Clithon oualaniense ( Lesson, 1831 )

Mustapha, Nurshahida, Baharuddin, Nursalwa, Tan, Siong Kiat & Marshall, David J., 2021, The neritid snails of Brunei Darussalam: their geographical, ecological and conservation significance, Ecologica Montenegrina 42, pp. 45-61 : 55-57

Clithon oualaniense ( Lesson, 1831)

Figs. 3K–L and 4M

Neritina oualaniensis Lesson, 1831: 379 .

Clithon oualaniense – Tan & Clements, 2008: 491, figs. 3-37, 3-38; Eichhorst, 2016a: 208, pls. 34–38; Zvonareva & Kantor, 2016: 405 , figs. 3M, 3N.

Material. UBDM.7.00120. Sample collected from PB.

Abundance. Very abundant.

Habitat. Mangroves. On muddy surfaces in mangrove stands.

Local distribution. PB.

Ecology and conservation. The species shows preference for seagrass turfs but occurs in lower density on muddy surfaces (see also Fong et al. 2018). Following the demise of locally dense seagrass turf, a vast population of relatively large C. oualaniense individuals is now lost from PB. This fast-growing species, suggested by their small size and thin shells, should rapidly recolonise when favourable habitat becomes available again. Although locally abundant, it is highly sensitive to habitat change, and can benefit from monitoring.

Biogeographical distribution. TR, AN, VN, TH and SG of Sunda Shelf. Generally distributed in Indo-West Pacific ( Eichhorst 2016a).

Biogeographical comparisons

Our study adds eight new records of neritid snails to the Palawan/ North Borneo (PNB) ecoregion ( Spalding et al. 2007) as follows, N. patula , N. insculpta , N. litterata , N. undata , N. histrio , Clithon oualaniense, Nerito. dubia and Nerip. spirale ( Table 1). Six of these species ( N. exuvia , N. insculpta , N. litterata , N. patula, Nerito. dubia and Nerip. spirale ) have been reported at 4 or fewer localities in the Western Coral Triangle and Sunda Shelf ( Table 1). Despite uncertainty among the records, these species at least appear to be regionally rare. The absence in previous reporting of N. patula , N. insculpta and Nerito. dubia could relate to their being overlooked in local diversity studies. Species identification also presents a problem, especially when based only on shell characteristics ( Carpenter & Niem 1998; Tan & Clements 2008; Zvonareva & Kantor 2016). N. litterata , for example was thought to be a form or polymorphic variation of Nerita polita (e.g., Tan & Clements 2008), so this might easily have been misidentified elsewhere.

Species numbers recorded in Brunei share similarity with other studies from the Western Coral Triangle (WCT): Sarangani (21 species) and Banyuwangi (16 species; Table 1). However, biogeographical comparisons between regions assume equality of sampling effort, which is rarely the case. Such discrepancies may arise from the range of local neritid habitats not being sampled in studies ( Burghardt et al. 2006; Frey 2010 a, 2010b; Dolorosa & Dangan-Galon 2014; Hombre et al. 2016; Ng et al. 2017; Baderan et al. 2019). More species (not listed here) are expected in regions centrally within the Coral Triangle, noting that Brunei is at the edge of this biogeographical region. However, an unusual complex interaction of ocean currents and gyres lies off the Brunei coast (SCS Southern Cyclonic Gyre, SCS Southern Anticyclonic Gyre, SE Vietnam Offshore Current, Gulf of Tonkin Surface Current), which should facilitate pelagic dispersal of species from widely across the South China Sea ( Liu et al. 2016).

Ecology and Conservation

Consistent with a previous observation, the natural rocky shore supports a much greater neritid species diversity along the Brunei coastline (n = 8 species) than the more extensive seawalls (n = 3 species; see Marshall et al. 2017). The observation of single specimens of three species ( Nerita patula , N. exuvia and N. histrio ) on the spatially restricted natural rocky shore (ES) suggests ‘seeding’ from neighbouring north- west Bornean (Malaysian states, Labuan, Sabah and Sarawak), though notably, N. patula has not been reported from Palawan/ North Borneo. Unlike Brunei, these regions comprise of extensive rocky shores which theoretically should support the species found in the present study, in addition to others. The inability of these scarce species to form significant populations relates to combinations of species interactions (competition) and life history features (growth, reproduction and dispersal capabilities), but ultimately this scarcity signals species likely to be at risk within the region. Two of the five single specimen records ( N. patula and Nerip. spirale ) are regionally rare ( Table 1).

Nerita chamaeleon is by far the most abundant and widespread rocky-shore species on the Brunei coast and often prevails on artificial shores. It is a habitat generalist and has been found to persist under stress caused by sediment loading of the rocks. This species appears to outcompete N. undata , which takes a stronghold on shores devoid of N. chamaeleon and apparently tolerates conditions higher on the shore. N. albicilla is a low-shore generalist that forms significant populations below N. chamaeleon . Nerita insculpta occurs in the highest intertidal zone and is specialized to withstand high temperature and prolonged air exposure. Nerita litterata is habitat specialist at the interface of the rock and sandy beach, and has shown to be vulnerable to habitat change. Within the study period, a flourishing population became locally extinct following landscaping and deforestation of the fringe habitat at ES, causing excavation, erosion and change in the physical structure of the sandy interface.

Less clarity surrounds the local vulnerability of the mangrove/estuarine neritid snails, given the extent of the mangrove stands in Brunei Bay and our limited sampling of this. However, the Brunei Bay and estuarine system have been the subject of significant habitat modification over the last decade, derived from the construction of Brunei’s landmark bridges (including SOAS Bridge). Notably, changes in populations have been observed for N. balteata and C. oualaniense . A very high C. oualaniense abundance at PB, declined in conjunction with declining seagrass cover at this site. Seagrass stabilises sediment, which benefits the snails aside from providing a preferred habitat/food source ( Lee et al. 2001; Fong et al. 2018). The decline of N. balteata at BM matched collecting by researchers and harvesting of this larger species for food. Recruitment of these snails to these mangroves was found to be negligible over a five-year period, likely relating to their expected long lifespan and low reproductive potential (see Somchai 1995; Matsuura et al. 2000; Köhler et al. 2012; Hamli et al. 2013; Kano & Fukumori 2019). Neripteron violaceum and Nerip. cornucopia are widespread at the higher estuarine reaches, and local populations increase at more homogeneous substrates (rocky surfaces), with Nerip. violaceum always dominating numerically. The low abundances of the other higher estuarine, Vittina coromandeliana, Nerip. spirale and Nerito. dubia , could relate to undersampling or to the limit to their sometimes more freshwater distribution. Neritodryas dubia is clearly a Nipa palm specialist, and locally is always found associated with this tree.

Overall, most of our neritid snails can be categorised as having little conservation concern, implying that their populations should not experience undue risk of local extinction in the near future ( N. albicilla , N. balteata , N. chamaeleon , N. insculpta , N. planospira , N. undata , C. oualaniense, Nerip. violaceum and Nerip. cornucopia ). Those of greater conservation concern are N. patula , N. histrio , N. exuvia and N. litterata . Local extinction risk is inconclusive from this study for V. coromandeliana, Nerito. dubia and Nerip. spirale . It is likely that the small-sized Nerip. spirale is often overlooked in local habitats.

Summary

This study updates the neritid snails of Brunei, North Borneo and Palawan. We suggest that these snails are likely sensitive to habitat loss and human exploitation, causing local population declines and species losses. However, they are likely to be useful bioindicators of environmental change through local land use and/or anthropogenic climate change. Short-term studies can easily overlook the low abundance (scarce) species.