Rathbunamon chumomrayense, Do, Van Tu, Dang, Van Dong, Cao, Thi Kim Thu & Hoang, Ngoc Khac, 2016

Do, Van Tu, Dang, Van Dong, Cao, Thi Kim Thu & Hoang, Ngoc Khac, 2016, A new species of semi-terrestrial freshwater crab (Crustacea: Decapoda: Brachyura: Potamidae) from the Central Highlands of Vietnam, Zootaxa 4179 (2), pp. 279-287 : 280-286

publication ID	https://doi.org/ 10.11646/zootaxa.4179.2.8
publication LSID	lsid:zoobank.org:pub:C51C4A03-B63C-4A14-B2F6-E7B6A6233D07
DOI	https://doi.org/10.5281/zenodo.6062676
persistent identifier	https://treatment.plazi.org/id/FD2187A1-FFEB-C442-24DC-FE3FB933FA84
treatment provided by	Plazi
scientific name	Rathbunamon chumomrayense
status	sp. nov.

Treatment

Rathbunamon chumomrayense View in CoL sp. nov.

( Figs. 2–5 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Material examined. Holotype: male (23.0 × 18.2 mm) (IEBR-FC KCx01), 11°22'50.5"N 108°47'49.0"E, Chu Mom Ray National Park, Kon Tum Province, Vietnam, 700 m, coll. A. T. Hoang, 6 August 2015 GoogleMaps . Paratype: 1 damaged male (12.9 × 11.1 mm) (IEBR-FC KCx02), same data as holotype. GoogleMaps

Comparative material. Dromothelphusa longipes (A. Milne-Edwards, 1869) : 1 male (39.0 × 31.6 mm) (IEBR-FC DLx01), 8°41'56"N 106°35'49.4"E, forest near Ma Lanh Thien bridge, Con Dao Island, Ba Ria-Vung Tau province, Vietnam, coll. V. T. Do, 23 August 2014 GoogleMaps .

Diagnosis. Carapace quadrate, dorsal surface flat, gently convex laterally, longitudinally; regions poorly defined; anterolateral area rugose; epigastric, postorbital cristae confluent, gently sinuous; frontal region strongly deflected downward, granular; epibranchial tooth poorly developed, almost absent; external orbital angle low but distinct, triangular. Third maxilliped with very short exopod, distinctly shorter than outer margin of ischium, without trace of tooth or flagellum. Finger of cheliped shorter than palm. Suture between male anterior thoracic sternites 2, 3 and sternites 3, 4 faint. Male telson broadly triangular with lateral margins almost straight. G1 slightly sinuous with distal part of subterminal segment narrow, neck-like; terminal segment conical with developed dorsal flap extending more than half length of segment. G2 with distinct distal segment, longer than half length of basal segment.

Description. Carapace quadrate, about 1.2 times broader than long, high; dorsal surface flat, smooth, glabrous; regions poorly defined, cervical grooves undiscernible, H-shaped depression shallow but distinct ( Figs. 2 View FIGURE 2 A, 3A). Epigastric cristae developed, smooth, more or less in line with, confluent with postorbital cristae; postorbital cristae cristate, not confluent with epibranchial tooth; regions behind epigastric, postorbital cristae from having faint granules to smooth ( Figs. 2 View FIGURE 2 A, 3A). Frontal margin broadly emarginate medially; frontal region strongly deflected downwards, granular; antennular fossae slit-like when viewed from front; orbital region narrow, lined with faint granules to almost smooth; supraorbital margin gently sinuous, cristate; infraorbital margins cristate, concave; eyes normal, filling orbit, cornea developed; subhepatic, sub-branchial regions smooth ( Figs. 2 View FIGURE 2 A, B). External orbital angle triangular, outer margin subequal to length of inner margin; epibranchial tooth indistinct, separated from external orbital angle by shallow cleft; anterolateral margin convex, smooth; posterolateral margin rounded, not convergent posteriorly; branchial region almost smooth, weak inflated; metabranchial region smooth ( Figs. 2 View FIGURE 2 A, B). Epistome anterior margin with median triangle; posterior margin with low median triangular tooth, slightly crenulated, with outer part slightly concave ( Fig. 2 View FIGURE 2 B).

Third maxilliped glabrous; ischium rectangular, about 1.7 times longer than broad, distinctly broader (1.3 times) than merus, with distinct longitudinal median sulcus; merus quadrate, about 1.3 broader than long, shorter than half (0.4 times) of ischium length, with concave outer surface; palp normal with tip exceeding proximal margin of merus; exopod very sort, reaching to just over a half of ischium, about 0.6 times length of ischium, distally tapered, without trace of tooth or flagellum ( Figs. 2 View FIGURE 2 B, 3B).

Chelipeds unequal, right side larger; outer surfaces of merus, carpus, palm rugose to granular ( Fig. 2 View FIGURE 2 B). Merus with small subterminal spine; carpus with robust, obliquely directed subdistal spine on inner margin. Right cheliped with length of palm plus pollex about 2.3 times palm height; finger shorter than palm (0.9 times), movable finger shorter than fixed finger, tips hooked, overlapping when closed, surface smooth, with several longitudinal rows of pits.

Ambulatory legs glabrous, relatively slender; dactylus elongated, slender; merus unarmed with upper margin weakly serrated; dactylus of second pair about 7.1 times longer than proximal width, propodus about 2.7 times longer than broad, about 0.9 times length of dactylus, carpus about 0.7 times length of dactylus, merus about 1.6 times longer than dactylus; dactylus of fourth pair about 8.2 times longer than proximal width, propodus about 0.8 times length of dactylus, carpus about 0.7 times length of dactylus, merus about 4.0 times longer than proximal width, about 1.3 times longer than dactylus ( Fig. 2 View FIGURE 2 A).

Sutures between male anterior thoracic sternites 2, 3, between sternites 3, 4 obscure ( Figs. 2 View FIGURE 2 D, 3C); thoracic sternites 5, 6 medially interrupted; sternites 7, 8 medially separated by distinct longitudinal median groove. Male sternoabdominal cavity reaching imaginary line joining middle points of cheliped coxae (at sternite 4) ( Fig. 2 View FIGURE 2 D). Tubercles of male abdominal-locking mechanism positioned on thoracic sternite 5.

Male abdomen triangular; telson broadly triangular, broader than long, lateral margins almost straight, tip rounded, slightly longer than somite 6; somite 6 with proximal width about 2.5 times length, lateral margins gently convex; lateral margins of somites 4, 5 straight; lateral margins of somite 3 gently convex ( Figs. 2 View FIGURE 2 C, 3D).

G1 slightly sinuous, terminal segment conical, gently curved outward, about 0.5 times length of subterminal segment, about 2.8 times longer than proximal width, dorsal flap semicircular, extending distally more than halfway along terminal segment, produced in proximal portion, tapering towards tip, about 0.2 times length of terminal segment (from ventral view); subterminal segment broad, with neck-like distal part, shallow subdistal cleft on outer margin ( Figs. 4 View FIGURE 4 A–H). G2 subequal to G1, distal segment about 0.6 times length of basal segment ( Figs. 4 View FIGURE 4 I–J).

Variation. The only other specimen available is a juvenile male and it was badly damaged.

Etymology. The new species is named after the type locality, Chu Mom Ray.

Coloration. In living specimens, the carapace (except frontal region) and chelipeds are orange-red; with the ambulatory legs reddish ( Figs. 5 View FIGURE 5 A, B).

Ecological notes. This species inhabits forested mountain areas. The larger specimen was found running around on the forest floor, far from the stream ( Fig. 6A View FIGURE 6. A – B ); however, the smaller specimen was found near the stream ( Fig. 6B View FIGURE 6. A – B ). It is believed that this species spends most of their time in the terrestrial environment.

Remarks. R. chumomrayense clearly belong to Rathbunamon by the following characteristics: dorsal surface of carapace is smooth, gently convex laterally and longitudinally; anterolateral margin is smooth; epibranchial tooth is very small, blunt, hardly discernible; epigastric and postorbital cristae are confluent; exopod of third maxilliped is short, distinctly shorter than outer margin of ischium; flagellum is absent; ischium is distinctly broader than merus; dactyli of ambulatory legs elongate; suture between thoracic sternites 2 and 3 is straight and incomplete; male abdomen is broadly triangular; distal part of subterminal segment of G1 is narrow, neck-like; dorsal fold is present in terminal segment; G2 distal segment is shorter than basal segment (Ng, 1996).

The main character which distinguishes Rathbunamon (including in the new species) from most other Asian potamid genera is the shortest exopod of the third maxilliped. Phaibulamon Ng, 1992 , and Pudaengon Ng & Naiyanetr, 1995 , also have a short third maxilliped exopod (not exceeding the length of the merus), but they possess different carapace features, gonopods and the suture separating thoracic sternites 2 and 3 are complete (obscure in Rathbunamon ) (Ng, 1992, 1996; Ng & Naiyanetr, 1995).

The new species is similar to the type species of the genus, Rathbunamon lacunifer (Rathbun, 1904) , with regards to its relative small adult size, very short exopod of the third maxilliped, structure of the thoracic sternum type and the shape of the G1. The new species can nevertheless be immediately distinguished from R. lacunifer by the following characteristics: 1) anterolateral regions are slightly rugose (in contrast to anterolateral regions smooth); 2) the confluent epigastric and postorbital cristae are sinuous (in contrast to the confluent epigastric and postorbital cristae are straight); 3) anterolateral margins are convex (in contrast to anterolateral margins strongly convex); 4) fingers of chelipeds are shorter than palm (in contrast to fingers subequal to, or slightly longer than palm); and 5) terminal segment of G1 is slightly curved outwards with narrower dorsal flap, about 0.2 times length of terminal segment (in contrast to terminal segment of G1 strongly curved outward with broader dorsal flap, about 0.3 times length of terminal segment). It is perhaps not surprising to discover a second species of Rathbunamon from the Central Highlands of Vietnam. Rathbunamon lacunifer was collected from somewhere in southern Laos, which is near the region the new species was found ( Fig. 1 View FIGURE 1 ). If we examine the zoogeography and distribution of the described species, which is important for Indochinese crabs, Vietnam and Laos belong to Indochina area in the Oriental zoogeographical region ( Yeo et al., 2008).

The new species is also superficially similar to some species of Dromothelphusa Naiyanetr, 1992 , owing to its high, quadrate carapace and prominent dorsal flap on the G1 terminal segment. In R. chumomrayense sp. nov., however, the sutures between male thoracic sternites 2 and 3, and sternites 3 and 4 are incomplete (in contrast to sutures between thoracic sternites 2 and 3 and sternites 3 and 4 complete); the exopod of the third maxilliped is very short, just reaching to over a half of ischium (in contrast to exopod of the third maxilliped long, exceeding distal margin of ischium), and the terminal segment of G1 and its dorsal flap are proportionately broader (in contrast to terminal segment and its dorsal flap narrower).

The new species is restricted to the Chu Mom Ray National Park, Central Highlands of Vietnam. It has not been found in other regions around this area during our surveys. We believe this species has a small distribution area. Although, the area of occupancy belongs to the protected area, deforestation is still continuing. Therefore, this species should be considered endangered due to limited range and impacts of deforestation on the IUCN Red List Categories and Criteria ( IUCN, 2001).