Tiwaripotamon caeruleum Dang & Do, 2025, sp. nov.
Creators: name: Dang, Khai
name: Do, Cuong
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Potamidae
Tiwaripotamon
Tiwaripotamon caeruleum

description: Tiwaripotamon caeruleum sp. nov.
(Figures 1–5)
Material examined. Holotype: male (40 mm × 31 mm), (Experta 2024.001), Ang Toong pass, Bac Kan city, Bac Kan Province, 22.105°N, 105.919°E, coll. Son Dinh, 9 August 2023. Paratypes: 2 males (43.5 mm × 34 mm, 39.5 mm × 31.5 mm), 1 female (40 mm × 32mm) (Experta 2024.002; 2024.003; 2024.004), 4 males (45.4 mm × 32.5 mm, 43.2 mm × 32.5 mm, 41.1 mm × 30.1 mm, 41.0 mm × 30.1 mm), 3 females (46.6 mm × 34.0 mm, 45.3 mm × 33.0 mm, 40.0 mm × 29.8 mm) (ZRC 2024.0311), Ang Toong pass, Bac Kan city, Bac Kan Province, coll. Son Dinh, 9 August 2023.
Diagnosis. Carapace transversely ovate, broader than long, dorsal surface glabrous, nearly flat (Fig. 2A, B); regions indistinctly defined, epigastric cristae very low, marked by low striae. Cervical grooves not visible, H-shaped groove shallow but distinct (Fig. 2A). External orbital tooth broadly triangular, demarcated from epibranchial tooth and the rest of anterolateral margin by distinct and wide cleft, epibranchial tooth sharp, triangular, anterolateral margin serrated (Fig. 2A). Frontal margin broadly emarginated medially (Fig. 2B). Posterior margin of epistome with distinct median lobe (Fig. 2B). Third maxilliped exopod with distinct flagellum that reaches half of the merus width (Fig. 3A, 5A). Chelipeds elongate, outer surface of chela slightly rugose and covered with scattered small, distinct granules on ventral surface (Fig. 2A, D, E). G1 relatively slender, curved outwards (Fig. 3B), subterminal segment about 3.1 times as long as terminal segment, terminal segment with wide lobe protrudes across the length of terminal segment on proximal part, not visible in dorsal view (Fig. 3I, 5E). G2 elongate, slightly longer than G1, distal segment two-thirds length of basal segment (Fig. 3D, 5D).
Description of holotype male: Carapace transversely ovate, markedly wider than long (width-to-length ratio 1.3); dorsal surface nearly flat from the frontal view, glabrous, regions indistinctly defined (Fig. 2A). Epigastric cristae very low, marked by low striae, separated by median shallow grooves (Fig. 2A, B); postorbital cristae not prominent, confluent with epibranchial tooth. Cervical grooves not visible, H-shaped groove shallow but distinct (Fig. 2A). Frontal margin broadly emarginated (Fig. 2B). External orbital tooth broadly triangular, demarcated from epibranchial tooth and the rest of anterolateral margin by distinct and wide cleft, epibranchial tooth sharp, triangular (Fig. 2A). Anterolateral margins convex, lined with small sharp granules, appear serrated. Posterolateral margin gently sinuous, converging towards posterior margin (Fig. 2A). Orbits ovate; eyes filling orbital space, eyes peduncle relatively short, stout; cornea large, round (Fig. 2B). Supraorbital margin sinuous. Suborbital margin convex, complete and smooth (Fig. 2A, B). Antennae very short; antennules narrow, folding transversely. Posterior margin of epistome sinuous with distinct median lobe (Fig. 2B).
Third maxillipeds relatively short, covering most of buccal cavity when closed (Fig. 2B); merus shorter and narrower than ischium; ischium subrectangular, surface glabrous and smooth, slightly concave in the middle (Fig. 3A); merus subquadrate, surface smooth; exopod with distinct flagellum that reaches half of the merus width (Fig. 3A, 5A).
Chelipeds asymmetrical (Fig. 2A). Anterior margin of basis-ischium smooth; margins of merus lined with sharp and prominent granules, appears strongly serrated. Outer surface of carpus weakly rugose with sharp and broad spine on inner side (Fig. 2A). outer surface of chela slightly rugose and covered with scattered small, distinct granules on ventral surface (Fig. 2D, E). Fingers short, stout, gently curved inwards, slightly shorter than palm, outer surface lined with shallow pits; cutting edges of both fingers with variously sized sharp teeth; dorsal margin of dactylus line with small pits (Fig. 2D, E).
Ambulatory legs relatively long, segments relatively slender; second pair longest (about 2.1 times longer than carapace width), last pair shortest (about 1.7 times longer than carapace width) (Fig. 2A). Merus slender, outer surface slightly rugose, dorsal margin uneven, subcristate, without subdistal spine or tooth (Fig. 2A); carpus with serrated margin in first and second legs but absence in third and fourth legs; propodus with all margins serrated, the fourth pair is the least serrated; dactylus relatively long, slender, gently curved, margins with short, sharp pectinate spines and short setose (Fig. 2A).
Thoracic sternum relatively broad, smooth (Figs. 2C, 3B). Sternites 1, 2 merged completely to form broadly triangular plate; separated from sternite 3 by distinct margin; sternites 3, 4 fused, with shallow, complete groove demarcating suture, sternite 3 rugose (Fig. 2C). Sternite 8 not visible when pleon closed. Sternopleonal cavity reaching to imaginary line joining median part of cheliped coxae (Fig. 3B). Male pleonal locking tubercle very small, almost invisible, located on submedian part of sternite 5 (Fig. 3B).
Pleon broadly triangular, all somites and telson free; telson broadly triangular, lateral margins nearly straight (Figs. 2C, 3C); somite 6 transversely rectangular with lateral margins gently convex, somites 4 and 5 trapezoidal, gradually decreasing in width, increasing in length (Fig. 3C); somite 3 widest, subrectangular; somite 2 slightly wider than somite 1; somite 1 wide, reaching to bases of coxae of fourth ambulatory legs (Fig. 3C).
G1 relatively slender, curved outwards distally (Fig. 3B, 3E, 3F, 5B, C); gently tapering distally; terminal segment slender, slightly curved upward, clearly separated from subterminal segment (Fig. 3E, F), tip pointed with wide, rounded lobe protrudes across the length of terminal segment on proximal part, not visible in dorsal view (Fig. 3I). Subterminal segment about 3.1 times of terminal segment (Fig. 3E, F). G2 straight, slightly longer than G1, distal segment two-thirds length of basal segment (Fig. 3D, 5D).
Description of paratype female: Female specimens with similar external morphological features to males except having more slender and smaller chelipeds. Pleon is subovate, wide and covers all of thoracic sternum when close (Fig. 4A). Vulvae on thoracic sternite 6, ovate, open inward, semicircular (Fig. 4B).
Etymology: The species name is taken from the specific epithet “caeruleum ” from the Latin meaning blue, referring to the colouration of living individuals.
Live colouration: Entire body is dark blue except the white chelipeds fingers and thoracic sternites, the merus of third maxillipeds are orangish-red. Some small individuals have white chelipeds or ivory white propodus and dactylus of the ambulatory legs. There is no distinct difference in morphological structure between individuals that are different in colours.
Habitat: The area where specimens were collected is an evergreen broadleaf forest on limestone mountains. There are many rotten leaves scattered on the forest floor and some large limestone rocks with many crevices around. Surrounding vegetation includes small herbaceous plants and ferns that grow on the cliffs and ground. They often hide beneath crevices in rocks and come out after heavy rains. Locals capture the crabs using a long stick to lure the crabs out of the rock crevices and catch by hand. Individuals are often seen moving on the forest floor and on large limestone boulders and run away very quickly when encountered during the rainy season, and are less common during the dry season.
Remarks. Based on external features, the new species most closely resembles Tiwaripotamon edostilus Ng & Yeo, 2001, and T. pluviosum Do, Shih & Huang, 2016, due to the flat carapace with sharp epibranchial tooth and serrated anterolateral margin (Fig. 2A, B; cf. Ng & Yeo 2001: fig. 4A, B; Do et al. 2016: fig. 3A, B). However, Tiwaripotamon caeruleum sp. nov. can be distinguished from these two species by the following: 1) T. caeruleum sp. nov. has distinct and wide cleft separate external orbital tooth from the rest of anterolateral margin, epibranchial tooth sharp, triangular (Fig. 2A) (versus T. edostilus with widely triangular epibranchial tooth with narrow and deep cleft separate external orbital tooth from the rest of anterolateral margin; cf. Ng & Yeo 2001: fig. 4A), and T. pluviosum has wide, low epibranchial tooth with indistinct cleft separate external orbital tooth from the rest of anterolateral margin; cf. Do et al. 2016: fig. 3A; Fig. 2A); 2) posterior margin of epistome of T. caeruleum sp. nov. sinuous with distinct median lobe (Fig. 2B) (versus low median triangular lobe in two other species; cf. Ng & Yeo 2001: fig. 4B; Do et al. 2016: fig. 3B); 3) third maxillipeds of the new species has distinct flagellum that reaches half of the merus width (Fig. 2A, 5A) (versus T. edostilus with flagellum shorter than half width of merus (cf. Ng & Yeo 2001: fig. 5A), and T. pluviosum also has flagellum that reaches half of the merus width; cf. Do et al. 2016: fig. 1C); 4) the G1 terminal segment of T. caeruleum sp. nov. slightly curved upward with wide dorsal lobe protrudes across the length of terminal segment on proximal part, not visible in dorsal view (Figs. 3E, F, I, 5B, C, E) (versus terminal segment distinctly upcurved with small lobe in T. edostilus and T. pluviosum; cf. Ng & Yeo 2001: fig. 5D, G, I, K; Do et al. 2016: fig. 2A, B, C, D); and 5) the life coloration of these species are also very different, the carapace of T. caeruleum sp. nov. is dark blue (Fig. 1A) (versus the carapace of T. edostilus is scarlet red (cf. Shih & Do 2014: fig. 7A, B, C, D) and the carapace of T. pluviosum varies from reddish brown, purple to orange; cf. Do et al. 2016: fig. 6A, B).
Tiwaripotamon caeruleum sp. nov. is also similar to T. pingguoense Dai & Naiyanetr 1994, from Guangxi province, China, in gonopod structure. However, it can be distinguished from this species by the following: 1) T. caeruleum sp. nov. has transversely ovate carapace with sharp, triangular epibranchial tooth (Fig. 2A) (versus T. pingguoense has subquadrate carapace with broadly triangular epibranchial tooth; cf. Ng 2024: figs. 6A, 7A); 2) posterior margin of epistome of T. caeruleum sp. nov. sinuous with distinct median lobe (Fig. 2B) (versus posterior margin with low and wide median lobe in T. pingguoense; cf. Ng 2024: fig. 7B); 3) the G1 characteristics of these two species are similar, but the terminal segment of T. caeruleum sp. nov. slightly curved upward with wide dorsal lobe protrudes across the length of terminal segment on proximal part, not visible in dorsal view (Figs. 3E, F, I, 5B, C, E) (versus T. pingguoense is visible in dorsal view; cf. Dai et al. 1980: fig. 2.3; Ng 2024: fig. 8B, D); and 4) the G1 terminal segment of T. caeruleum sp. nov. is relatively shorter, about 3.1 times of terminal segment (Figs. 3E, F, G, H, 5B, C, E) (versus 3.4 times in T. pingguoense; cf. Dai et al. 1980: fig. 2.2; Ng 2024: fig. 8A, B, C, D); 5) the female vulvae shape of these two species is also very different, the vulva opening of the new species is semicircular (Fig. 4B) (versus that of T. pingguoense is narrowly elliptical, directed obliquely inwards; cf. Ng 2024: fig. 7D).
Nguyen (2017) reviewed the known species of genus Tiwaripotamon in Vietnam and discussed their distribution in his unpublished Master of Science thesis. Among the species in the unpublished thesis, several specimens were collected in Chi Lang, Lang Son province and identified as “ Tiwaripotamon araneum ” (Nguyen 2017). In the original description of this species, however, it was reportedly collected in "massif mont de Tai Kinh, near Hanoi " (Rathbun 1905). This type locality of T. araneum could be located in the west of Hanoi, which is very far from Chi Lang, Lang Son province and the specific location remains unknown, and therefore, the Tiwaripotamon species collected in Lang Son province is unlikely to be T. araneum but may be a different species instead (Peter K.L. Ng, pers. comm.). Indeed, some morphological characteristics including the shape of the carapace, pleon, prominent lobe of epistome and third maxilliped flagellum of this species closely resemble T. caeruleum sp. nov. (cf. Nguyen 2017: figs. 3.6A, B, C, 3.7D), but the G1 structure of these two species still have significant differences in the terminal segment (Figs. 3E, F, G, H, 5B, C, E; cf. Nguyen 2017: fig. 3.7A, B, C), the regions on the carapace of T. caeruleum sp. nov. are also less distinct, and the cheliped merus of the new species is more strongly serrated (Fig. 2A; cf. Nguyen 2017: fig. 3.6A). The distribution areas of these two species are also about 150 km apart. Therefore, specimens of the species “ Tiwaripotamon araneum ” will need to be examined before any conclusion can be made.
description: Published as part of Dang, Khai & Do, Cuong, 2025, A new species of genus Tiwaripotamon Bott, 1970 (Crustacea, Decapoda, Brachyura, Potamidae) from northern Vietnam, with notes on a similar species identified as " Tiwaripotamon araneum ", pp. 585-593 in Zootaxa 5631 (3) on pages 586-591, DOI: 10.11646/zootaxa.5631.3.11, http://zenodo.org/record/15370783