Mekhongthelphusa menglongensis, Yeo Hongying Sun, 2021

Mekhongthelphusa menglongensis View in CoL sp. nov.

( Figs. 1–3 View Figure 1 View Figure 2 View Figure 3 )

Zoobank: urn:lsid:zoobank.org:act:E87171F9-B10F-4301-BC5F-B0F6C87ABD25

Type material. Holotype: male (35.1 × 30.4 mm) ( NNU 16 C-MM01), Menglong Town , Jinghong City, Yunnan Province, China, 21°35’17”N 100°39’05”E, coll. Da Pan and Ruxiao Wang, 28 Sept. 2019. GoogleMaps

Paratypes: 1 male (34.3 × 29.2 mm, NNU 16 C-MM02), same data as holotype GoogleMaps . 4 females (34.7 × 28.9 mm) ( NNU 16 C-MM03), (33.8 × 28.4 mm) ( NNU 16 C-MM04), (33.5 × 27.3 mm) ( NNU 16 C-MM05), (35.9 × 29.2 mm) ( ZRC 2021.0531 View Materials ), same data as holotype GoogleMaps .

Comparative material examined. The present new species was compared against literature as well as the specimens listed below:

Mekhongthelphusa brandti - Holotype, male (21.8 × 17.8 mm) ( SMF 4405 About SMF ), Tad San Falls , 61 km from Loei to Dan Sai, Loei Province, Thailand, coll. R. Brandt, 18 Dec.1967.

Mekhongthelphusa kengsaphu - Holotype, male (33.7 × 26.5 mm) ( ZRC 1995.286 View Materials ), under water plants, Mun River , Kengsaphu, Amphoe Phibun Mamgsahan, Ubon Ratchatani , Thailand, coll. Somluck Kuntarphrug, 27 Mar.1991.

Mekhongthelphusa neisi - Lectotype, male (22.6 × 18.5 mm) ( MNHN-B 5311 ), Cochinchine, coll. Harmand, 1876 . Paralectotypes: 1 male (28.4 × 23.0 mm), 1 female ( MNHN-B 5311 ), same data as lectotype. The lectotype designation and status of these type specimens will be discussed in an upcoming revision of the genus (D.C.J. Yeo, unpublished data) .

Mekhongthelphusa tetragona - Lectotype, female (24.9 × 21.1 mm) ( MNHN-B 5318 ), no other data (unknown locality) . Paralectotypes: 3 females ( MNHN-B 5318 ) , 2 females (larger 24.8 × 21.7 mm) ( USNM 30018 About USNM ), same data as lectotype. Others : 1 male (24.5 × 21.0 mm), 2 females (larger 33.5 × 28.1 mm) ( ZRC 1995.285 View Materials ), along bank, under stones, on muddy sand, Mekong River , Amphoe Maung, Changwat Mukdahan, northeastern Thailand, coll. P. Naiyanetr, 2 Dec. 1982 .

Diagnosis. Carapace trapezoidal, slightly broader than long, dorsal surface smooth, slightly convex; posterolateral margins gently converging; H-shaped groove distinct; epigastric cristae distinctly anterior of postorbital cristae, outer edge slightly overlapping inner edge of postorbital cristae; postorbital cristae distinct, entire, sharp, very gently sinuous, curving obliquely posterolaterally, reaching but not confluent with base of third epibranchial teeth; anterolateral margin with 3 distinct epibranchial teeth. Third maxilliped ischium rectangular, pitted; exopod long, reaching beyond midpoint of merus, with long, well-developed f lagellum. Male pleon T-shaped, somites 5 and 6 together constricted medially, with lateral margins concave. Male chelipeds strongly asymmetrical, cutting edge lined with small, rounded teeth, gap distinct when fingers closed. G1 strongly curved outwards, distal part slender, tip longitudinally twisted, slightly bent, distinctly hooked in appearance, basal part dilated up to least half length of G1, with broadest part approximately one-third length of G1.

Description. Carapace trapezoidal, slightly broader than long, dorsal surface smooth, slightly convex, regions defined ( Fig. 1A View Figure 1 ); H-shaped groove distinct; frontal margin cristate, slightly sinuous, anterior of external orbital angle confluent with supraorbital margin ( Fig. 1A View Figure 1 ), with distinct, complete frontal median triangle, frontal region not deflexed downwards, broad, smooth; supraorbital margin sinuous, cristate; infraorbital margin curved, cristate; orbital region relatively broad; eyes normal; subhepatic and subbranchial regions with sparse low granules or rugae; epigastric cristae distinct, sharp, slightly oblique, separated by deep, distinct groove, distinctly anterior of postorbital cristae, separated from postorbital cristae by distinct groove, outer edge slightly overlapping inner edge of postorbital cristae ( Fig. 1A, C View Figure 1 ); postorbital cristae distinct, entire, sharp, very gently sinuous, curving obliquely posterolaterally, reaching, but not confluent with, base of third epibranchial teeth; regions behind epigastric and postorbital cristae smooth; external orbital angle distinct, triangular, inner margin shorter than outer margin, outer margin convex, with small gap separating it from first epibranchial tooth ( Fig. 1A View Figure 1 ); anterolateral margin short, with 3 distinct epibranchial teeth, epibranchial teeth relatively narrow, first and second teeth subequal in size, third tooth smallest, with distinct small gap between second and third teeth, confluent with posterolateral margin( Fig. 1A View Figure 1 ); posterolateral margins straight, slightly converging posteriorly, posterolateral region lined with oblique striae ( Fig. 1A View Figure 1 ). Epistome posterior margin median tooth distinct, broadly triangular, outer parts not concave, sloping downwards, lateral parts gently sinuous; median endostomial ridge not clearly visible from frontal view ( Fig. 1C View Figure 1 ).

Third maxilliped ischium rectangular, distinctively pitted, about 1.5 × longer than broad, with distinct longitudinal sulcus; merus subrectangular, shorter than half ischium length,with concave outer surface; exopod long, beyond midpoint of merus, with well-developed f lagellum, longer than merus width ( Fig. 2A View Figure 2 ).

Male thoracic sternum smooth, pitted, suture between thoracic sternites 2 and 3 complete, distinct, suture between thoracic sternites 3 and 4 indistinct, with groove at lateral edges ( Fig. 1B View Figure 1 ); male sternopleonal cavity reaches an imaginary line joining anterior edges of bases of chelipeds ( Fig. 1B, H View Figure 1 ); male pleon T-shaped ( Fig. 1B, F View Figure 1 ); telson tongueshaped, about 1.1 × broader than long, subequal in length to somite 6, tip broadly blunt, lateral margins slightly concave ( Fig. 1B,F View Figure 1 ); somites 5 and 6 together constricted medially; somite 6 distal width about 1.1 × length, distal margin 1.2 × longer than proximal margin, lateral margins concave; somite 5 proximal width about 2.2× length, proximal margin 1.6 × longer than distal margin, lateral margins concave ( Fig. 1B, F View Figure 1 ). Female pleon broadly oval ( Fig. 1G View Figure 1 ).

Male chelipeds strongly asymmetrical, right cheliped larger in holotype; manus of major chela surface smooth, fingers longer than palm, moveable finger strongly curved downward, immovable finger gently curved upward, cutting edges lined with small, rounded teeth; gap distinct when fingers closed, tips slightly overlapping ( Fig. 1A, D, E View Figure 1 ); fingers of minor chela slightly gaping when closed ( Fig. 1D View Figure 1 ); carpus smooth, with distinct, obliquely directed subdistal spine on inner margin; merus with low subterminal spine ( Fig. 1A, D, E View Figure 1 ). Female chelipeds slightly asymmetrical.

Ambulatory legs glabrous, smooth; second ambulatory legs longest, merus about 1.4 × as long as dactylus, with distinct dorsal subdistal spine, propodus with small spines on both margins, dactylus slender, with short chitinous spines ( Fig. 1A View Figure 1 ).

G1 strongly curving outwards, terminal and subterminal segments not clearly demarcated, tapering distally; distal part slender, smoothly and strongly curving outwards ( Figs. 2B, C View Figure 2 , 3A, B View Figure 3 ), tip longitudinally twisted, slightly bent, distinctly hooked in appearance ( Figs. 2D, E View Figure 2 , 3A, B View Figure 3 ); basal part dilated, up to at least half length of G1, gently tapering distally, with broadest part approximately one-third length of G1( Figs.2D, E View Figure 2 , 3A, B View Figure 3 ); G2 subterminal segment basal part inflated, terminal segment slender, distinctly shorter than half of subterminal segment ( Fig. 2F View Figure 2 ). Female vulvae round, with operculum, located slightly below suture of sternite 5/6 ( Fig. 1I View Figure 1 ).

Remarks. This new species is assigned to Mekhongthelphusa s. l. ( Ng et al., 2008; D.C.J. Yeo et al., unpublished data) based on the following characters: carapace slightly broader than long; epigastric cristae distinctly anterior of postorbital cristae, with outer edge slightly overlapping inner edge of postorbital cristae; postorbital cristae curving obliquely posterolaterally, reaching but not confluent with base of third epibranchial teeth; male pleon T-shaped; and G1 curved outwards, with dilated basal part, and tip distinctly longitudinally twisted, superficially resembling a very short “terminal segment” ( Naiyanetr, 1994; Naiyanetr and Ng, 1995; D.C.J. Yeo, unpublished data).

Mekhongthelphusa menglongensis View in CoL can be easily distinguished externally from M. tetragona View in CoL and M. kengsaphu View in CoL by its relatively more transverse carapace being distinctly broader ( Fig. 1A View Figure 1 ) (vs. carapace more squarish, cf. Naiyanetr and Ng, 1995: figs. 1, 4, 6B) and slightly convex carapace dorsal surface ( Fig. 1A, C View Figure 1 ) (vs. carapace dorsal surface comparatively flat, cf. Naiyanetr and Ng, 1995: figs. 1, 2A, 4, 5A, 6A, B), as well as by its relatively more slender and more strongly curved G1 ( Figs. 2B–E View Figure 2 , 3A, B View Figure 3 ) (vs. G1 relatively broader and more gently curved, cf. Naiyanetr and Ng, 1995: figs. 3B–E, 5D–G, 6D, E).

Mekhongthelphusa menglongensis View in CoL is superficially most similar to M. brandti View in CoL and M. neisi View in CoL in carapace morphology. Mekhongthelphusa menglongensis View in CoL , however, can be distinguished externally from M.neisi View in CoL by its relatively narrower external orbital angle and epibranchial teeth ( Fig.1A View Figure 1 ) (vs. external orbital angle and epibranchial teeth relatively broader, Fig. 4B View Figure 4 ; cf. Rathbun, 1905: fig. 61, pl. XI fig. 5), and furthermore by its stouter and more strongly curved G1 with tip appearing distinctly hooked ( Figs. 2B–E View Figure 2 , 3A, B View Figure 3 ) (vs. G1 distinctly more slender and gently curved, with tip weakly hooked in appearance, Fig. 4E, F View Figure 4 ).

Mekhongthelphusa menglongensis View in CoL closely resembles M. brandti View in CoL in its G1 being strongly curved, with a distinctly hooked tip.The new species can nevertheless be separated from M. brandti View in CoL by the relatively more strongly curved G1 ( Figs. 2B–E View Figure 2 , 3A, B View Figure 3 ) (vs. G1 less strongly curved, Fig. 4C, D View Figure 4 ; cf. Naiyanetr, 1994: fig. 4C), and basal part being dilated up to at least half length of G1, with broadest part at approximately one-third length of G1 ( Figs. 2B, C View Figure 2 , 3A, B View Figure 3 ) (vs. basal part being dilated up to less than half length of G1, with broadest part at less than one-third length of G1, Fig. 4C, D View Figure 4 ; cf. Naiyanetr, 1994: fig. 4C).

In China, species of Somanniathelphusa Bott, 1968 View in CoL , also have a T-shaped male pleon ( Dai,1999). However, M. menglongensis View in CoL sp. nov. can be easily distinguished morphologically from Somanniathelphusa species by its low and relatively squarish carapace, with relatively flat dorsal surface, gently converging posterolateral margins, and long postorbital cristae, extending beyond cervical grooves to reach anterolateral margin (vs. carapace high, distinctly broader than long, with strongly convex dorsal surface, distinctly converging posterolateral margins, and short postorbital cristae, notexceedingcervicalgroovesorreachinganterolateral margins); proportionately broader male pleon, with somites 5 and 6 having gently concave lateral margins (vs. male pleon narrower, with somites 5 and 6 having more distinctly concave lateral margins); and distally gradually tapered G1, from an expanded basal part into a narrower distal part, with a longitudinally twisted tip of G1 (vs. G1 abruptly narrowing, from expanded basal part into narrow distal part, and lacking longitudinally twisted tip) ( Figs.1A–C View Figure 1 , 2B–E View Figure 2 ; cf. Naiyanetr, 1994: figs. 1, 4, 5).

Etymology. The species name is derived from the type locality, Menglong Town, Jinghong City, Xishuangbanna Dai Autonomous Prefecture, Yunnan Province, China.

Color in life. The dorsal surface of carapace is light-brownish with dark-brown patches at progastric regions bordered posterolaterally by the cervical grooves. The ambulatory legs are dark-brown all over.

Habitat. Specimens were found in a small gravelbed river, slightly turbid, slow flowing, about 5 m in width. The crabs were collected from the middle of the river.

Distribution. The species is so far only known from Menglong Town, Jinghong City, Xishuangbanna Dai Autonomous Prefecture, Yunnan Province, China.