The discovery underscores how little is still known about freshwater biodiversity in Southeast Asia. More than half of all freshwater crab species known to science have been described only since 1980, and experts consider the global fauna to remain in a discovery phase. Vietnam, with its heterogeneous topography, varied climate and ecosystem types, and a long history of limited biological surveys, was expected to harbour a substantial number of undescribed potamid crabs. Indeed, recent years have brought a string of new taxa from the country, including Xestomon tacu in 2021, Dacuomon quynhi in 2023 and Tayninhon nuibaden in 2025. With the latest additions, Vietnam’s freshwater crab fauna now stands at 65 recognised species in 24 genera.

Angiangmon nuicoto was collected near the summit of Co To Mountain, a forested peak where no streams are present. The crab is a medium-sized species, with adult males ranging from about 18 to 32.5 millimetres in carapace width, and it is instantly striking in life, with a reddish-purple dorsal surface and purple walking legs. Its carapace is subquadrate and almost perfectly smooth on the upper surface, with a serrated anterolateral margin lined with widely spaced granules. The genus name combines An Giang Province with the traditional freshwater crab genus name Potamon, while the species name honours the type locality, Nui Co To.

What sets Angiangmon apart from its relatives is a suite of anatomical characters, particularly in the feeding appendages and reproductive structures. The third maxilliped, one of the mouthparts, carries a flagellum on its exopod that is very short or reduced to a vestigial knob, an unusual condition among potamid crabs. The walking legs are long and slender, with the second pair the longest, and the fourth merus has a length-to-width ratio of about 4.2. The male first gonopod, the intromittent organ used in mating, has a terminal article that curves inwards at its distal end, while the second gonopod is longer than the first. In adult males, the fingers of the larger claw leave a conspicuous gap when closed.

Among the closest relatives, the new genus most resembles Neolarnaudia, a group distributed in South Central Vietnam, but the differences are numerous. Adult male Angiangmon are smaller, their carapaces smoother, the external orbital angle broadly rather than acutely triangular, the maxilliped flagellum reduced, the ambulatory legs slender rather than stout, and the telson has slightly convex rather than concave lateral margins. Intriguingly, the reduced flagellum may reflect an ecological shift. According to work by Ng and Yeo, adults of more terrestrial crab species show reduced reliance on this structure, and Angiangmon is indeed semiterrestrial: the specimens were found on land away from water, crawling on large rocks, whereas Neolarnaudia species inhabit mountain streams. The carapace also resembles that of Dalatomon, but that genus has a well-developed flagellum, a hidden suture between sternites 3 and 4, and a differently shaped, outwardly curved gonopod tip with a dorsal flap.

The second new taxon, Lamdongmon tanhlinhensis, was collected in terrestrial habitat a few metres to several hundred metres from streams in Ong Mountain Nature Reserve. Smaller than its newly described cousin, with adult males measuring 9.2 to 26 millimetres in carapace width, it wears a very different colour scheme: a dark yellow carapace with bright yellow chelipeds and legs. Its ambulatory legs are normal in proportions rather than elongated, and they are covered in dense setae, giving the animal a distinctly hairy-legged appearance. The species is presumed not to burrow, as no crab burrows attributable to it were observed; instead, the crabs likely shelter in pre-existing crevices and cavities.

Lamdongmon is defined above all by its remarkable male first gonopod, which is prominently sinuous: slender, curved outwards along its basal two-thirds, and then abruptly directed upwards at the distal end, with a pointed tip and no dorsal flap. Other diagnostic traits include a subquadrate carapace in which the epibranchial tooth is indistinct, an external orbital tooth that is not clearly demarcated, low and discontinuous postorbital cristae, a subquadrate merus on the third maxilliped with a well-developed flagellum, and completely fused sternites 3 and 4 separated only by oblique grooves. The genus name derives from Lam Dong Province combined with Potamon, and the species epithet references the Tanh Linh locality.

Distinguishing Lamdongmon from superficially similar genera required careful comparison of fine morphological detail. It resembles Dalatomon in general carapace appearance but differs in the indistinct epibranchial tooth, the discernible suture between sternites 3 and 4, and the long, slender, sinuous gonopod terminal article. It also recalls Dacuomon, described only in 2023, yet differs in the rounded rather than sharp postorbital crista, the indistinct external orbital and branchial teeth, an acutely triangular median lobe on the epistome, a maxilliped flagellum shorter than the width of the merus, a nearly smooth outer chela surface, a distinct groove between sternites 3 and 4, and the upwardly directed gonopod tip. In crab taxonomy, where genera are often separated by such characters, the gonopod architecture is especially decisive because it evolves rapidly and tends to be species-specific.

The field methods behind the descriptions were thoroughly modern. Live specimens were photographed with a digital camera before being preserved in 70 to 95 percent ethanol, then examined under a stereo microscope in the laboratory, with detailed images taken of the carapace, claws, sternum and gonopods. The type material is deposited at the Institute of Biology of the Vietnam Academy of Science and Technology. The holotype of Angiangmon nuicoto, an adult male 32.5 by 25.3 millimetres, was collected in May 2018, while the Lamdongmon holotype, a 26 by 21 millimetre male, was collected in September 2019, each accompanied by a series of male and female paratypes that document the range of variation within each species.

Perhaps the most sobering aspect of the discovery is what it suggests about vulnerability. Unpublished survey data across Vietnam indicate that each species is restricted to a single mountain locality, with estimated areas of occupancy of less than 50 square kilometres for Angiangmon and less than 250 square kilometres for Lamdongmon. The forested summit of Co To Mountain may represent the only remaining habitat for the first species, and it faces mining, rapidly expanding tourism infrastructure, intensive agriculture in the surrounding foothills, droughts linked to climate change, and a forest fire in April 2024. The authors note that most threatened freshwater crabs are narrow-range endemics, often semiterrestrial, inhabiting ecosystems increasingly affected by deforestation, fires, altered drainage and pollution. They argue that immediate conservation measures are warranted, centred on habitat protection and further field surveys to assess population sizes and ecological requirements, and that conserving natural habitats alongside improvements in local livelihoods should be a priority for biodiversity efforts in the region. The two crabs, hidden in plain sight on their respective peaks, are a vivid reminder that some of the planet’s most distinctive animals may be discovered and lost within a single generation.

Subject of Research: Taxonomic description of two new genera and species of potamid freshwater crabs from southern Vietnam

Article Title: Two new genera and two new species of freshwater crab ( Crustacea , Brachyura , Potamidae , Potamiscinae ) from southern Vietnam

Article References: Do, V. T., Nguyen, T. C., Dang, V. D., Bui, T. L., & Le, H. A. (2026). Two new genera and two new species of freshwater crab (Crustacea, Brachyura, Potamidae, Potamiscinae) from southern Vietnam. Zoosystematics and Evolution, 102(4), 1065-1078. https://doi.org/10.3897/zse.102.187291

Image Credits: AI Generated

DOI: 10.3897/zse.102.187291

Keywords: freshwater crabs, Potamidae, new species, Vietnam, taxonomy, biodiversity, endemism, conservation, An Giang, Lam Dong, carcinology, semiterrestrial crabs