Tortomon Huang, Wang & Shih, 2020

Ng, Peter K. L. & Tri, Ngo Van, 2023, Two new species of semiaquatic freshwater crabs of the genera Tortomon and Aiyunamon from karst forests in northern Vietnam (Crustacea: Brachyura: Potamidae), Zootaxa 5284 (2), pp. 364-376 : 365

Genus Tortomon Huang, Wang & Shih, 2020

Type species. Tortomon puer Huang, Wang & Shih, 2020 , by original designation.

Diagnosis. (Emended from Huang et al. 2020: 2) Carapace width less than 25 mm; carapace wider than long, dorsal surface smooth, evenly convex, regions indistinct; epigastric cristae and postfrontal cristae very low to inconspicuous, smooth; external orbital tooth not demarcated from rest of anterolateral margin, no trace of epibranchial tooth, margin smooth to weakly cristate; median lobe of epistome triangular. Third maxilliped with quadrate to rectangular ischium, exopod strongly tapering, reaching beyond anterior edge of ischium, no trace of flagellum. Cheliped merus margins smooth, surface of chela smooth. Ambulatory legs slender, unarmed. Male anterior thoracic sternum relatively wide, sternites 3 and 4 fused but demarcated by shallow but visible oblique grooves, distal edge of sternopleonal cavity with low ridge; sternal tubercle of male pleonal locking mechanism relatively small or large, on submedian part or distal third of sternite 5. Male pleon variously triangular; telson with gently to strongly concave lateral margins. G1 relatively short, almost straight to gently curved, relatively stout, terminal segment short, stout, conical to more sharply tapering. G2 subequal or longer than G1, flagellum half to two-thirds length of basal segment, with proximal part tubular and gently twisted, distalmost part very slender, prominently coiled. Vulvae with relatively wide space between them, ovate, convex, medium-sized, positioned on proximal half or one-third of sternite 6, pushing onto suture with sternite 5.

Remarks. Huang et al. (2020) described Tortomon on the basis of two new species from southern Yunnan in China, T. puer Huang, Wang & Shih, 2020 , and T. gejiu Huang, Wang & Shih, 2020 . Both are relatively small species (carapace width less than 25 mm) with a smooth carapace having entire anterolateral margins. The exopod of their third maxilliped lacks a flagellum, a feature typically associated with the genus Potamiscus Alcock, 1909 . Unlike Potamiscus s. lato, however (cf. Dai 1999; Yeo & Ng 2007; Ng et al. 2019; Pati et al. 2020), Tortomon is unique in possessing an unusual male G 2 in which the flagellum is twisted, especially at the distalmost part, which is very slender and twisted. Such a male G2 form is not known in any other potamid genus.

A note on the G1 is useful. The G1 of Tortomon is relatively short and stout, and the terminal segment is not demarcated from the subterminal segment by deep sutures. In most potamids, the suture separating the two segments is deep and prominent, visible from both ventral and dorsal views. In the case of Tortomon , the suture is shallow and more obvious in dorsal view ( Fig. 3H , 4D ). The G2 flagellum is unusual in that proximal part of the structure is normal in thickness and only slightly twisted medially; but the distalmost part tapers suddenly to form a flagellum-like structure, which is strongly coiled like a corkscrew.

Figures

FIGURE 3. Tortomon distinctum n. sp.A–D, G–I, holotype male (19.2 × 13.0 mm) (ZRC 2022.797), Vietnam; E, paratype male (18.3 × 12.4 mm) (ZRC 2022.798), Vietnam; F, paratype male (19.5 × 12.8 mm) (ZRC 2022.798), Vietnam. A, buccal cavity, anterior thoracic sternum and pleon; B, sternopleonal cavity; C, right fourth ambulatory leg; D–F, pleon; G, left G1 (ventral view); H, left G1 (dorsal view); I, left G2, ventral view). G–I, same scale.

FIGURE 4. Tortomon distinctum n. sp., holotype male (19.2 × 13.0 mm) (ZRC 2022.797), Vietnam. A, left G1 (ventral view); B, distal part of left G1 (ventral view); C, distal part of left G1 (dorsomesial view); D, distal part of left G1 (dorsal view); E, left G2 (ventral view); F, distal part of left G2 (ventral view); G, distal part of left G2 (dorsal view). Scales: A, E = 0.5 mm; B–D, F, G = 0.2 mm.