 View TABLE 1 ; Dai 1995).

Other Indochinese potamiscine genera that have swollen, smooth, eggshaped carapaces similar to Laevimon are Thaipotamon Ng & Naiyanetr, 1993 [ Thailand], Thaiphusa Ng & Naiyanetr, 1993 [ Thailand], and Pudaengon Ng & Naiyanetr, 1995 [ Thailand, Laos]. Laevimon is immediately differentiated from these by its unique suite of characters of the third maxilliped, thoracic sternum and G1 (see Table 1 View TABLE 1 ).

Crabs of the genus Tiwaripotamon Bott, 1970 [central Vietnam to southern China] also have a relatively smooth carapace ( Bott 1970, Ng & Yeo 2001). However, the genus can immediately be distinguished by their distinctly lower and flatter carapace; slender, elongated legs; and third maxilliped with a squarish ischium having a shallow or no sulcus, and a short exopod with a short flagellum. This contrasts with Laevimon species, all of which have a high, swollen carapace; ambulatory legs that are not unusually elongated; and a third maxilliped with a rectangular ischium possessing a distinct sulcus, and a long exopod with a long flagellum ( Figs. 2 View FIGURE 2 A, B, 3A, 4A, B; cf. Bott 1970, Dai 1999, Ng & Yeo 2001).

Distribution. Hai Phong, Thanh Hoa Province, northern Vietnam; Nghe Tinh Province, central Vietnam ( Dang & Tran 1992).

Laevimon Hainanpotamon Thaipotamon Thaiphusa Pudaengon G1 terminal Subcylindrical; Conical, with tip Conical, with Conical, with Conical, with segment tip broad and comparatively tip compara tip compara tip comparashape truncate, with slenderer and tively slen tively slen tively slen

inner distal tapered, lacking derer and derer and derer and

angle produced projection on tapered, lack tapered, lack tapered, lackas sharp, narrow inner distal angle ing projection ing projection ing projection projection on inner distal on inner distal on inner distal angle angle angle

G1 terminal Narrow, hump Narrow, hump Broad, large, Broad, low, Broad, large, segment dor like, in proximal like, in proximal semicircular, distinctly semicircular, sal flap part, distinctly part, distinctly distinctly longer than distinctly

shorter than half shorter than half longer than half length of longer than length of termi length of termi half length of terminal seg half length of nal segment nal segment terminal seg ment terminal segment ment

Laevimon kottelati , new species ( Figs. 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 )

Material examined. Holotype: male (45.1 by 33.4 mm) ( ZRC 2004.695), unnamed stream on road from National Park HQ to Gia Luan (Ra Luan), about 1 km, Cat Ba Island, Hai Phong, northern Vietnam, coll. M. Kottelat et al., 25 Sep.1998. Paratypes: 1 female (44.9 by 33.5 mm) ( ZRC 2004.696), unnamed stream on road from National Park HQ to Gia Luan, about 2–3 km, northern Vietnam, coll. M. Kottelat et al., 26 Sep.1998; 3 males (largest 27.6 by 20.9 mm) ( ZRC 2004.697), Cat Ba Island, 20º4757” N 107º00’05” E, northern Vietnam, coll. T. Whitten, 1998.

Description. Carapace ( Fig. 2 View FIGURE 2 A, B) distinctly transverse, 1.32–1.35 times broader than long, high; dorsal surface strongly convex longitudinally and transversely, glabrous; regions indistinct; cervical grooves faint; Hshaped groove also indistinct. Epigastric cristae poorly developed, rounded, smooth, very weakly separated by faint groove, slightly anterior to postorbital cristae, confluent with postorbital cristae; postorbital cristae indistinct, straight, rounded, smooth, confluent with epibranchial teeth; regions behind epigastric and postorbital cristae smooth. Frontal margin very gently sinuous, cristate; frontal region strongly deflexed downwards, smooth; antennular fossae slitlike when viewed from front; supraorbital margin very gently sinuous, cristate; infraorbital margin straight, cristate; orbital region relatively narrow; eyes normally developed; subhepatic and subbranchial regions rugose. External orbital angle well developed, acutely triangular, outer margin subequal to inner margin, margins cristate, with shallow, narrow, rounded cleft separating it from epibranchial tooth; epibranchial tooth weak, low, rounded; anterolateral margin strongly convex, indistinctly cristate, almost smooth, entire, not confluent with posterolateral margin, running inwards posteriorly; posterolateral margin strongly converging posteriorly, entire, gently convex; branchial region smooth, inflated; metabranchial region smooth, lacking oblique striae. Epistome with median triangle on anterior margin; median tooth on posterior margin very low, broadly triangular, lateral and outer parts forming continuous sinuous margin, with gently concave outer parts and convex lateral parts; median endostomial ridge hardly visible ( Fig. 2 View FIGURE 2 B).

Third maxilliped glabrous; ischium broadly rectangular, with distinct longitudinal median sulcus; merus squarish, subequal to half of ischium length, with concave outer surface, with smooth margins; palp threesegmented, with tip exceeding proximal margin of merus, lacking any distinctive feature; exopod long, exceeding distal margin of ischium, straight, distally tapered, inner margin of distal part produced as a tooth, flagellum well developed, longer than or subequal to width of merus ( Figs. 2 View FIGURE 2 B, 3A).

Chelipeds unequal, outer surface of merus, carpus and palm weakly rugose; fingers strongly gaping, longer than palm, tips hooked and overlapping, carpus armed with robust, obliquely directed subdistal spine on inner margin; merus without subterminal spine ( Fig. 2 View FIGURE 2 A). Chelipeds similar in males and females.

Ambulatory legs glabrous, relatively short and stout; dactyli elongated, slender; merus unarmed, with upper margin weakly serrated; dactylus of fourth ambulatory leg about 1.3 times as long as propodus, about 6.2 times longer than proximal width; propodus, carpus and merus rugose ( Fig. 2 View FIGURE 2 A).

Suture between thoracic sternites 2 and 3 distinct, complete, straight; groove or suture between thoracic sternites 3 and 4 demarcated by distinct, strong, complete ridge, fused with tip of margin of abdominal cavity; thoracic sternites 5 and 6 medially interrupted; thoracic sternite 7 completely separated by longitudinal median line; thoracic sternite 8 completely separated by longitudinal median line, lacking transverse ridge; abdominal cavity reaching imaginary line joining median part of coxae of chelipeds ( Figs. 1 View FIGURE 1 , 2 View FIGURE 2 C).

Male abdomen narrowly triangular; telson narrowly triangular, longer than or subequal to proximal width, slightly longer than sixth segment, lateral margins gently concave, tip rounded, proximal margin almost straight; segment 6 elongate rectangular or trapezoidal, median length about half of proximal width, proximal margin almost straight, lateral margins convex; lateral margins of segments 3 to 5 convex ( Fig. 2 View FIGURE 2 C).

G1 gently sinuous, slender; terminal segment clearly separated from subterminal segment, relatively short and stout, about 0.3 times length of subterminal segment, subcylindrical, straight, without longitudinal torsion, without swelling on inner margin, dorsal flap present in proximal part, relatively high, narrow, humplike, tip truncate with inner distal angle produced as sharp, narrow projection; groove for accommodation of G2 marginal; subterminal segment slender, not distinctly constricted or necklike distally, without shelf on upper part of outer margin ( Fig. 3 View FIGURE 3 B–E). G2 with distal segment distinctly longer than half of basal segment, slender, tapering distally, without distal projection; outer margin of basal segment gently convex ( Fig. 3 View FIGURE 3 F).

Etymology. The species is named after its collector, Dr. Maurice Kottelat.

Remarks. Laevimon kottelati , new species, is very similar to L. tankiense ( Dang & Tran, 1992) . Laevimon kottelati can, however, still be easily separated from L. tankiense by the differences of the carapace and G1: i) carapace relatively less broad, 1.32–1.35 times broader than long (versus carapace broader, 1.43–1.52 times broader than long); ii) anterolateral and posterolateral margins relatively less strongly convex and less strongly converging, respectively (versus anterolateral and posterolateral margins relatively more strongly convex and more strongly converging, respectively); iii) cleft separating external orbital angle and epibranchial tooth relatively narrower, less distinct (versus cleft relatively broader, more distinct); iv) epibranchial tooth relatively lower and weaker (versus epibranchial tooth relatively higher and more strongly developed); v) ridge demarcating suture between thoracic sternites 3 and 4 complete, fused with tip of margin of abdominal cavity (versus ridge demarcating suture between thoracic sternites 3 and 4 incomplete, broken medially, not fused with tip of margin of abdominal cavity); vi) male abdominal segment 6 lateral margins distinctly convex (versus male abdominal segment 6 lateral margins very gently convex to almost straight); vii) G1 terminal segment relatively more slen der, with a relatively higher and narrower dorsal flap (versus G1 terminal segment relatively stouter, with a relatively lower and broader dorsal flap); and viii) G1 subterminal segment is relatively more slender (versus subterminal segment relatively broader) ( Figs. 2–6 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 ).

Ecological note. The specimens of Laevimon kottelati were collected from along the stream banks as well as a short distance from the streams (M. Kottelat, pers. comm.). This semiterrestrial habit is unusual for freshwater crabs with such carapace physiognomy (high, swollen carapace), instead normally being associated with more terrestrial forest habitats (e.g., Terrapotamon abbbotti , Thaiphusa sirikit , Thaipotamon chulabhorn , Pudaengon spp.) or poorlyoxygenated aquatic habitats (e.g., Somanniathelphusa spp.)

( Naiyanetr 1992, 1993; Ng 1993; Ng & Naiyanetr 1993, 1995, 1998; Ng & Kosuge 1997; pers. observ.).

Distribution. Hai Phong, northern Vietnam.

Laevimon tankiense ( Dang & Tran, 1992) View in CoL ( Figs. 4–6 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Orientalia tankiensis Dang & Tran, 1992: 19 , Fig. 2 View FIGURE 2 ; Dang & Ho, 2001: 74, Fig. 9. Hainanpotamon tankiensis .— Yeo & Ng, 1999: 641.

Material examined. Holotype: male (40.0 by 26.4 mm) (VNCNST), Tan Ky District, Nghe Tinh Province, northern Vietnam, coll. Jul.1987. Paratype: 1 female (33.5 by 23.4 mm) (VNCNST), same data as holotype.

Diagnosis. Carapace distinctly transverse, 1.43–1.52 times broader than long, high; dorsal surface strongly convex longitudinally and transversely, smooth; epigastric cristae very weak, indistinct; postorbital cristae indistinct, rounded; external orbital angle acutely triangular, separated from epibranchial tooth by distinct, broad rounded cleft; epibranchial tooth low but distinct, rounded; anterolateral and posterolateral margins very strongly convex and strongly converging, respectively. Third maxilliped exopod reaching beyond distal margin of ischium, flagellum well developed, subequal to or exceeding width of merus. Ambulatory dactyli long, slender. Suture between thoracic sternites 3 and 4 demarcated by strong incomplete ridge, not fused with tip of margin of abdominal cavity. Male abdomen narrowly triangular; segment 6 with lateral margins very gently convex. G1 terminal segment about 0.3 times length of subterminal segment; subcylindrical, straight, tip truncate with inner distal angle produced as sharp, narrow projection, with narrow, humplike, dorsal flap in proximal part; subterminal segment relatively slender. G2 with distal segment longer than halflength of basal segment.

Remarks. Laevimon tankiense ( Dang & Tran, 1992) differs from its only congener, L. kottelati , new species, by several characters of the carapace, thoracic sternum and G1 (see Remarks for Laevimon kottelati ). The first author examined the types of this species while visiting the VNCNST but, unfortunately, was unable to illustrate specimens. Dang & Tran’s (1992) figure 2 and Dang & Ho’s (2001) figure 9 are therefore reproduced here in Figs. 5 View FIGURE 5 and 6 View FIGURE 6 for comparison. Although the drawings by Dang & Tran (1992) and Dang & Ho (2001) are rather diagrammatic, they nevertheless agree relatively well with the types reexamined. Dang & Tran’s (1992: fig. 2) drawing of the G1 was made with the structure tilted on its side and hence their figure may be somewhat misleading. In any case, the G1 of L. kottelati , when viewed and compared from all possible orientations, neither matches the G1 of the holotype examined by the first author nor the published illustrations of L. tankiense ( Dang & Tran 1992, Dang & Ho 2001). Therefore, we are certain that we are dealing with interspecific differences between L. kottelati and L. tankiense .