Megapleonum falx, Huang & Shih & Ahyong, 2025

Huang, Chao, Shih, Hsi-Te & Ahyong, Shane T., 2025, Four new freshwater crab species of the genus Megapleonum Huang, Shih & Ahyong, 2018 (Crustacea, Decapoda, Potamidae) from Guangdong, China, ZooKeys 1244, pp. 1-27 : 1-27

publication ID	https://doi.org/10.3897/zookeys.1244.148112
publication LSID	lsid:zoobank.org:pub:CCCA8237-6587-44BD-A25E-41BD00928A11
DOI	https://doi.org/10.5281/zenodo.15791866
persistent identifier	https://treatment.plazi.org/id/FEBCE5AD-8B23-5E49-A600-56E5B8027C78
treatment provided by	ZooKeys by Pensoft
scientific name	Megapleonum falx
status	sp. nov.

Treatment

Megapleonum falx sp. nov.

Figs 1 View Figure 1 , 2 View Figure 2

Type material.

Holotype: • SYSBM 002142 , male (19.6 × 15.8 mm), Baima Village , Huidong County, Huizhou City, Guangdong Province, China, 23.03°N, 115.07°E, hill stream under rock, coll. Song-Bo Wang, August 2019. GoogleMaps

Diagnosis.

Carapace broader than long, dorsal surface slightly convex, lateral margins covered in dense setae; postorbital, epigastric cristae confluent (Fig. 1 A View Figure 1 ). Maxilliped 3 sparsely covered in long setae; merus width ~ 1.4 × length; ischium width ~ 0.7 × length; exopod reaching beyond anterior edge of ischium, flagellum as long as merus width (Fig. 2 A View Figure 2 ). Ambulatory legs with dense setae; pereiopod 5 dactylus ~ as long as propodus (Fig. 1 A View Figure 1 ). Male anterior thoracic sternum very broad, width ~ 1.8 × length (Fig. 1 C View Figure 1 ). Male pleon large, broadly triangular, pleonite 6 width ~ 2.6 × length, telson width ~ 1.8 × length (Fig. 1 D View Figure 1 ). G 1 large, strongly sinuous, tip exceeding suture between thoracic sternites 4 / 5 in situ (Fig. 1 E View Figure 1 ); subterminal segment length ~ 3.2 × length of terminal segment (Fig. 2 C View Figure 2 ). Subterminal segment outer margin strongly concave; terminal segment strongly curved inwards, strongly tapering, sickle shaped, tip pointed downwards in dissected view (Fig. 2 C, F, G View Figure 2 ). G 2 subterminal segment tapering, bent outwards distally, flagelliform terminal segment ~ 1.8 × length of subterminal segment, apex blunt (Fig. 2 B View Figure 2 ).

Description of male holotype.

Carapace broader than long, ~ 1.2 × as wide as long; regions not pronounced, dorsal surface slightly convex, finely pitted; dense setae along lateral margins (Fig. 1 A View Figure 1 ). Frontal margin almost straight, deflexed (Fig. 1 A, B View Figure 1 ). Epigastric cristae and postorbital cristae relatively smooth, confluent; bifurcated groove between epigastric cristae (Figs 1 A View Figure 1 , 2 A View Figure 2 ). Branchial regions not swollen (Fig. 1 A, B View Figure 1 ). Cervical groove shallow but visible (Fig. 1 A View Figure 1 ). Mesogastric region flat (Fig. 1 A View Figure 1 ). External orbital angle broadly triangular, outer margin slightly convex, confluent with anterolateral margin (Fig. 1 A, B View Figure 1 ). Epibranchial tooth granular, indistinct (Fig. 1 A, B View Figure 1 ). Anterolateral margin lined with indistinct single or partially fused granules; posterolateral margin posteriorly convergent (Fig. 1 A View Figure 1 ); posterolateral surface slightly rugose (Fig. 1 A View Figure 1 ). Orbits regular; supraorbital margins weakly cristate, infraorbital margins lined with fused granules (Fig. 1 B View Figure 1 ). Eyes normal (Fig. 1 A, B View Figure 1 ). Sub-orbital, pterygostomial and sub-hepatic regions generally smooth, pitted (Fig. 1 B View Figure 1 ). Antennules large, folded within broad fossae; antennae very short (Fig. 1 B View Figure 1 ). Median lobe of epistome buccal margin broadly triangular, lateral margins slightly sinuous (Fig. 1 B View Figure 1 ).

Maxilliped 3 sparsely covered in long setae, merus subtrapezoidal, with slight median depression, width ~ 1.4 × length; ischium subtrapezoidal with shallow median sulcus, distomesial margin rounded, width ~ 0.7 × length. Exopod reaching proximal one-third of merus; flagellum as long as merus width (Fig. 2 A View Figure 2 ).

Chelipeds (pereiopod 1) subequal (Fig. 2 D, E View Figure 2 ). Merus trigonal in cross-section, surfaces generally smooth; outer dorsal and ventral margins slightly crenulated, inner margin lined with large granules (Fig. 1 A, B View Figure 1 ). Carpus dorsal surface slightly rugose, with large spike at inner-distal angle, spinule at base (Fig. 1 A View Figure 1 ). Major cheliped palm length ~ 1.5 × height; dactylus 0.7 × palm length (Fig. 2 D, E View Figure 2 ). Palm surface pitted, occlusal margin of fingers with 7–9 irregular blunt teeth, with small gape when closed (Fig. 2 D, E View Figure 2 ).

Ambulatory legs (pereiopods 2–5) slender, covered with setae (Fig. 1 A View Figure 1 ). Pereiopod 3 merus 0.6 × CL (Fig. 1 A View Figure 1 ). Pereiopod 5 propodus length 1.7 × height, approximately as long as dactylus (Fig. 1 A View Figure 1 ).

Male thoracic sternum generally smooth, pitted, setae at margins and sparsely on sternites; sternites 1–4 width ~ 1.8 × length; sternites 1, 2 fused to form broad triangle; fused sternites 1, 2 demarcated from sternite 3 by sinuous transverse sulcus, sulcus lined with setae; sternites 3, 4 fused without obvious demarcation other than a line of setae (Fig. 1 C View Figure 1 ). Male sterno-pleonal cavity reaching anteriorly slightly beyond mid-length of cheliped coxa (Fig. 1 C View Figure 1 ). Male pleonal locking tubercle positioned at mid-length of sternite 5 (Fig. 1 E View Figure 1 ).

Male pleon large, broadly triangular; somites 3–6 progressively narrower, not entirely confluent with each other; somite 6 width approximately 2.6 × length; telson width 1.8 × length; lateral margins almost straight, apex rounded (Fig. 1 D View Figure 1 ).

G 1 large, strongly sinuous, tip exceeding suture between thoracic sternites 4 / 5 in situ (Fig. 1 E View Figure 1 ); subterminal segment length ~ 3.2 × length of terminal segment (Fig. 2 C View Figure 2 ). Subterminal segment outer margin strongly concave; terminal segment strongly curved inwards, strongly tapering, sickle shaped, tip pointed downwards in dissected view (Fig. 2 C, F, G View Figure 2 ). G 2 subterminal segment tapering, bent outwards distally, flagelliform terminal segment ~ 1.8 × length of subterminal segment, apex blunt (Fig. 2 B View Figure 2 ).

Colour in life.

Generally camouflage-brown all over.

Habitat.

Unknown. The only specimen collected was found in the lower reaches of the hill stream, but further collection efforts in the same area and further upstream yielded no crabs at all. We consider it likely that the primary habitat of this species is higher up the mountain, which reaches above 1000 m above sea level, and that this lone specimen that was collected was washed downstream in a flooding event.

Distribution.

Baima Village, Huidong County, Huizhou City, Guangdong Province, China.

Etymology.

The species name is the Latin word falx which means sickle-shaped. It alludes to the sickle-shaped G 1 terminal segment of this species.

Remarks.

The wide male anterior thoracic sternum, large and broadly triangular male abdomen and large and sinuous G 1 of Megapleonum falx sp. nov. fit the diagnosis of the genus. In possessing thick setae on the carapace margins and ambulatory legs, M. falx sp. nov. is most similar to M. ferrumequinum sp. nov. The sickle-shaped G 1 terminal segment of M. falx sp. nov., however, is unlike any other congener and immediately distinguishes it (Fig. 2 C, F, G View Figure 2 ). Apart from the G 1, M. falx sp. nov. also has a flatter carapace dorsal surface when compared to M. ferrumequinum sp. nov. (Figs 1 B View Figure 1 , 3 B View Figure 3 ). The flagellum of the maxilliped 3 exopod is present in M. falx sp. nov. whereas it is absent in M. ferrumequinum sp. nov. (Fig. 2 A View Figure 2 vs Fig. 4 A View Figure 4 ). The male abdomen is also obviously wider in M. falx sp. nov., with the somite 6 width approximately 2.6 × length (Fig. 1 D View Figure 1 ) (vs narrower in M. ferrumequinum sp. nov., sixth somite width approximately 2.5 × length, Fig. 3 D View Figure 3 ). The relative length of the pereiopod dactylus to the propodus is also different, being approximately the same length in M. falx sp. nov. and in M. ferrumequinum sp. nov., having a shorter dactylus (Figs 1 A View Figure 1 , 3 A View Figure 3 ). More detailed comparisons can be found in Table 1 View Table 1 .