A checklist of land snails from the west coast islands of Sabah, Borneo (Mollusca, Gastropoda)

Abstract Sabah, situated in one of the world’s biodiversity hotspots, has the largest number of islands in Malaysia with more than 500 of various sizes and degrees of isolation. However, information on the islands’ biodiversity is limited. This study provides an up-to-date checklist of land snail species found on 24 west coast islands in Sabah. A total of 67 species (nearly 20% of the total number of land snail species in the state) representing 37 genera and 19 families is enumerated based on systematic field surveys of 133 sampling plots, BORNEENSIS database records and species checklists published between 2000 and 2016. The number of species on the islands ranges from four to 29. Labuan Island has the highest number of species (29), followed by Tiga Island (25), Mantanani Besar Island (24) and Gaya Island (23). However, the populations of some land snail species may have declined as several previously recorded species on the islands were not found in a recent systematic field sampling. This checklist is provided as a baseline inventory for future island land snail studies and to better inform biodiversity conservation plans of marine parks and other islands on the Sabah west coast.


Introduction
The island of Borneo is recognised as one of the mega biodiversity hotspots in the world (Myers et al. 2000). Sabah, a state in Malaysia and formerly known as British North Borneo, is situated on the northwestern part of the island. Documentation of the biodiversity in Sabah began in the 19 th century with explorations by European naturalists in this region. One of the first comprehensive checklists of land snails was published by Issel (1874) who documented an estimated 100 species from Borneo. In the subsequent decades, explorations and collections of land snails in Sabah were conducted mainly by British naturalists such as A. Everett and H. Cuming. Their explorations resulted in several published reports and descriptions of a number of new species (Pfeiffer 1852(Pfeiffer , 1863Godwin-Austen 1891;Smith 1894). In the 20 th century, information on Sabah land snails was mainly contributed by Jaap J. Vermeulen through taxonomic revisions of several land snail genera (Vermeulen 1991(Vermeulen , 1993(Vermeulen , 1994(Vermeulen , 1996(Vermeulen , 1999. In the 21 st century, knowledge on Sabah land snails expanded greatly with the publication of detailed taxonomic revisions of certain land snail groups (Liew et al. 2009;Vermeulen et al. 2015), inventories of Borneo land snails (Schilthuizen and Vermeulen 2003;Schilthuizen et al. 2011;Uchida 2013) and ecological studies (Schilthuizen and Rutjes 2001;Schilthuizen et al. 2002Schilthuizen et al. , 2003aSchilthuizen et al. , 2003bSchilthuizen et al. , 2005Liew et al. 2008Liew et al. , 2010. To date, the number of land snail species documented in Sabah is approximately 350.
Many of these studies were conducted in ecosystems on the Sabah mainland, covering mountains (Liew et al. 2010), tropical lowland rainforests (Schilthuizen and Rutjes 2001;Schilthuizen et al. 2002;Liew et al. 2008;Uchida et al. 2013) and limestone outcrops (Schilthuizen et al. 2003a(Schilthuizen et al. , 2003b(Schilthuizen et al. , 2005Schilthuizen and Vermeulen 2000). To date, there have been very few systematic surveys on the land snail diversity in the island ecosystem. In the 19 th century, several new species from Labuan Island, Tiga Island and Usukan Island on the west coast of Sabah were described (Pfeiffer 1863; Adams 1865; Issel 1874; Godwin-Austen 1889; Godwin-Austen 1891; Fulton 1896). It was only a century later that land snail biodiversity studies were conducted on islands such as the Mantanani Island group on the west coast, Balambangan Island and Banggi Island on the north coast, and Bohey Dulang Island, Tetagan Island, Bod Gaya Island, Sebangkat Island, Maiga Island, Mantabuan Island, Pulau Sibuan and Selakan Island on the east coast (Liew et al. 2008;Schilthuizen et al. 2011Schilthuizen et al. , 2013. Despite these efforts, knowledge on land snail biodiversity on the islands remains inadequate as previous studies covered less than 3% of the estimated 500 islands in Sabah (JUPEM 2005). Of these number, 45 islands located on the west coast of Sabah should be prioritised for a land snail survey since (1) they are close to major cities in Sabah and therefore subjected to more intensive tourism activities and economic development, (2) 11 of these 45 have been gazetted as marine parks, and basic biodiversity knowledge is thus vital for park management, and (3) there is already a land snail species at risk of extinction -Plectostoma decrespignyi-from Labuan Island, and this indicates that island land snails are very vulnerable to human activities (Schilthuizen and Vermeulen 2004b).
In view of these, a systematic sampling for land snails on 24 west coast islands was conducted and the data compiled together with land snail inventory data from literature and previously collected specimens deposited in the BORNEENSIS collection at Universiti Malaysia Sabah. Although only about half of the total number of islands on this part of Sabah were involved, the selection included the entire geographical extent of islands off the west coast of Sabah (see Figure 1). The aim of this paper is to provide a comprehensive and updated checklist of land snails by consolidating data from literature and database, including photographs of all species, in order to present a baseline species inventory for future island land snail studies. Interpretation of species diversity pattern in term of island biogeography will be discussed in upcoming analytical papers.

Site of study
The Malay language term for 'island'-pulau -will henceforth be used as many of the formally gazetted island names are in the local language. Land snails from 24 islands located off the Sabah west coast were investigated (see Figure 1). The sizes of these islands range from 0.005 km² (Pulau Peduk) to 87 km² (Pulau Labuan) (see Table 1). The distance between these islands and the mainland ranges from 0.25 km to 60 km. Eleven of the selected islands are in the three gazetted parks, namely Tunku Abdul Rahman Park, Pulau Tiga Park and Labuan Marine Park.

Land snail sampling and sorting
Field work was carried out between December 2015 and November 2016 with standard sampling conducted within a 20m × 20m plot (Schilthuizen et al. 2013). The number of plots varied from two to 14 depending on island size. Direct search for living land snails in their habitats such as vegetation, decaying logs and leaf litter on forest floor at each plot was carried out by two person-hours. Land snails encountered opportunistically along these trails were also collected. In addition, 5 liters of leaf litter and top soil were collected at each plot. The soil samples were then oven-dried for three days and subjected to a series of sieves. The empty shells were subsequently sorted out from the soil samples under a dissecting microscope.

Land snail identification and accession
All the specimens were preserved and identified to species level based on shell characteristics by referring to the manuscript of Field Guide to Land Snails of Sabah (Ver-  Vermeulen et al. (2015), literature with original species descriptions, and the BORNEENSIS collection at the Institute for Tropical Biology and Conservation at Universiti Malaysia Sabah. After identification, specimens were then catalogued in the database, labelled and stored in the BORNEENSIS collection.

Species checklist compilation
This checklist enumerates the land snail fauna sampled and recorded from the west coast islands between 2000 and 2016 based on literature, BORNEENSIS collections and recent systematic samplings (Table 1). Taxonomy classification of the species in this checklist followed the system used in the recent systematic paper on Sabah land snails (Vermeulen et al. 2015). The checklist comprised information about the type locality of the species, examined materials with accession number, species distribution in Sabah and general remarks for the species. For morphospecies that could not be assigned to an existing taxonomic name, a provisional species name was given (for example, sp. 1). Photographic images comprising four views of the representative specimen (i.e. apertural, lateral, dorsal and ventral views) for each species were presented.

Type localities
The type locality of each species was written in the format of "country: state: districtlocation" if their exact location were known. For those without the exact location specified, a general description of the location (e.g. "Borneo") was mentioned. "Not stated" was given to species with no locality indicated in their original description. For provisional species name, type locality was indicated as "not applicable".

Examined materials
For each species, the accession number of referenced specimens from the west coast islands was listed out. The abbreviation "BOR/MOL" indicated specimens from the BORNEENSIS collection while "V" referred to the private collection of Jaap J. Vermeulen of Leiden in the Netherlands. "Not seen" was mentioned for materials based solely on literature that could not be examined.

Distribution in Sabah
The distribution of each species on the Sabah islands and mainland was compiled from the BORNEENSIS collection which currently houses more than 12,000 records. The distribution of each species was indicated in the format of "Island: [West]; [North]; [East]. Mainland:" Islands in Sabah were grouped into three categories: [West] for islands located within the West Coast Division and Interior Division; [North] for those located within Kudat Division, and [East] for ones situated within the Sandakan Division and Tawau Division. The distribution of the species on the mainland was according to five divisions: West Coast Division, Kudat Division, Sandakan Division, Tawau Division and Interior Division ( Figure 1A).

Remarks
Additional information such as species status as either endemic or introduced to Sabah or Borneo was stated for species well supported by literature. Conversely, those with no information from the literature were not mentioned. Other relevant notes for the species were also provided.
Among the 24 west coast islands, Pulau Labuan had the highest species richness (see supplementary file 1) and the highest number of species unique to the island. This could probably be due to its larger size compared to other islands. The second largest island on the west coast, Pulau Gaya, ranked fourth in species richness. Pulau Tiga and Pulau Mantanani Besar also recorded remarkable species richness, with 25 species from Pulau Tiga and 24 from Pulau Mantanani Besar. These two islands differed from other west coast islands in having limestone outcrops (in Pulau Mantanani Besar) and mud volcanoes (in Pulau Tiga), which potentially housed more diverse habitats leading to the existence of more land snail species.
The decline of land snail species (especially endemics) on the west coast islands could be attributed to two reasons. First, the field sampling period coincided partly with El Nino which had caused six months of drought in Sabah between January 2016 and June 2016 and might have negatively influenced sampling effectiveness. Second, forested habitats on many islands have vanished due to rapid urban development and tourism activities, particularly in Pulau Labuan (Mohamad and Samion 2016). Many of the species from Pulau Labuan were described or recorded from limestone outcrops in the south-eastern part of the island, all of which had since been quarried away for the construction of Labuan Airport in the 1960s. Hence, there was a high possibility that limestone-restricted species from Pulau Labuan could have been extinct in local.
However, six introduced land snail species were encountered across the 24 west coast islands -Subulina octona, Achatina fulica, Huttonella bicolor, Macrochlamys indica, Quantula striata and Bradybaena similaris. A large quantity of Macrochlamys indica and Quantula striata were found only on Pulau Labuan and Pulau Papan with Quantula striata a new record for Sabah land snails. The impact of the invasion by these introduced species on island land snail communities remains unknown. Therefore, future research should focus on the effects of introduced species on native species.
Although only 24 out of the 500 islands in Sabah were included, this study has managed to document nearly 20% of the total land snail species in Sabah. West coast islands are significant as habitats for land snails, particularly for several endemic species. In view of this, islands within the Labuan Marine Park, Pulau Tiga Park, Tunku Abdul Rahman Park, Pulau Labuan and Mantanani Island group should be prioritised for land snail conservation. Particular attention should also be accorded to island endemic species such as Plectostoma jucundum (endemic to Pulau Mantanani Besar) and Ganesella tigaensis (endemic to Pulau Tiga). Biodiversity conservation plans are therefore advocated not only for marine parks, but also for other west coast islands in Sabah.   Remarks. Endemic to Borneo. The examined sample was collected in 2000 and not found in the current surveys. Figure 3B Type locality. "N. Borneo" (Fulton, 1905) Examined materials.   Remarks. Only found in Pulau Mantanani Besar and Pulau Balambangan, which are close to the Palawan archipelago in the Philippines. Figure 3E Type locality. Japonia kinabaluensis "Malaysia: Sabah: Ranau-Mt. Kinabalu" (Smith, 1895); Japonia trilirata "Malaysia: Sabah: Labuan" (Pfeiffer, 1852) Examined materials. Remarks. Widespread in Sabah. The difference between Japonia kinabaluensis (E.A. Smith, 1895) and Japonia trilirata (Pfeiffer, 1852) is ambiguous as the shell shape and size of the two species are highly variable and intermediate shell forms have been found. Therefore, we considered J. kinabaluensis and J. trilirata as Japonia trilirata/ kinabaluensis species complex.      Remarks. Endemic to Borneo. Figure 4C Type locality. "Brunei: Bandar Seri Begawan" (Habe, 1965) Examined materials. Remarks. There is no recent record of this species from the mainland. The only record on the mainland is in the original description of the species by Smith (1895).  Remarks. Endemic and widespread in Sabah.

Arinia borneensis E.A. Smith, 1894
Arinia sp. Figure 5C Type locality. Not applicable. Examined materials. Pulau Rusukan Besar: BOR/MOL 12252. Remarks. Unknown status. Does not match with any other Arinia species in Borneo treated in the revision by Vermeulen (1996).   Remarks. Unknown status. Relatively scarce information about the distribution in Sabah as its habitat at coastal areas has rarely been surveyed. Presumably widespread along coastal areas since this species was known from across the Indo-pacific region (Clench and Turner 1948). Figure 6B Type locality. "Malaysia: Sabah: Labuan" (Küster, 1855) Examined materials.   Figure 7 Type locality. "Malaysia: Sabah: Kota Belud-Usukan island" (Godwin-Austen 1889) Examined materials. Remarks. Endemic and widespread in Sabah.   Remarks. Only found in Pulau Mantanani Besar and Pulau Balambangan, which are close to the Palawan archipelago in the Philippines. This species is reported in Schilthuizen et al. (2013) but the specimen could not be found in the BORNEENSIS collection.  Figure 9A Type locality. "Philippines: Luzon-near Manila" (Dohrn 1863  Remarks. Rather widespread in Sabah.