Megapleonum shenzhen, Huang & Mao, 2021

Huang, Chao & Mao, Siying, 2021, Martesia fragilis Verrill and Bush 1898, Zoological Studies 60 (66), pp. 1-12 : 2-7

Family Potamidae Ortmann, 1896 Subfamily Potamiscinae Bott, 1970 (sensu Yeo & Ng 2003)

Megapleonum shenzhen sp. nov. ( Figs. 1–3 , 4G, H ) urn:lsid:zoobank.org:act:819F9B5B-C2E1-4583-8CFC-567AFA689BFB

Material examined: Holotype: SYSBM 001983 , male (18.6 × 14.4 mm), Pingpu (22.64°N, 114.53°E), Longgang District, Shenzhen, Guangdong, China, small hillstream, 130 m a.s.l., coll. C. Huang, November, 2018 . Paratypes: SYSBM 001984, 1 male (15.9 × 12.4 mm), same data as holotype. SYSBM 001985- 001987, 3 females (22.4 × 16.7 mm, 19.2 × 14.2 mm, 15.1 × 10.9 mm), same data as holotype. SYSBM 001988, male (16.2 × 12.4 mm), Maluan Mountain (22.61°N, 114.33°E), Yantian District, Shenzhen, Guangdong, China, small hillstream, 130 m a.s.l., coll. C. Huang, November, 2018. SYSBM 001989-001990, 2 males (16.6 × 12.8 mm, 15.9 × 12.4 mm), Luowutian Reservoir (22.66°N, 114.46°E), Yantian District, Shenzhen, Guangdong, China, small hillstream, 80 m a.s.l., coll. C. Huang, August, 2019. SYSBM 001991, 1 female (17.0 × 12.6 mm), same data as above. AM P105613, 1 male (17.2 × 13.1 mm), same data as holotype.

Description: Small sized (CW <30 mm). Carapace broader than long, width 1.3–1.4 × length (n = 10), regions indistinct; dorsal surface finely pitted, convex ( Fig. 1 ). Front deflexed, margin slightly ridged in dorsal view ( Fig. 1 ). Epigastric cristae distinct, separated by narrow gap ( Fig. 1 ). Postorbital cristae sharp, laterally expanded, fused with epigastric cristae but not with epibranchial teeth ( Fig. 1 ). Branchial regions flat; cervical groove shallow; mesogastric region slightly convex ( Fig. 1 ). External orbital tooth granular, indistinct; external orbital angle fused with anterolateral margin with almost no gap ( Figs. 1 , 2A ). Epibranchial tooth granular, indistinct ( Fig. 1 ). Anterolateral margin cristate, lined with 15–19 granules ( Fig. 1 ). Posterolateral margin with lined striae ( Fig. 1 ). Orbits small, supraorbital and infraorbital margins ridged ( Fig. 2A ). Sub-orbital, sub-hepatic and pterygostomial regions divided by sutures; surfaces pitted ( Fig. 2A ). Epistome median lobe broadly triangular, posterior margin almost straight ( Fig. 2A ).

Maxilliped 3 merus width about 1.2 × length; ischium width about 0.7 × length; merus subtrapezoidal with slight median depression; ischium subtrapezoidal, with median sulcus, mesial margin rounded; exopod tapering, reaching to proximal quarter of merus height, flagellum vestigial ( Fig. 3A ).

Chelipeds slightly unequal ( Figs. 1 , 3F–G ). Merus trigonal in cross section, margins crenulated ( Fig. 2A ). Carpus with sharp spine at inner-distal angle, spinule at base, dorsal surface with weak striae ( Fig. 1A ). Major cheliped palm length about 1.4–1.5 × height in males (n = 3), 1.4–1.6 × height in females (n = 3); dactylus 0.9 × palm length in males (n = 3), 0.7–0.9 in females (n = 3) ( Fig. 3F–G ). Palm surface generally pitted. Occlusal margin of fingers lined with triangular teeth of different sizes; small gape when closed ( Fig. 3F–G ).

Ambulatory legs (P2–5) stout, short, carpus with sparse short setae; propodus and dactylus with relatively denser setae ( Fig. 1 ). P3 merus 0.6–0.7 × carapace length in males (n = 3), 0.7 × in females (n = 3). P5 propodus 1.7–1.9 × as long as broad in males (n = 3), and 1.7–2.1 × in females (n = 3), shorter than dactylus ( Fig. 1 ).

Male thoracic sternum generally pitted; sternites 1–4 wide, width about 1.9 × length; sternites 1, 2 fused, forming a triangular structure; sternites 2, 3 fused, separated by a deep transverse sulcus; sternites 3, 4 fused, with median sulcus ( Fig. 2B ). Male sternopleonal cavity reaching anteriorly beyond level of mid-length of chelipeds coxae base ( Fig. 2B ); median longitudinal groove separating sternites 7, 8 deep ( Fig. 2D ). Male pleonal locking tubercle positioned at mid-length of sternites 5 ( Fig. 2D ). Female vulva ovate, medium-sized, reaching to the suture of sternites 5/6, relatively widely spaced between one another ( Fig. 2F ).

Male pleon large, broadly triangular; somite 3–6 progressively broader longitudinally, lateral margins convex; somite 6 width about 2.6 × length; telson 2 × as broad as long, apex rounded ( Fig. 2C ). Female pleon broadly ovate ( Fig. 2E ).

G1 sinuous, reaching beyond suture of sternites 4/ 5 in situ ( Fig. 2D ). Subterminal segment 2.2–2.5 × as long as terminal segment (n = 3), bent inwards, outer-distal part with distinct lobe. Terminal segment stout, goose-head-shaped; tip pointed, pointing inner-upwards; distal part forming lobe; ventral side with large bulging curved flap ( Figs. 2D , 3C–E , 4G ). G2 slender, subterminal segment relatively thick basally, tapering distally, subterminal segment 1.8 × as long as terminal segment ( Fig. 3B ).

Etymology: The species is named after the locality where the species was first collected, Shenzhen, Guangdong, China.

Colour in life: Dorsal surface generally mottled brown, blending in well with the rocks and plant debris in the hillstream where it is usually found ( Fig. 4H ).

Habitat: This species was collected from under rocks in hillstreams at around 100 m a.s.l. No other crabs could be found with the new species in Pingpu. In the Maluan Mountain collection site, the new species was extremely rare, whereas Eriocheir hepuensis and Nanhaipotamon hongkongense were common. We suspect that the main population of M. shenzhen n. sp. resides in the upper reaches of that hillstream where there would presumably be fewer E. hepuensis . At Luowutian Reservoir, N. hongkongense were also found in the hillstream where we collected the new species. For reasons not yet known, the N. hongkongense found here seemed to be more aquatic in habit than most other populations, as all were found in the water and oddly no crab holes could be seen. The first author observed a population with similar aquatic habitats in Bowen Hill, Hong Kong.

Remarks: Externally, Megapleonum shenzhen n. sp. is very similar to M. ehuangzhang . Both species have a large and wide male pleon with convex lateral margins, which is the most noticeable external characteristic of the genus ( Fig. 2C ; Huang et al. 2018a: fig. 2C). Similarly, the anterior thoracic sternum is broad in both species, width 1.9 × as length ( Fig. 2B ; Huang et al. 2018a: fig. 4A). Another main feature of the genus is the unique G1 morphology, which is characterized by the overall sinuous shape and the curved flap on the terminal segment ventral side ( Figs. 2D , 3C–E , 4G ; Huang et al. 2018a: figs. 3B, C, E, F, H).

Despite the many similarities, the new species can easily be distinguished from M. ehuangzhang . The G1 of M. shenzhen n. sp. reaches well beyond the pleonal locking tubercle and even beyond the suture of sternites 4/ 5 in situ ( Fig. 2D ), while that of M. ehuangzhang barely exceeds the pleonal locking tubercle ( Huang et al. 2018a: fig. 2D). The unique G1 terminal segment of the new species is also substantially different from M. ehuangzhang : the outer-distal part of the subterminal segment has a distinct raised lobe in the new species, whereas such a lobe is absent in M. ehuangzhang ; the distal part of the terminal segment also forms a lobe in the new species, whereas such a lobe is also lacking in M. ehuangzhang ( Figs. 2D , 3C–E , 4G ; Huang et al. 2018a: figs. 3B, C, E, F, H). Additionally, the G2 terminal segment in M. shenzhen n. sp. is slenderer and has a pointed tip, whereas that of M. ehuangzhang is stouter and has a blunt tip ( Fig. 3B ; Huang et al. 2018a: fig. 3A, G). The new species also has vestigial flagellum on the third maxilliped exopod, while M. ehuangzhang completely lacks this character ( Fig. 3A ; Huang et al. 2018a: fig. 3D). However, this may not be a reliable character in separating species as some species can have individuals that totally lack the flagellum or have a very short flagellum ( Huang et al. 2020).

With the discovery of this new species, the genus definition in Huang et al. 2018a, must also be adjusted to: Third maxilliped exopod with very short flagellum or no flagellum (instead of flagellum absent) and G2 with flagelliform terminal segment (instead of flagelliform terminal segment with blunt tip).

Figures

Fig. 1. Dorsal habitus. (A) Megapleonum shenzhen n. sp., male holotype (18.6 × 14.4 mm), SYSBM 001983; (B) female paratype (22.4 × 16.7 mm), SYSBM 001985.

Fig. 2. (A–D) Megapleonum shenzhen n. sp., male holotype (18.6 × 14.4 mm), SYSBM 001983; (E–F) female paratype (22.4 × 16.7 mm), SYSBM 001985. (A) Cephalothorax, anterior view; (B) anterior thoracic sternum; (C) anterior thoracic sternum and pleon, ventral view; (D) sterno-pleonal cavity with G1 in situ, ventral view; (E) pleon, ventral view; (F) vulvae, ventral view.

Fig. 3. (A–C, F–G) Megapleonum shenzhen n. sp., male holotype (18.6 × 14.4 mm), SYSBM 001983; (D) male paratype (17.2 × 13.1 mm), AM P.105613; (E) male paratype (16.2 × 12.4 mm), SYSBM 001988. Left maxilliped 3 (A); left G2, ventral view (B); left G1, ventral view (C–E); major cheliped (F); minor cheliped (G). Scale bars: A–E = 1.0 mm, F–G = 5.0 mm.

Fig. 4. (A–F) Left G1, ventral view. (A) Nanhaipotamon aculatum Dai, 1997, male (34.9 × 28.3 mm), SYSBM 001176, Bao’an, Shenzhen; (B) Nanhaipotamon aculatum, male (40.4 × 31.5 mm), SYSBM 001179, Bao’an, Shenzhen; (C) Nanhaipotamon aculatum, male (37.5 × 30.0 mm), SYSBM 001179, Shenzhen; (D) Nanhaipotamon hongkongense (Shen, 1940), male (32.0 × 25.4 mm), SYSBM 001780, Yantian, Shenzhen; (E) Nanhaipotamon hongkongense, male (31.0 × 25.4 mm), SYSBM 001781, Yantian, Shenzhen; (F) Nanhaipotamon hongkongense, male (28.0 × 22.5 mm), SYSBM 001782, Yantian, Shenzhen. (G) Megapleonum shenzhen n. sp., male holotype (18.6 × 14.4 mm), SYSBM 001983. Left G1 terminal segment, ventral view. (H) Megapleonum shenzhen n. sp., female paratype (22.4 × 16.7 mm), SYSBM 001985, colour in life. (I) Hillstream in Pingpu, Shenzhen, where Megapleonum shenzhen n. sp. is found. Scale bars: A–G = 1.0 mm.