Journal of Threatened Taxa | | 26 January 2018 | 10(1): 11156–11165



Distribution of Nanhaipotamon hongkongense (Shen, 1940) (Crustacea: Brachyura: Potamidae), a freshwater crab endemic to Hong Kong


David John Stanton 1, Michael Robertson Leven 2 & Tommy Chung Hong Hui 3


1,2,3 AEC Ltd., 127 Commercial Centre, Palm Springs, Yuen Long, Hong Kong

1 (corresponding author), 2, 3








Abstract: Nanhaipotamon hongkongense (Shen, 1940) is a tropical freshwater crab currently considered endemic to Hong Kong. The species is more widely distributed in Hong Kong than previously published photographic records from Guangdong Province require further survey and corroboration; these would be the first records of this species outside of Hong Kong. Nanhaipotamon hongkongense prefers terrestrial environs in close proximity to clean watercourses shaded by secondary woodland, and records from this study indicate it is also found at lower elevations than previously published. The habitats of this semi-aquatic species are under threat due to development. It is hoped that understanding of the species’ distribution will aid in its conservation and encourage further study of this species and its habitat uses.

Keywords: Crabs, Crustacea, endemic, freshwater, habitat loss, Hong Kong, tropical.

Chinese abstract: 香港南海溪蟹是一種目前被認為是香港特有的熱帶淡水蟹。本文研究發現香港南海溪蟹在香港的分佈較以往認知的更為廣泛。另外,曾有報告稱於廣東省發現本種棲息,惟仍須進一步調查及確認。如有關報導屬實,將成為本種於香港境外的首筆確實紀錄。香港南海溪蟹偏好次生林地中的清澈溪流附近的陸地生境,本研究亦發現香港南海溪蟹棲息地的海拔高度下限似乎較過去所知的低。香港南海溪蟹的棲息地正面臨各種發展的威脅。本文希望增加對本種分佈的了解,可以幫助及促進本種的保育工作,及促進更多有關本種及其生境使用的研究。







Only known from Hong Kong, Nanhaipotamon hongkongense (Shen, 1940) is a tropical freshwater crab (Image 1). This species is found mostly in secondary forest; it is very terrestrial and rarely occurs in water, instead inhabiting the dry areas beyond the banks of streams, though smaller crabs and juveniles appear to stay closer to pools or patches of wet ground (Ng & Dudgeon 1992; Cumberlidge 2008a). They excavate burrows during dry periods; following rainfall the adult crabs move out of their burrows, even during daylight (Ng & Dudgeon 1992; Cumberlidge 2008a).

The species is listed under the Least Concern category of the IUCN Red List because there are no known long-term threats (Cumberlidge 2008a). It was last collected in 1991 and its extent of occurrence is probably less than 20,000km² (Cumberlidge 2008a). Its published range under Cumberlidge (2008a) is limited to three locations, Tai Po Kau Forest Reserve and Nai Chung stream in New Territories and near Victoria Peak on Hong Kong Island, though the range map shows the distribution extending outside of Hong Kong into neighbouring Guangdong, northwards up the Pearl River estuary and eastwards along the coast, though no supporting evidence or rationale for this suggested wider range is provided.

IUCN stated that major threats to this species are habitat loss and pollution and that no conservation measures are known to be in place for this species, and incorrectly, that it is not found in a protected area (Cumberlidge 2008a). According to a local conservation assessment, the species is listed as being of Potential Global Concern (Fellowes et al. 2002).

While some Chinese freshwater crabs have been quite well studied, most species are either known only from the type locality or from just a few localities. In these situations, further collections are necessary to ascertain their actual distributions (Cumberlidge et al. 2010). Therefore, we have made field observations in Hong Kong in order to provide additional information on the distribution of N. hongkongense.

Materials and Methods

Study Area

The present study area, Hong Kong Special Administration Region (SAR), People’s Republic of China (PRC) (22.15000000–22.616666670N & 113.8333333–114.50000000E) is situated on the southern China coast to the east of the Pearl River (Zhujiang) estuary (Fig. 1). Hong Kong occupies an area of 1,100km2 and is made up of a section of the Chinese Mainland (Kowloon and the New Territories, 793km2) and islands, of which Hong Kong and Lantau are the largest (78km2 and 147km2, respectively). The topography of Hong Kong is generally rugged with little flat land; much of the flatter areas (c. 60km2) are a result of land reclamation (Dudgeon & Corlett 2004). The Shenzhen River to the north largely separates Hong Kong from the Shenzhen Special Economic Zone of the PRC.

The climate of Hong Kong is distinctly monsoonal and despite its subtropical nature has well-defined seasons associated with the East Asian monsoons (Carey et al. 2001). During winter, the continental high-pressure region over Siberia and Mongolia result in north or northeasterly winds that bring cool, dry air to Hong Kong (Dudgeon & Corlett 2004).

Literature review

Literature was reviewed to examine the known distribution of N. hongkongense. Full details of this review and sources can be seen in Appendices 1 and 2. Hong Kong SAR has a robust Environmental Impact Assessment (EIA) process and numerous developments requiring EIA studies have taken place in lowland Hong Kong; potentially affecting streams where this crab occurs. Such EIA studies invariably require surveys of the streams that may be affected. Accordingly, desktop studies of EIA reports were made from the documents available at the Environmental Protection Department website ( in order to comprehensively review the available ecological findings from these studies. Additional data were obtained from unpublished studies and the authors’ own unpublished results of previous survey findings and from observations submitted on the online initiative iNaturalist (, an online citizen science network that collates and presents observations of wildlife.


A review of 126 EIA reports, published between 2002 and 2017, was undertaken and these are listed in Appendix 1. The findings of the present review, combined with additional data obtained from unpublished studies, have revealed that N. hongkongense is known from at least 34 locations at 24 sites in Hong Kong (Appendix 1, Fig. 1). The type locality is The Peak on Hong Kong Island (Shen 1940).

Nanhaipotamon hongkongense has been found mostly in or near fast-flowing watercourses, within semi-mature secondary woodland with limited anthropogenic influences, and with altitudinal range from 17–514 m. Examples of typical habitats where the crab has been observed are shown in Image 2. Field observations by the authors are that it is generally recorded singularly; in captivity, as this species is aggressive, specimens, particularly conspecifics, cannot be kept together (Ng & Dudgeon 1992). It has, however, been recorded in the same watercourses as Cryptopotamon anacoluthon, another endemic freshwater crab (Ng & Dudgeon 1992).

Anecdotal evidence supports the published statement that the crab is largely terrestrial with records from residential areas and school buildings, well away from any streams (Alex McMillan in lit. 08.ix.2016). The species was also observed climbing on a tree in Tai Po Kau Forest Reserve, at a height of over a metre from the ground (pers. obs.). Furthermore, it is understood that it may “occur wherever there are reasonably clean streams under almost completely closed canopy. They don’t seem to care if the stream has polystyrene and plastic bags as long as the water is flowing well, so is clean” (Paul Crow in lit. 13.ix.2016).

Outside of Hong Kong, there are records of this species and its congener, N. aculatum, from neighbouring Shenzhen over the past 10 years, though further details of habitat type and actual locations are apparently unpublished thus far. A record of land crabs (to date of unknown species) from the Dapeng Peninsula of Guangdong (Jonathan Martinez in lit. 15.xi.2017) is to be further investigated. These crabs were recorded from good quality lowland fung shui woodland behind a village on a hillside. Whilst this site in Dapeng is only 18km from the closest Hong Kong record, they are separated by the extensive seawaters of Mirs Bay.



Distribution and habitat requirements of

N. hongkongense

Cumberlidge (2008a) stated that N. hongkongense occurred in three locations in Hong Kong, at elevations from 50–100 m; however, from the present review, it is clear that the species is more widespread than previously thought, i.e., with 34 identified locations and approximately 400km2 area of occupancy in Hong Kong. In addition, the records from Shenzhen confirm that this species occurs beyond Hong Kong, albeit the extent of its range in southern China remains uncertain. Cumberlidge (2008a) suggested that the population of N. hongkongense is not severely fragmented, though the habitat mosaics in which N. hongkongense occurs are often fragmented by developed areas (Stanton & Leven 2016; Stanton et al. 2017) and do not share downstream confluences or natural habitat linkages, a result of urbanisation. Many watercourses have been piped or channelised in their lower sections and suitable secondary woodlands lack connectivity, thus potentially inhibiting the movement of crabs. Hence, it is likely that within this area of occupancy there are now a number of more or less isolated sub-populations.

Mitigation and Conservation

According to IUCN, no conservation measures are known to be in place for N. hongkongense, and the species is not found in a protected area (Cumberlidge 2008a). One of the sites listed by Cumberlidge (2008a), Tai Po Kau Reserve, however, was in fact protected at that time, as it is today as is the type locality, which falls within Aberdeen Country Park. Given its habitat requirements, many of the sites occur within upland hillstreams within secondary wooded habitats, which are largely situated within Country Parks or Protected Areas (e.g., Tai Po Kau, Fung Yuen). Those sites zoned ‘Green Belt’ under local planning guidelines are under pressure for housing developments (Stanton et al. 2017). It should be noted that the species does also occur at lower elevations, particularly on the islands of Kau Sai Chau and Lamma (see Appendix 2, Fig. 1).

Currently, there is no mechanism in place to protect the ecology of entire rivers and their catchments in Hong Kong (Dudgeon & Chan 1996; Cheung et al. 2010), and there is an urgent need for protection of the remaining rivers in their natural state (Hong Kong Birdwatching Society 2013); a similar situation is occurring in much of the rest of Asia (Cumberlidge et al. 2009, 2010).

When mitigation is prescribed through the EIA process in Hong Kong, it is usually in the form of watercourse preservation and the inclusion of riparian buffers and/or translocation exercises. Currently, there are no stringent guidelines for implementation of habitat management, riparian buffer zones or conducting species translocation (Stanton & Leven 2016; Lau et al. 2017); though based on detailed studies of the semi-aquatic Hong Kong Newt Paramesotriton hongkongensis in Hong Kong, Lau et al. (2017) proposed fixed-width buffer zones of 113m away from stream margins to protect the terrestrial habitat for this species. But projects for reducing habitat loss and fragmentation by watercourse restoration, recreation or enhancement and faunal conservation programs are being started or are in progress (e.g., Cumberlidge et al. 2009, 2010; Hong Kong Birdwatching Society 2015) in Hong Kong and elsewhere in the south China region. Furthermore, with the implementation of actions in the Hong Kong Biodiversity Strategy and Action Plan 2016–2021, there is scope for several specific actions that address habitat management and enhancement of watercourses and country parks and potentially help to improve our knowledge of this, and other, endemic freshwater species. This could also benefit its little known congener, Nanhaipotamon aculatum, which is only known from a few localities in the Northwestern New Territories in Hong Kong (H.K. Chan in lit. xi.2016) and also in Shenzhen. It is listed as Data Deficient in view of the absence of further information on its extent of occurrence, ecological requirements, population size, population trends, and long-term threats (Cumberlidge 2008b).

The restricted range of many crab species from China, together with the ongoing human-induced loss of habitat in many parts of the region are a cause for concern, and it is considered that conservation activities should be aimed primarily at preserving the integrity of sites and habitats while closely monitoring key populations at the same time (Cumberlidge et al. 2010).

Many of the sites in Hong Kong are isolated, fragmented by a combination of developed areas (where downstream sections have been lost) and physical topography, and have few ecological linkages suitable for a predominantly aquatic species to exploit. Protection of known sites is therefore important, so that these can ensure the continued survival of the species, and suitable habitat management would also be beneficial either by providing increased habitat area or by providing corridors to link populations.

IUCN Red List Status

The present study is not intended to constitute a review of the IUCN listing of N. hongkongense Nevertheless, we suggest that the IUCN Red List status of N. hongkongense should be revisited in the light of our findings. While it is most unlikely that the population size and the extent of occurrence or area of occupancy meet the IUCN criteria for the listing of N. hongkongense as ‘Vulnerable’ , the species is still known only from Hong Kong and probably with a relatively small, fragmented and declining population largely restricted to upland areas.

The mapped range of N. hongkongense shown by IUCN extends considerably beyond Hong Kong into Guangdong Province, though as noted above no justification for this wider range is provided by Cumberlidge (2008a). Since much of the purported range comprises the intensely developed urban centres of Shenzhen and Dongguan it is considered most unlikely that it is present throughout this area. However, there are patches of potentially suitable habitat, highlighted by the photographic evidence from Shenzhen and it would be prudent to search for N. hongkongense in such areas to better determine the extent of species’ occurrence.

Cumberlidge et al. (2010) stated that the existing IUCN Red List status can be updated by gathering current data on the distribution, natural history, population trends, threats, and endemism of China’s highly diverse freshwater crabs. Once the IUCN Red List is updated, the conservation strategies can be developed for these understudied, diverse and potentially threatened fauna. It is hoped that the information gathered during the present study will help to feed into this process.


Nanhaipotamon hongkongense is widely distributed within Hong Kong; recorded throughout the New Territories, Hong Kong, Lamma and Lantau Islands. So far, there are no fully documented observations outside of Hong Kong and photographic records from Guangdong Province require further survey and corroboration. Generally, N. hongkongense prefers terrestrial environs in close proximity to clean watercourses shaded by secondary woodland. The species, however, has also been recorded in lower elevations from several locations, notably on the islands of Lamma and the Sai Kung Area (including the island of Kau Sai Chau). Watercourses and woodland in which this species occurs are largely natural with limited anthropogenic impacts such as channelisation or modification, but as the requirement of land increases for development, such areas will be under threat.


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