Tiwaripotamon caeruleum, Dang & Do, 2025

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 ”, Zootaxa 5631 (3), pp. 585-593 : 586-591

publication ID	https://doi.org/10.11646/zootaxa.5631.3.11
publication LSID	lsid:zoobank.org:pub:2F863D2F-61DE-4CEB-845D-E89BA9689601
DOI	https://doi.org/10.5281/zenodo.15374780
persistent identifier	https://treatment.plazi.org/id/038587AC-FFA5-3A5F-02EE-41B2E79AFDD2
treatment provided by	Plazi
scientific name	Tiwaripotamon caeruleum
status	sp. nov.

Treatment

Tiwaripotamon caeruleum sp. nov.

( Figures 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 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 GoogleMaps . 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 View Materials ), 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 View FIGURE 2 ); regions indistinctly defined, epigastric cristae very low, marked by low striae. Cervical grooves not visible, H-shaped groove shallow but distinct ( Fig. 2A View FIGURE 2 ). 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 View FIGURE 2 ). Frontal margin broadly emarginated medially ( Fig. 2B View FIGURE 2 ). Posterior margin of epistome with distinct median lobe ( Fig. 2B View FIGURE 2 ). Third maxilliped exopod with distinct flagellum that reaches half of the merus width ( Fig. 3A View FIGURE 3 , 5A View FIGURE 5 ). Chelipeds elongate, outer surface of chela slightly rugose and covered with scattered small, distinct granules on ventral surface ( Fig. 2A, D, E View FIGURE 2 ). G1 relatively slender, curved outwards ( Fig. 3B View FIGURE 3 ), 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 View FIGURE 3 , 5E View FIGURE 5 ). G2 elongate, slightly longer than G1, distal segment two-thirds length of basal segment ( Fig. 3D View FIGURE 3 , 5D View FIGURE 5 ).

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 View FIGURE 2 ). Epigastric cristae very low, marked by low striae, separated by median shallow grooves ( Fig. 2A, B View FIGURE 2 ); postorbital cristae not prominent, confluent with epibranchial tooth. Cervical grooves not visible, H-shaped groove shallow but distinct ( Fig. 2A View FIGURE 2 ). Frontal margin broadly emarginated ( Fig. 2B View FIGURE 2 ). 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 View FIGURE 2 ). Anterolateral margins convex, lined with small sharp granules, appear serrated. Posterolateral margin gently sinuous, converging towards posterior margin ( Fig. 2A View FIGURE 2 ). Orbits ovate; eyes filling orbital space, eyes peduncle relatively short, stout; cornea large, round ( Fig. 2B View FIGURE 2 ). Supraorbital margin sinuous. Suborbital margin convex, complete and smooth ( Fig. 2A, B View FIGURE 2 ). Antennae very short; antennules narrow, folding transversely. Posterior margin of epistome sinuous with distinct median lobe ( Fig. 2B View FIGURE 2 ).

Third maxillipeds relatively short, covering most of buccal cavity when closed ( Fig. 2B View FIGURE 2 ); merus shorter and narrower than ischium; ischium subrectangular, surface glabrous and smooth, slightly concave in the middle ( Fig. 3A View FIGURE 3 ); merus subquadrate, surface smooth; exopod with distinct flagellum that reaches half of the merus width ( Fig. 3A View FIGURE 3 , 5A View FIGURE 5 ).

Chelipeds asymmetrical ( Fig. 2A View FIGURE 2 ). 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 View FIGURE 2 ). outer surface of chela slightly rugose and covered with scattered small, distinct granules on ventral surface ( Fig. 2D, E View FIGURE 2 ). 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 View FIGURE 2 ).

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 View FIGURE 2 ). Merus slender, outer surface slightly rugose, dorsal margin uneven, subcristate, without subdistal spine or tooth ( Fig. 2A View FIGURE 2 ); 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 View FIGURE 2 ).

Thoracic sternum relatively broad, smooth ( Figs. 2C View FIGURE 2 , 3B View FIGURE 3 ). 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 View FIGURE 2 ). Sternite 8 not visible when pleon closed. Sternopleonal cavity reaching to imaginary line joining median part of cheliped coxae ( Fig. 3B View FIGURE 3 ). Male pleonal locking tubercle very small, almost invisible, located on submedian part of sternite 5 ( Fig. 3B View FIGURE 3 ).

Pleon broadly triangular, all somites and telson free; telson broadly triangular, lateral margins nearly straight ( Figs. 2C View FIGURE 2 , 3C View FIGURE 3 ); somite 6 transversely rectangular with lateral margins gently convex, somites 4 and 5 trapezoidal, gradually decreasing in width, increasing in length ( Fig. 3C View FIGURE 3 ); 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 View FIGURE 3 ).

G1 relatively slender, curved outwards distally ( Fig. 3B, 3E, 3F View FIGURE 3 , 5B, C View FIGURE 5 ); gently tapering distally; terminal segment slender, slightly curved upward, clearly separated from subterminal segment ( Fig. 3E, F View FIGURE 3 ), tip pointed with wide, rounded lobe protrudes across the length of terminal segment on proximal part, not visible in dorsal view ( Fig. 3I View FIGURE 3 ). Subterminal segment about 3.1 times of terminal segment ( Fig. 3E, F View FIGURE 3 ). G2 straight, slightly longer than G1, distal segment two-thirds length of basal segment ( Fig. 3D View FIGURE 3 , 5D View FIGURE 5 ).

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 View FIGURE 4 ). Vulvae on thoracic sternite 6, ovate, open inward, semicircular ( Fig. 4B View FIGURE 4 ).

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 View FIGURE 2 ; 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 View FIGURE 2 ) (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 View FIGURE 2 ); 2) posterior margin of epistome of T. caeruleum sp. nov. sinuous with distinct median lobe ( Fig. 2B View FIGURE 2 ) (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 View FIGURE 2 , 5A View FIGURE 5 ) (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 View FIGURE 3 , 5B, C, E View FIGURE 5 ) (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 View FIGURE 1 ) (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 View FIGURE 2 ) (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 View FIGURE 2 ) (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 View FIGURE 3 , 5B, C, E View FIGURE 5 ) (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 View FIGURE 3 , 5B, C, E View FIGURE 5 ) (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 View FIGURE 4 ) (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 View FIGURE 3 , 5B, C, E View FIGURE 5 ; 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 View FIGURE 2 ; 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.