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

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Shenzhen over the past 10 years, though further details of habitat type and actual locations are apparently unpublished thus far.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.

Discussion

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.

Conclusions

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.

References

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