Journal of Threatened Taxa | www.threatenedtaxa.org | 26 March 2016 | 8(3): 8564–8574

 

 

 

Distribution, habitat utilisation and conservation status of the freshwater crab, Somanniathelphusa zanklon Ng & Dudgeon, 1992 (Crustacea: Brachyura: Gecarcinucidae) endemic to Hong Kong

David J. Stanton 1 & Michael R. Leven 2

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

1 djs@aechk.hk (corresponding author), 2 mrl@aechk.hk

 

 

 

doi: http://dx.doi.org/10.11609/jott.2070.8.3.8564-8574 | ZooBank: urn:lsid:zoobank.org:pub:EC3285D0-85F2-4121-8C27-61610077A2AB

 

Editor: Anonymity requested. Date of publication: 26 March 2016 (online & print)

Manuscript details: Ms # 2070 | Received 15 June 2015 | Final received 14 October 2015 | Finally accepted 27 February 2016

 

Citation: Stanton, D.J. & M.R. Leven (2016). Distribution, habitat utilisation and conservation status of the freshwater crab, Somanniathelphusa zanklon Ng & Dudgeon, 1992 (Crustacea: Brachyura: Gecarcinucidae) endemic to Hong Kong. Journal of Threatened Taxa 8(3): 8564–8574; http://dx.doi.org/10.11609/jott.2070.8.3.8564-8574

 

Copyright: © Stanton & Leven 2016. Creative Commons Attribution 4.0 International License. JoTT allows unrestricted use of this article in any medium, reproduction and distribution by providing adequate credit to the authors and the source of publication.

 

Funding: None.

 

Conflict of Interest: The authors declare no competing interests.

 

Author Details: David J. Stanton and Michael. R. Leven are professional ecologists and directors at AEC Ltd. based in Hong Kong. They conduct surveys for a wide range of faunal groups and input into a range of large scale Environmental Impact Assessments and Strategic Planning Studies in Hong Kong and Asia.

 

Author Contribution: DJS and MRL both participated in the design of the study, acquisition of data, analysis and interpretation of data, and drafting of the manuscript. Both read and approved the final manuscript. Both the authors have contributed equally to this paper.

 

Acknowledgments: We would like to thank Graham Reels, Carlos Chan, Tommy Hui, Philip Tang, and Kevin Leung (Drainage Service Department) for providing information on additional sites of occurrence of S. zanklon. Thanks to Jenny Hui for preparing the map, Grace Yang for additional Chinese translation and to Paul Leader on comments on this. We would also like to thank Professor David Dudgeon for valuable comments and feedback on an earlier draft of this manuscript. Special thanks go to Philip Yip for stimulating discussion on the species and extensive assistance with earlier field surveys, collation of water quality data, provision of Chinese language abstract, and species photographs.

 

 

 

Abstract: Somanniathelphusa zanklon Ng & Dudgeon, 1992 is a tropical freshwater crab currently considered endemic to Hong Kong. The present study shows that S. zanklon has been found in a variety of lotic and lentic habitats, mostly lowland, slow moving streams and marshes. The species is more widely distributed than previously known, and potentially occurs outside Hong Kong. However, its habitat is under threat from development and channelization of watercourses. The life cycle of the crab requires both slow moving streams and marshes for adults and adjacent terrestrial habitats for brooding females, highlighting the need for integrated conservation of lowland streams and their riparian corridors. It is hoped that understanding of the species habitat requirements will encourage others to further explore lowland watercourses in the Pearl River Delta and aid in habitat conservation.

 

 

Keywords: Crustacea, endemic, habitat loss, Hong Kong, marsh, tropical, watercourses.

 

 

 

 

Introduction

 

Somanniathelphusa zanklon Ng & Dudgeon, 1992 is a tropical freshwater crab (see Image 1a,b), which occurs in a variety of lotic and lentic habitats of Hong Kong (Ng & Dudgeon 1992; Esser & Cumberlidge 2008). Somanniathelphusa zanklon was confused with S. sinensis (H. Milne Edwards, 1953) until Ng & Dudgeon (1992) recognised them as distinct species. S. zanklon is currently considered an endemic freshwater crab of Hong Kong and is listed under the Endangered category of the IUCN Red List due to a limited extent of occurrence (less than 5,000km2, known only from the lower course of the Lam Tsuen River and Su Kwun) and probable decline in habitat quality from application of pesticides in rice fields (Esser & Cumberlidge 2008). While the threat assessment made by IUCN must be viewed as hypothetical as there were no rice fields in Hong Kong when this assessment was made (Environmental Resources Management 2009), there is ongoing loss and channelization of lowland wetland habitats and watercourses within the species’ known range in Hong Kong (Hyder-Mott Connell 2007; Hill 2011). Under a local conservation assessment, the species is listed as being of global concern (Fellowes et al. 2002).

Somanniathelphusa zanklon has a similar life cycle to other true freshwater crabs of Hong Kong, i.e., with a suppressed planktonic development and hatchlings that resemble the adult. Brooding female crabs are reclusive and their juveniles are released at the onset of wet season (Ng & Dudgeon 1992; Dudgeon & Corlett 1994). This species is considered to be omnivorous in the wild (Ng & Dudgeon 1992; Dudgeon & Corlett 1994, 2004), and an individual has been observed eating a seed of Bauhinia variegata (a common Hong Kong tree) (Philip Yip pers. comm. 20 October 2013). In laboratory conditions the species selectively prefers thin-shelled gastropod prey (Dudgeon & Cheung 1990), and there is evidence that the abundance of thin-shelled gastropods may be directly influenced by its presence, with populations of pulmonates being reduced where the crab is found (Dudgeon & Corlett 1994). This suggests that thin-shelled snails might be a preferred prey in the wild. As a corollary, the absence of thin-shelled gastropods in streams may affect distribution of S. zanklon, albeit it appears that the former are more widespread than the latter (Dudgeon & Corlett 1994). Therefore, pollution may be a more important limiting factor (either directly, or indirectly due to low dissolved oxygen levels in polluted waters) than prey availability for the distribution of S. zanklon (Dudgeon & Corlett 1994). However, the species is reported to be fairly tolerant to organic pollution (Ng & Dudgeon 1992). Ng & Dudgeon (1992) found the crab in irrigation ditches and flooded furrows and sometimes dwelling among the roots of floating plants such as the exotic water hyacinth, Eichhornia crassipes or the trailing roots and stems of riparian grasses and other plants.

Field observations by the authors indicated that this species actually uses a wider range of habitats than has been described previously, and is distributed quite widely in the northern and western New Territories and Lantau Island of Hong Kong. This study was implemented in order to fill gaps in the knowledge of the distribution of this species.

 

 

 

Materials and Methods

 

Study area

The study area is the Hong Kong Special Administration Region, People’s Republic of China (PRC), which lies on the south China coast between 22009’–22037’N to the east of the Pearl River (Zhujiang) estuary (113050’–114030’E) (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 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 cyclical advance and retreat of the east Asian monsoons (Carey et al. 2001). In winter, the continental high-pressure region over Siberia and Mongolia results in north or north-easterly winds that bring cool, dry air to Hong Kong (Dudgeon & Corlett 2004).

 

 

Literature review

A detailed review of literature was undertaken to examine the known distribution of S. zanklon. 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 Departments website (http://www.epd.gov.hk/eia/) in order to comprehensively review the available ecological findings from these studies. Additional data was obtained from unpublished studies (Dudgeon & Chan 1996; Drainage Services Department 2014), communication with other freshwater surveyors and the authors’ own unpublished results of previous survey findings.

Field study

Among the localities surveyed during the previous EIA studies where S. zanklon was known to occur, five sites were selected to conduct water quality sampling during the present study (Image 2). Five sampling stations were chosen at each selected site. Five water quality parameters (turbidity, salinity, dissolved oxygen, pH and water temperature) were tested for each of the sites and the range of each water quality parameter calculated.

 

 

Results

Distribution of S. zanklon

Previously, specimens of S. zanklon were collected from the main Lam Tsuen River and also from the irrigation ditches of the nearby Su Kwun village of New Territories, Hong Kong (Ng & Dudgeon 1992). During the present review, no recent record of the species was found from these locations.

A review of 104 EIA reports, published between 2002 and 2015, was undertaken and these are listed in Appendix 1. The findings from this review, combined with additional data obtained from unpublished studies have revealed that S. zanklon is known from at least 43 sites in Hong Kong (Appendix 2). These sites are widely located in the northwestern as well as northeastern New Territories and Lantau Island (Fig. 1). The occurrence limit of the species is thus extended to 250km2.

 

 

 

Habitat requirements of S. zanklon

Somanniathelphusa zanklon was mostly found either in lowland watercourses or wetlands/marsh, with all records below 220m. All the sites are characterised by slow flowing, low-gradient streams or wetland/marsh with varying degrees of water quality. All the sites have been subjected to anthropogenic influence, either through modification and channelization of watercourses or by point pollution and water quality impacts. At none of the sites the crab was listed as, or found to be, abundant. Hence, it appears that S. zanklon occurs at a low population density.

 

 

 

 

Water quality

Among the localities surveyed during the previous EIA studies, five sites were selected to conduct water quality sampling during the present study. The range of each water quality parameter is shown in Table 1. Full results of water quality sampling can be seen in Appendix 3.

 

 

 

Potential predators and preys of S. zanklon

From the literature review and authors’ knowledge, potential predatory fishes, from watercourses where S. zanklon occurs, include Channa spp., Carassius auratus, Clarias spp., Oreochromis niloticus, and Hemiculter leucisculus (Ove Arup & Partners 2002, 2007, 2009, 2013a,b; Mott MacDonald 2010; Meinhardt 2012). It is not clear whether S. zanklon forms a part of the diet of these fishes or not; however, in view of the nature of the known diet of these species, this is likely.

Gastropods typically recorded from watercourses and wetlands where S. zanklon occurred include, but are not limited to, Sinotaia quadrata, Melanoides tuberculata, Radix sp., Pomacea lineata, Bellamya sp., Biomphalaria straminea, Gyraulus sp., and Physella acuta.

Discussion

 

Distribution and habitat requirements of S. zanklon

Esser & Cumberlidge (2008) stated that the crab occurred in less than five localities in Hong Kong; however from the present review, it is clear that the species is more widespread than previously thought with 43 identified sites in an area of occupancy of approximately 250km2. Watercourses within this area are often fragmented, piped or channelized—modifications that are likely to inhibit movement of crabs, particularly upstream movements, both within and between catchments. Hence it is likely that within this area of occupancy there are now a number of more or less isolated sub-populations.

The most important factor influencing the distribution of S. zanklon appears to be the physical features of the site, i.e., slow-flowing, low-gradient streams or wetland/marsh together with water quality parameters. Predation by fish does not appear to be a limiting factor though this may require further study. All the sites appeared to have a good assemblage of gastropods; thin-shelled gastropods are preferred prey for this species in laboratory conditions. Given the apparent suitability of other lowland watercourses of the New Territories, it is likely that the species is found more widely including areas, such as the eastern New Territories, where the developmental pressure (and hence the requirement to undertake surveys under the EIA process) is lower.

For the implementation of conservation actions, prior identification of potential sites of species occurrence is required. Misconceptions about its habitat requirements, notably the suggestion that it is associated with rice fields, and observer bias may have contributed to the species being overlooked at other sites. It is likely that the range of S. zanklon extends beyond Hong Kong, given its occurrence in the Hong Kong catchment of the Shenzhen River, and searches of lowland watercourses may result in the discovery of more populations. As such, it should be looked for more widely in the Pearl River Estuary and in other lowland river catchments in South China.

 

 

Influence of water quality parameters on S. zanklon

Testing of water quality was only made from five sites where S. zanklon occurs, as part of this review. These samples were made in order to provide an overview of water quality requirements for the species. From these limited findings, it is difficult to draw any firm conculsions on the water quality requirements of S. zanklon. Whilst it does appear that the species is tolerant of high turbidity and low dissolved oxygen levels, further work is required to investigate this in more detail.

Threats to global population

The use of pesticides in rice fields, which was described as a major threat to this species by Esser & Cumberlidge (2008), remains a hypothetical threat as S. zanklon has yet to be found in rice-growing areas. Instead, it is considered here that the development of lowland watercourses and wetland habitats is a serious threat to the species. Lowland habitats are under severe pressure from development or to reduce flood risk (Cheung et al. 2010; Hong Kong Birdwatching Society 2013). Watercourses are subject to modification, channelization, fragmentation or complete loss by placing them in underground pipes. 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 Asia (Cumberlidge et al. 2009). Undiscovered populations of S. zanklon may be in danger of being totally lost or in a state of population decline, as a result.

 

 

Mitigation and Conservation

IUCN stated that no conservation measures were known to be in place for this species and it was not found in a protected area (Esser & Cumberlidge 2008); this remains largely correct, with only one site afforded protection through conservation designation. 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. However, projects for reducing habitat loss and fragmentation by watercourse restoration, recreation or enhancement and faunal conservation programs are in progress or are being studied (e.g., Cumberlidge et al. 2009; Drainage Service Department 2014) in Hong Kong and elsewhere in the southern China region.

Studies on other endangered freshwater crabs of this region showed that these crabs are able to readily colonize newly created habitats (Cumberlidge et al. 2009). Many of the sites in Hong Kong are isolated, fragmented by a combination of developed areas 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 S. zanklon. Nevertheless, we suggest that the IUCN Red List status of S. zanklon should be revisited in the light of our findings: it is most unlikely that the population size or its rate of decline and the extent of species occurrence or area of occupancy meet the IUCN criteria for the listing of S. zanklon as ‘Endangered’. Conversely, the species is still known only from Hong Kong and probably with a relatively small, fragmented and declining population.

Conclusions

Somanniathelphusa zanklon is widely distributed within Hong Kong, recorded from the northwestern and northeastern New Territories and Lantau Island. This species has so far not been recorded outside of Hong Kong. It appears that the species prefers low-gradient, slow-flowing tributaries and freshwater marshes as opposed to larger rivers or smaller fast-flowing watercourses. Watercourses in which this species occurs are of diverse ecological value and states of naturalness, and are under threat of loss and modification from urban development. Given the life cycle of the species, with suppressed planktonic development, adjacent terrestrial habitats within the riparian corridor or marsh-edges are as important as the aquatic water bodies; as such, channelization of watercourses can be detrimental to species recruitment. Watercourse restoration projects provide an opportunity to conserve habitat of this species’, and its habitat requirements should be taken into account, when restoration measures are planned for potentially suitable watercourses.

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