Flabellamon ngankee, Tan & Yeo, 2024

Tan, Zhi Wan & Yeo, Darren C. J., 2024, On the genus Flabellamon Ng, 1996, with descriptions of two new species (Crustacea: Brachyura: Potamidae), Zootaxa 5476 (1), pp. 434-446 : 438-442

Flabellamon ngankee sp. nov.

( Figs. 1–3 )

Ranguna (Ranguna) turgidula .— Bott 1970: 164 (part), pl. 38 fig. 37, pl. 51 fig. 54 (not Potamon (Potamon) turgidulum Alcock, 1909 ).

Type material. Holotype, male (37.5 × 28.8 mm) ( ZRC 2024.0272 ), Huay Ya U village/stream, about 50 km from Mae Sot to Tak, Mae Sot District , Tak Province, Thailand, coll. H. H. Tan and H. H. Ng, 27 May 1999 . Paratypes: 1 male (20.9 × 16.3 mm) , 1 female ( NMB 19 a), Meetun , Tenasserim, Myanmar, coll. L. Fea, no date (“ Viaggo in Birmania di L. Fea 1885–89”) ; 1 male (18.1 × 14.4 mm) ( NMB 19 b), Kokareet [= present day Kawkareik], Tenasserim, Myanmar, coll. L. Fea , no date; 1 female (25.1 × 18.8 mm) ( ZRC 2024.0273 ), Huai Mae Lamao , north basin of Salween, Tak Province, Thailand, coll. M. Kottelat, 31 Mar 1980 .

Diagnosis. Carapace ( Fig. 1A ) transversely sub-ovate, low, broader than long (CW/CL ratio: 1.30); epigastric cristae well developed, not sharp, rugose, slightly anterior to postorbital cristae; postorbital cristae well developed, relatively sharp, rugose, confluent with epibranchial teeth epibranchial tooth blunt, broad, separated from external orbital tooth by deep broad V-shaped cleft; external orbital tooth lateral margin slightly longer than mesial margin. Third maxilliped exopod flagellum length equal to width of merus ( Fig. 3F ). Male pleon ( Fig. 2A ) with sixth somite lateral margins convex. G1 ( Fig. 3C, D ) terminal segment conical in appearance, dorsal flap high, with rounded apex slightly skewed proximally, second lower apex slightly skewed distally, abruptly tapered proximally and distally, almost perpendicular.

Description of male holotype. Carapace ( Fig. 1A ) transversely sub-ovate, distinctly wider than long, (CW/ CL ratio: 1.30); dorsal surface gently convex. Frontal region gently granulose; orbital regions smooth; lateral parts of anterolateral region weakly rugose; lateral parts of branchial regions weakly covered with oblique striae; mesogastric, urogastric, cardiac, intestinal regions sparsely punctate, otherwise smooth; suborbital region weakly granulose; pterygostomial region granulose at lateral parts; subhepatic, sub-branchial regions strongly rugose, striated. Epigastric cristae distinct, rugose, not cristate, separated by narrow, median inverted Y-shaped furrow; epigastric cristae just anterior of postorbital cristae, separated by short furrow; postorbital cristae distinct, margin uneven, prominently raised, gently sloping posterolaterally, lateral edge prominent, confluent with anterolateral margins. Cervical grooves distinct, not reaching lateral margins, not connected to distinct H-shaped median gastric groove. Frontal margin sinuous, appears entire in dorsal view, gently sinuous in frontal view; divided into 2 low lobes, separated by broad, very shallow concavity; margin of each lobe gently convex. External orbital tooth distinct, sharp, triangular, lateral margin straight, much longer than length of mesial margin, demarcated from rest of anterolateral margin by deep V-shaped cleft; epibranchial tooth well defined.Anterolateral margins convex, cristate, lined with sharp granules, appears serrated. Posterolateral margin gently concave, converging towards weakly convex posterior carapace margin. Orbits ( Fig. 1C ) ovate; eye filling most of space; ocular peduncle relatively short; cornea large, ovate, pigmented. Supraorbital margin nearly straight, entire. Suborbital margin very gently concave, entire. Antennae very short, tip not reaching cornea of eyes; antennules short, folding transversely in very wide fusiform fossa. Posterior margin of epistome with distinct median triangle, lateral margin weakly concave.

Third maxillipeds ( Figs. 1D , 3F ) covering most of buccal cavity when closed; ischium subrectangular, with distinct median groove, surface scattered with pits and short setae; merus subquadrate, slightly wider than long, surface slightly pitted, margins cristate, antero-external tooth angular, produced; exopod slender, reaching to about proximal one-third length of merus, with elongate flagellum equal to merus width.

Chelipeds ( Figs. 1A, E, F ) asymmetrical, left cheliped larger in male holotype.Anterior margin of basis-ischium lined with few small sharp granules; margins of merus lined with low sharp granules, appears weakly serrated. Outer surface of carpus gently rugose, inner distal angle with large sharp tooth and low basal tooth. Outer surfaces of chelae weakly rugose, upper part covered with tubercles, lower part pitted; major chela similar in proportions to minor chela. Fingers of major chela long, slender, longer than upper margin of palm, outer surface lined with 3 rows of pits; forming small gape when fingers closed; cutting edges of both fingers with variously sized sharp teeth and denticles; dorsal margin of dactylus with prominent tubercles and granules. Fingers of minor chela similar to major chela.

Ambulatory legs ( Fig. 1A ) not elongated, segments relatively stout; second pair longest, last pair shortest. Merus not elongate, outer surface very gently rugose, dorsal margin uneven, subcristate, without subdistal spine or tooth; carpus very weakly rugose, dorsal margin lined with low spines, outer surface with low submedian crista on first to third legs, that on fourth leg smooth; dorsal margin of propodus with crista, outer surface with shallow, submedian groove; dactylus relatively long, gently curving, quadrate in cross section, margins with short, sharp pectinate spines.

Male thoracic sternum ( Fig. 2A, B ), notably sternites 3, 4, relatively broad, surface with few short, stiff setae; sternites 1-2 completely fused to form broadly triangular plate; separated from sternite 3 by distinct, straight suture; sternites 3, 4 completely fused, with shallow groove demarcating suture. Sutures between sternites 4/5, 5/6, 6/7, and 7/8 medially interrupted; median longitudinal groove on sternites 7 and 8. Male sternopleonal cavity ( Fig. 2B ) deep, reaching imaginary line joining anterior part of cheliped coxae. Male pleonal locking tubercle low, round, on midpoint of sternite 5.

Male pleon along with telson ( Fig. 2A ) triangular, all somites and telson free; telson triangular, lateral margins distinctly concave; somite 6 trapezoidal, much wider than long, lateral margins convex; somites 3–5 trapezoidal, gradually decreasing in width, increasing in length; somites 1 and 2 subrectangular, wide, reaching to bases of coxae of fourth ambulatory legs, thoracic sternite 8 not visible when pleon closed.

G1 ( Figs. 3A–D ) sinuous, stout; subterminal segment sinuous, proximal part very broad, tapering strongly at median part, lateral margin without cleft, separated from terminal segment by dilation; terminal segment conical, almost straight from base to tip, bent outwards, tapering uniformly to pointed tip, dorsal flap high, narrow, with rounded apex slightly skewed proximally, second lower apex slightly skewed distally, abruptly tapered proximally and distally, almost perpendicular.

Female. Female pleon and telson ( Fig. 2C ) broad; somites with convex lateral margins. Vulva ( Fig. 2D ) ovate, large, occupying distal 2/3 of sternite 6, strongly recessed, opening directed mesially, located mesial end of sternite 6, impinging on suture of sternite 5/6, lateral end of sternite 6 produced with relatively short, soft membranous flaplike protrusion, opening covered with membrane. The female specimen is similar to the males in most other key non-sexual characters.

Etymology. This species is named after the late Dr Ng Ngan Kee, in honour of her contributions and achievements in brachyuran taxonomy. Used as noun in apposition.

Remarks. The present specimens from NMB [abbreviated formerly as MBA] were placed under Ranguna (Ranguna) turgidula (Alcock, 1909) , with figures of a G1 and a specimen that closely resemble the present species, but not clearly indicating where the specimen was from ( Bott 1970 b: pl. 38 fig. 37, pl. 51 fig. 54). Bott’s (1970: pl. 38 figs. 36, 37, pl. 51 figs. 53, 54) figures, however, are not of Potamon (Potamon) turgidula [now in Badistemon Yeo & Ng, 2007 as Badistemon turgidulum (Alcock, 1909) ], but refer instead to two distinct species: F. kuehnelti and the present species, F. ngankee sp. nov. To further complicate matters, Bott (1970: 165) may have made a mistake with the localities and catalogue numbers of the NMB material as those he cited do not tally with the present material. Bott (1970) had included “ 2 males MBA 19a” [= NMB 19a] in his material from “Tenasserim, Meelan” and “ 1 male 1 female MBA 19b” [= NMB 19b] in his material from “Kokareetberge bei Meelan”. However, in the NMB material examined in the present study, NMB 19a consists of 1 male and 1 female specimen from “Meetun, Tenasserim” and NMB 19b consists of 2 males from “Kokareet, Tenasserim”. There is no mentioned of “Meelan”. The two NMB 19a specimens and the smaller (18.1 × 14.4 mm) of the two specimens in NMB 19b are hereby assigned to the present species, F. ngankee , while the larger (25.7 × 20.2 mm) NMB 19b specimen is hereby assigned to F. kuehnelti ( Pretzmann, 1963) .

Flabellamon ngankee sp. nov. most closely resembles F. wanchaii Takeda, Sanpool & Intapan, 2019 , in terms of G1 morphology, but nonetheless they can be differentiated by the following differences: areas behind epigastric, postorbital cristae and surfaces of chela less distinctly rugose and granulose ( Fig. 1A, E, F ) (versus areas behind epigastric, postorbital cristae and surfaces of chela strongly rugose and granulose, cf. Takeda et al. 2019: figs. 1A, C, 2); anterolateral margin of the carapace relatively less convex ( Fig. 1A ) (versus anterolateral margins of the carapace being relatively more convex; cf. Takeda et al. 2019: fig. 1C); external orbital tooth more strongly developed, acute ( Fig. 1A ) (versus external orbital tooth less strongly developed, more obtuse; cf. Takeda et al. 2019: fig. 1A, 2B); male pleon somite 6 lateral margins relatively more strongly convex ( Fig. 2A ) (versus male pleon somite 6 lateral margins weakly convex; cf. Takeda et al. 2019: fig. 1D); G1 terminal segment more stout, about 3.2 times longer than broad ( Fig. 3C, D ) (versus G1 terminal segment more slender, about 3.9 times longer than broad; cf. Takeda et al. 2019: fig. 3B); and G1 terminal segment dorsal flap more strongly produced, about 0.3 times length of terminal segment, with dorsal flap apex at proximal third of dorsal flap ( Fig. 3C, D ) (versus dorsal flap less strongly produced, lower, about 0.17 times length of terminal segment, with dorsal flap apex at proximal and distal third of dorsal flap, appearing flat topped; cf. Takeda et al. 2019: fig. 3B).

Distribution. THAILAND: Tak province. MYANMAR: Meetun, Kawkareik and Kayin state.

Figures

FIGURE 1. Flabellamon ngankee sp. nov., male holotype, 37.5 × 28.8 mm, ZRC 2024.0272: A, dorsal view; B, ventral view; C, frontal view; D, buccal cavity and third maxillipeds; E, minor chela (lateral view); F, major chela (lateral view).

FIGURE 2. Flabellamon ngankee sp. nov.: A, B, male holotype, 37.5 × 28.8 mm, ZRC 2024.0272; C, D, female paratype, 25.1 × 18.8 mm, ZRC 2024.0273.A, male anterior thoracic sternum, pleon and telson; B, male sternopleonal cavity (G1 and G2 removed); C, female anterior thoracic sternum, pleon and telson; D, female sternopleonal cavity and vulvae.

FIGURE 3. Flabellamon ngankee sp. nov., male holotype, 37.5 × 28.8 mm, ZRC 2024.0272:A, right G1 (dorsal view); B, right G1 (ventral view); C, terminal segment of right G1 (dorsal view); D, terminal segment of right G1 (ventral view); E, right G2 (dorsal view); F, left third maxilliped. Scale bar = 2.0 mm.