Ptychognathus lipkei N.K. Ng, 2010

Komai, Tomoyuki, Maenosono, Tadafumi, Saeki, Toshifumi & Naruse, Tohru, 2021, Redescription of the brackish water crab Ptychognathus glaber Stimpson, 1858 based on the topotypic material from the Ogasawara Islands and new record of P. lipkei N. K. Ng, 2010 from Japan (Decapoda: Brachyura: Varunidae), Zootaxa 5048 (1), pp. 58-76 : 66-70

Ptychognathus lipkei N.K. Ng, 2010

[New Japanese name: Kenaga-hiraiso-modoki]

( Figs. 6–8 )

Ptychognathus glaber .—de Man 1892: 324, pl. 19, fig. 11.— Tesch 1918: 87 (key).—N.K. Ng 2010: 548 (part).— Nakasone & Irei 2003: 274 (key to varunid species in Ryukyu Islands).

Ptychognathus lipkei N.K. Ng, 2010: 548 , figs. 1–3 [type locality: Kawasan Falls , Matutinao River mouth, Matutinao, Cebu Island, Philippines].

Material examined. All island /islands listed here belong to the Ryukyu Islands. Tokara Islands , Nakanoshima Island : CBM-ZC 4477, 1 male (10.6× 13.6 mm), estuary of small stream, 28 May 1997, coll. T. Komai. Yaeyama Islands , Ishigaki Island : CBM-ZC 16570, 3 males (8.1×10.2, 9.2×11.4, 11.2× 14.3 mm), Todoroki River downstream, 26 March 2016, coll. T. Maenosono; RUMF-ZC-6846, 2 males (9.2×11.1, 9.3× 11.5 mm), same locality, 25 March 2016, coll. T. Saeki; RUMF-ZC-6848, 1 male (8.3×10.0 mm), same locality, 28 March 2016, coll. T. Maenosono; RUMF-ZC-7113, 3 males (8.1×10.0, 8.8×10.5, 11.0× 13.3 mm), same locality, 4 February 2021, coll. T. Saeki; RUMF-ZC-6847, 1 male (9.2× 11.4 mm), Tsuru River downstream, under rock, 26 March 2016, coll. T. Maenosono & T. Saeki; RUMF-ZC-7114, 1 male (10.6× 13.1 mm), Kara River downstream, under rock, 5 February 2021, coll. T. Saeki; Iriomote Island: CBM-ZC 16571, 1 male (9.8× 12.3 mm), Honera River, downstream, under rock, 25 February 2017, coll. T. Saeki, DNA voucher .

Description of Japanese specimens. Male. Carapace ( Figs. 6A ; 7A ) subrectangular, 1.2–1.3 times as wide as long, substantially flattened; dorsal surface flat, with scattered fine punctae; regions poorly defined; cervical groove shallow, but clearly discernible, H-shaped; epigastric cristae rudimentary or absent; postorbital cristae absent; no distinct ridge present at region just adjacent to junction between posterolateral margin and posterior carapace margin. Anterolateral margin lined with faint granules, not clearly demarcated from posterolateral margin, bearing 1 small blunt tooth defined by shallow notch; external orbital angle triangular with subacute, forwardly directed tip; posterolateral margins slightly converging posteriorly, almost straight; supraorbital margin slightly oblique and slightly sinuous, faintly crenulate; frontal margin in dorsal view with broad, shallow median notch. Front ( Fig. 7B ) abruptly deflexed, very narrow, demarcated dorsally with transverse line of minute granules, medially shallowly concave, bisected by low transverse, granulated ridge continuing to upper margins of antennular fossae, attaching ventrally with upper margin of epistome; infraorbital margins ( Fig. 7C ) each with about 10 low, blunt granules, not congruent with anterolateral margin, inner orbital tooth small, granulated, not extending to lateral angle of front; subortbial region with scattered granules; pterygostomial region with row of long plumose setae near cheliped coxa.

Inter-antennular septum ( Fig. 7B ) broadly triangular, medially slightly depressed, upper margins bordered with minute granules.

Thoracic sternum glabrous, with scattered punctae. Sternite 1 ( Fig. 7D ) small, narrowly triangular, somewhat depressed; surface obscured by setae. Sternite 2 ( Fig. 7D ) widely trapezoidal, clearly demarcated from sternite 3. Sternites 3 and 4 ( Figs. 6B ; 7D ) fused, subtrapezoidal; lateral margin with small triangular process at base of maxilliped 3. Sternite 5–7 lateral parts subquadrate, respectively; lateral parts of sternite 8 exposed, subrectangular. Sterno-pleonal cavity deep, anterior end reaching anterior 0.2 of sternite 4 ( Fig. 7D ); press buttons reduced, obsolescent.

Eyes ( Fig. 6A ; 7A ) well developed; cornea distinct, darkly pigmented. Antennular peduncle basal articles ( Fig. 7B ) broad, widely separated; distal 2 peduncular articles folded slightly obliquely into antennular fossa. Antennal peduncle ( Fig. 7B ) entering into orbital hiatus.

Maxilliped 3 ( Fig. 7E ) broad, outer surface with small punctae. Exopod distinctly broader than endopodal ischium, mesial margin noticeably convex, lateral margin less convex than mesial margin. Ischium-merus lateral margin strongly concave to accommodate mesial margin of exopod; mesial margin with small notch at articulation; articulation between ischium and merus slightly sinuous. Ischium mesial margin gently convex, outer surface with shallow median sulcus. Merus with broad longitudinal sulcus on outer surface along mesial margin, antero-external angle strongly produced into roundly triangular projection. Carpus articulated at concave distomesial margin of merus. Dactylus longer than propodus, tapering to blunt apex.

Chelipeds ( Figs. 6A ; 8A, B ) usually symmetrical. Ischium convex on lower surface, with row of small tubercles on lower anterior margin. Merus without spines; upper margin convex, with deep subdistal transverse groove and long soft plumose setae proximally; lower surface flat, glabrous, inner lower margin bordered with small tubercles, outer lower margin bluntly carinate, smooth. Carpus sub-globular, smooth on surfaces; inner surface slightly produced, angular. Palm outer surface gently convex, microscopically granulate, glabrous, with scattered minute punctae; inner surface also gently convex, glabrous; no tuft of setae at base of fingers. Fixed finger without ridge on outer or inner surface, tapering to rounded, hollowed apex bordered by chitinous ridge; occlusal margin with row of distinct, rounded teeth, devoid of membranous protuberance proximally. Dactylus distinctly longer than palm measured along upper margin, gently arched, terminating in blunt, hollowed apex bordered by chitinous ridge; occlusal margin with row of blunt teeth, becoming obsolete distally.

Ambulatory legs (pereopods 2–5) ( Figs. 6A ; 8C, D ) relatively slender for genus, pereopods 3 and 4 longest, all similarly compressed. Pereopod 2–4 meri each with shallow transverse groove on anterior margin subdistally, extending onto upper and lower surfaces, and row of moderately long, soft feathered setae on anterior margin; upper and lower surfaces almost glabrous except for minute black setae on anterodistal margin; posterior surfaces each with row of long stiff setae along upper and lower margins and scattered minute black setae at least on proximal half. Carpi each with dense covering of minute black setae and 2 rows of widely separated, long stiff setae on extensor surface, minute black setae extending onto lower surface; extensor distal angle produced into small process interacting with extensor proximal part of propodi; upper surface with few minute black setae; flexor surfaces each with 2 rows of widely spaced, long stiff setae. Propodi each with covering of minute black setae and long stiff setae arranged in 2 rows on extensor and flexor surfaces; scattered moderately long stiff setae at least on extensor and flexor surfaces; upper surfaces with longitudinal row of stiff setae on midline (pereopod 2 and 3), no long setae (pereopod 4); lower surfaces with 2 lines consisting of minute black setae and row of widely spaced long stiff setae (pereopod 2 and 3), few minute black setae and row of widely spaced long stiff setae along midline (pereopod 4); flexor surfaces with few minute black setae; propodus of pereopod 4 2.1 times as long as wide. Dactyli faintly curved, terminating in sharp corneous claws; surfaces alternated with longitudinal lines of minute black setae and naked line; pereopod 4 dactylus 1.3 times as long as propodus, 5.6 times as long as basal width.

Pereopod 5 ( Figs. 6A ; 8D ) having setation generally similar to preceding pereopods. Merus slightly narrowing distally, with shallow transverse groove subdistally; posterior surface flattened in distal half, with few long stiff setae arranged in 2 rows on proximal part; upper surface with minute black setae adjacent to posterior margin; lower surface glabrous. Carpus with numerous minute black setae and widely spaced, long stiff setae, arranged in 2 rows, on extensor surface; upper surface almost glabrous except for minute black setae on proximal part and distal margin; lower and posterior surface glabrous. Propodus approximately as long as wide; extensor surface with covering of minute black setae and few long stiff setae; upper surface with minute black setae arranged in irregular lines; lower surface glabrous; flexor surface with covering of minute black setae and few long stiff setae. Dactylus slightly upturned, 1.2 times as long as propodus, about 3 times as long as wide; extensor and flexor surfaces with covering of black minute setae extending onto upper and lower surfaces, former with few long stiff setae, midlines of upper and lower surfaces naked.

Pleon ( Figs. 6B ; 7F ) relatively wide, surface smooth, margins with short setae. Pleomere 1 widest. Pleomeres 2 and 3 subequal in width. Pleomeres 3–5 becoming gradually narrower toward distal. Pleomere 6 subtrapezoidal, 2.0–2.3 times as wide as long, distolateral angle forming sloping shoulder. Telson tongue-shaped, 1.1–1.3 times long as pleomere 6, 1.1–1.2 times as long as wide; lateral margins slightly concave basally; distal margin rounded, with sparse setae not forming prominent tuft.

Male gonopod 1 ( Fig. 8E, F ) slender, slightly curving dorsally, reaching suture between thoracic sternite 4 and 5; tip roundly truncate, forming short chitinous structure ( Fig. 8F ); subdistal lobe obtuse, partially obscured by stiff setae; gonopod 2 shorter than 0.25 length of gonopod 1.

Female. No specimens available for examination.

Colouration in life. Body and pereopods generally brown on dorsal or upper side, paler on ventral or lower side ( Fig. 6A ).

Distribution. Cebu, the Philippines; Flores, Indonesia; Ryukyu Islands (Tokara and Yaeyama Islands), Japan (see Discussion).

Habitat. In the downstream of Todoroki River, Ishigaki Island, Yaeyama Islands, observation on the habitat of Ptychognathus lipkei was made. Individuals were seen in the upper tidal area and freshwater area just upper to the tidal area, hidden under rocks or among gravel. Individuals of P. ishii occurred sympatrically, although those of P. lipkei seem to be restricted to fast-flowing shoal. Other decapods also seen nearby the habitat of P. lipkei in Todoroki River include: Pyxidognathus granulosus A. Milne-Edwards (Brachyura: Varunidae ); Bresedium brevipes (de Man) (Brachyura: Sesarmidae ); Leptarma liho (Koller, Liu & Schubart) (Brachyura: Sesarmidae ); Parasesarma dumacense (Rathbun) (Brachyura: Sesarmidae ); and Sesarmops imperator Ng, Li & Shih (Brachyura: Sesarmidae ) ( Maenosono & Naruse 2015, 2016; Maenosono et al. 2015). It is interesting to note that those five brachyurans were also recorded from the type locality of P. lipkei: Kawasan Fall, Matutinao , Cebu Island ( Rahayu & Ng 2010; Rahayu & Li 2013; Li et al. 2019, 2020; Shih et al. 2019; Ng et al. 2020; Shahdadi et al. 2020).

Remarks. In the original description of Ptychognathus lipkei, N.K. Ng (2010) stated that the epigastric cristae on the carapace are “low, indistinct”, but in her figure (fig. 2A), the epigastric cristae are illustrated as distinct ridges. On the other hand, in the photo of the holotype (fig. 1A), epigastric cristae are not discernible. In our specimens, epigastric cristae are at most rudimentary, or absent.

Figures

FIGURE 6. Ptychognathus lipkei N.K. Ng, 2010. A, male (8.3×10.0 mm), Ishigaki Island, Yaeyama Islands, RUMF-ZC-6848, habitus in dorsal view; B, male (10.6×13.1 mm), Ishigaki Island, RUMF-ZC-7114, cephalothorax, ventral view.

FIGURE 7. Ptychognathus lipkei N.K. Ng, 2010, male (10.6×13.6 mm), Nakanoshima Island, Tokara Islands, CBM-ZC 4477. A, carapace with eyes and right antennule, dorsal view (setae on left side of surface omitted); B, front, antennules and left antenna, anterior view; C, left inferior orbital margin, ventral view; D, thoracic sternites 1–4, ventral view; E, left maxilliped 3, outer view; F, pleon, outer view.

FIGURE 8. Ptychognathus lipkei N.K. Ng, 2010, male (10.6×13.6 mm), Nakanoshima Island, Tokara Islands, CBM-ZC 4477. A, left cheliped, merus, ventral view; B, same, chela, outer view; C, left pereopod 4, carpus to dactylus, dorsal view; D, left pereopod 5, carpus to dactylus, dorsal view; E, left gonopod 1, ventral view; F, same, distal part, dorsal view.