Two new species of semiaquatic freshwater crabs of the genera Tortomon and Aiyunamon from karst forests in northern Vietnam (Crustacea: Brachyura: Potamidae) Author Ng, Peter K. L. Lee Kong Chian Natural History Museum, Faculty of Science, National University of Singapore, 2 Conservatory Drive, Singapore 117377, Republic of Singapore. Author Tri, Ngo Van National Key Laboratory, Institute of Tropical Biology, Vietnamese Academy of Sciences and Technology, 9 / 621 Hanoi Highway, Thu Duc City, Ho Chi Minh City, Vietnam. text Zootaxa 2023 2023-05-11 5284 2 364 376 http://dx.doi.org/10.11646/zootaxa.5284.2.8 journal article 55193 10.11646/zootaxa.5284.2.8 b4bc4bdf-927c-43d9-ac2b-c39d33137048 1175-5326 7923530 770F88D2-FA38-499B-AC81-61B8BE61992F Tortomon distinctum n. sp. ( Figs. 1–5 ) Type material . HOLOTYPE : male (19.2 × 13.0 mm) ( ZRC 2022.797 ), Phu Tra limestome forest, Khun Há commune, Tam Ðý ờng district, Lai Châu province , Vietnam , 22°12’N , 103°36’E , 1800 m asl , coll. V . T . Ngo, 5 July 2022 . PARATYPES : 6 males (14.8 × 10.2 mm , 17.6 × 12.2 mm , 18.2 × 12.5 mm , 18.3 × 12.4 mm , 19.5 × 12.8 mm , 20.4 × 13.6 mm ), 3 females (16.1 × 11.2 mm , 16.9 × 11.6 mm , 18.0 × 12.3 mm ) ( ZRC 2022.798 ), same data as holotype . Diagnosis . Carapace transversely ovate, proportionately very wide carapace, width 1.44–1.52 times length ( Fig. 2A, B ); epigastric cristae very low, evident only as flattened rugae, postfrontal cristae not visible ( Fig. 2A, B ); anterolateral margin with very low crista, surface appears rounded in frontal view ( Figs. 1B , 2C ); median lobe of posterior margin of epistome relatively acutely triangular ( Fig. 2C, D ). Third maxilliped distinctly elongate, ischium almost twice as long as wide ( Fig. 2E ); inner angle of carpus of cheliped with low tubercle ( Figs. 2A , 5A ). Adult major male chela forming distinct basal gape between closed fingers ( Figs. 1C , 2F ). P2–5 meri slender, elongate ( Figs. 2A , 3C ). Sternal tubercle of male pleonal locking mechanism relatively small, submedian in position on sternite 5 ( Fig. 3B ). Male pleon distinctly wide, triangular, proximal margin almost twice length of somite 6; lateral margin of telson gently concave ( Fig. 3A, D ). G1 distinctly bent, terminal segment relatively slender, tapering distally ( Figs. 3G, H , 4A–D ). G2 with long flagellum, ca. two-thirds length of basal segment ( Figs. 3I , 4E ). Vulva ovate, large, on proximal half of sternite 6, pressing on suture with sternite 5 ( Fig. 5C ). FIGURE 1. Tortomon distinctum n. sp. , colour in life. Paratype male (18.3 × 12.4 mm) (ZRC 2022.798), Vietnam. A, overall dorsal habitus; B, frontal view; C, major chela; D, ventral surface and gonopods. Female characters and variation . The prominence of the large teeth on the cutting edges of the fingers of the major male chela is variable: in the holotype male, the dactylus has two or three larger teeth, the largest being on the proximal one-third, while there is one large tooth on the basal part of the pollex ( Fig. 2F ); in a paratype male, there are two very large teeth on each of the proximal halves of the dactylus and pollex ( Fig. 1C ). Both specimens are of similar sizes and there are no other major differences. Females of T. distinctum n. sp. have distinctly smaller and symmetrical chelae ( Fig. 5A ). The length/width proportions of the male pleon varies and are not taxonomically reliable. In the holotype male, somite 6 is relatively wider, with the proximal margin almost twice the length ( Fig. 3D ), but in two similar sized-paratype males, somite 6 is distinctly longer ( Fig. 3E, F ). Similar variations in male pleonal proportions have been reported in the Myanmarese potamid Indawmon Ng, Win Mar & Shih, 2020 (see Ng et al. 2020: 535 , 537). Colour . Carapace, chelipeds and ambulatory legs are dark purple; with the ventral surfaces somewhat paler and the distal half of the fingers of the chelae orange ( Fig. 1 ). Etymology . The species is named for its many distinctive features, easily distinguishing it from its Chinese congeners. Remarks . Tortomon distinctum n. sp. can easily be distinguished from T. puer and T. gejiuin having a proportionately wider carapace with the width 1.44–1.52 times length ( Fig. 2A, B ) (versus proportionately less wide, width 1.28–1.45 times length; Huang et al. 2020 : figs. 1, 4); the median lobe of the posterior margin of the epistome is more acutely triangular ( Fig. 2C, D ) (vs. median lobe proportionately wider; Huang et al. 2020 : figs. 2A, 5A); the third maxilliped is distinctly elongate, with the ischium almost twice as long as wide ( Fig. 2E ) (vs. length of ischium of third maxilliped ca. 1.5 times width; Huang et al. 2020 : figs. 3A, 6A); the inner angle of the carpus of the cheliped only has a low tubercle ( Figs. 2A , 5A ) (vs. with a sharp tooth; Huang et al. 2020 : figs. 1, 4); the adult major male chela forms a distinct basal gape when the fingers are closed ( Figs. 1C , 2F ) (vs. without distinct gape; Huang et al. 2020 : figs. 3F, 6F); the P2–5 meri are proportionately more slender and longer ( Figs. 2A , 3C ) than in T. puer and T. gejiuin ( Huang et al. 2020 : figs. 1, 4); the sternal tubercle of the male pleonal locking mechanism is distinctly smaller and less prominent ( Fig. 3B ) than in T. puer and T. gejiuin ( Huang et al. 2020 : figs. 2D, 5D); the male pleon is distinctly broader and the lateral margin of the telson is gently concave ( Fig. 3A, D ) (vs. male pleon proportionately narrower and lateral margin of telson strongly concave; Huang et al. 2020 : figs. 2B, C, 5B, C); the G1 is more distinctly bent, with the terminal segment relatively slender, tapering distally ( Figs. 3G, H , 4A–D ) (vs. G1 gently curved to almost straight, the terminal segment being relatively stouter, shorter, conical; Huang et al. 2020 : figs. 3C–E, 6C–E); the G2 is distinctly longer than the G1 ( Figs. 3G–I , 4A, E ) (vs. G2 subequal in length to G1; Huang et al. 2020 : figs. 3B–E, 6B–E); the G2 has a distinctly longer flagellum which is ca. two-thirds the length of the basal segment ( Figs. 3I , 4E ) (vs. G2 flagellum shorter, ca. half the length of basal segment; Huang et al. 2020 : figs. 3B, 6B); and the vulva is smaller and less prominent ( Fig. 5C ) ( Huang et al. 2020 : figs. 2F, 5F). FIGURE 2. Tortomon distinctum n. sp. , holotype male (19.2 × 13.0 mm) (ZRC 2022.797), Vietnam. A, overall dorsal habitus; B, dorsal view of carapace, C, frontal view of cephalothorax; D, epistome and antennules; E, right third maxilliped; F, chelae. 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. The characters of T. distinctum n. sp. , notably in the longer third maxilliped, male pleonal shape and more elongate G2, may argue for placing the species in a separate genus from Tortomon . That being said, the distinctive structure of the G1 and twisted G2 flagellum suggest treating all three species as congeneric for now. To this effect, we have amended the diagnosis for the genus to reflect these observations. The smooth ovate carapace, entire anterolateral margins and the absence of a flagellum on the exopod of the third maxilliped of T. distinctum n. sp. also allies it with Xestomon tacu Ng, 2021 , described from a mountain in southern Vietnam ( Ng 2021 ). The form of the posterior margin of the epistome, male anterior thoracic sternum, male pleon, G1, and most significantly, the G2, however, are completely different. The colour of T. puer is brownish purple and most similar to the distinct purple colour of T. distinctum n. sp. , being distinct from the greyish-white colour of T. gejiu . Ecology. This is a semi-aquatic freshwater crab, inhabiting the banks of a small stream in a small karst outcrop in a black cardamon plantation ( Amomon sp., Zingiberaceae ) at high altitudes ( 1800 m asl). Conservation. This is a small species and does not appear to be under immediate threat. The area it occurs in, however, is not protected, and until its distribution is better understood, it should be regarded as a vulnerable species with a very restricted range (cf. Cumberlidge et al. 2009 ).