South American Apple Snails, Pomacea Spp
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South American apple snails, Pomacea spp. (Ampullariidae), in Singapore T.H. Ng1, S.K. Tan2 and D.C.J. Yeo1 1Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543. Email: [email protected], [email protected] 2Lee Kong Chian Natural History Museum, National University of Singapore, 2 Conservatory Drive, Singapore 117377 Abstract South American apple snails, Pomacea spp. (Ampullariidae) have been established in Singapore since the late 1980s. Based on molecular analyses and shell morphology, two species of Pomacea (P. canaliculata and P. maculata) have been found. The introduction of Pomacea to Singapore was probably accidental via the aquarium trade. Their current status and distribution in Singapore are summarised herein. In highly urbanised Singapore, the species have not caused obvious environmental damage, in contrast to the agricultural damage they have caused in rice plantations throughout neighbouring Southeast Asian countries. Nevertheless, the introduced Pomacea are now widespread throughout the island city-state, and may be competing with the native Southeast Asian ampullariid, Pila scutata, which has declined since the arrival of Pomacea. Other potential impacts of Pomacea spp. in Singapore include modification of wetland habitat and acting as vectors of human disease. Some measures to remove Pomacea in localised areas have been initiated. However, eradication of these introduced apple snails appears difficult, if not impossible. Additional keywords: introduced, invasive, gastropod, freshwater, Mollusca 221 Introduction There are some 25 species of freshwater gastropods in the island city-state of Singapore, the largest of which are species of the family Ampullariidae (Tan & Woo, 2010; Tan et al., 2012). There are currently at least three species of ampullariids extant in Singapore: the native Southeast Asian species Pila scutata, and two introduced South American species Pomacea canaliculata and Pomacea maculata (Ng et al., 2014; Fig. 1). Pomacea canaliculata has been listed among 100 of the world’s worst invasive species (Lowe et al., 2000) and is typically regarded as a pest in countries where it has been introduced, damaging crops and threatening native ecosystems (Naylor, 1996; Cowie, 2002; Carlsson et al., 2004). Although introduced apple snails are most commonly called Pomacea canaliculata, more than one species is in fact present in Asia (Hayes et al., 2008, 2012). Hayes et al. (2008) had identified Pomacea species from Singapore to be Pomacea maculata (as Pomacea insularum in Hayes et al. 2008), but the specimens used in that study were obtained from a local aquarium shop rather than from locally established populations (R.C. Joshi, pers. comm.). The aquarium shops in Singapore mostly sell imported non-native species, rather than locally collected ones (Ng, pers. obs.). While molecular methods have previously been the most effective means of distinguishing between Pomacea canaliculata and Pomacea maculata (see Hayes et al., 2012), in Southeast Asia, the situation has recently become more complicated, as Fig. 1. Ampullariidae in Singapore. A: Pomacea canaliculata; B: Pomacea introduced populations of the two species maculata;C: Pila scutata. Scale bars appear to be hybridising (Matsukura et al., 10mm.(Photos: S.K. Tan [A and C] and 2013). Snails collected in Singapore were T.H. Ng [B]) 222 BIOLOGY AND MANAGEMENT OF INVASIVE APPLE SNAILS recently re-examined, and individuals of Pomacea maculata and Pomacea canaliculata were distinguished based on both conchological characteristics and molecular methods (Ng et al., 2014; unpublished data). Unlike neighbouring Southeast Asian countries, e.g. Malaysia (see Yahaya et al., 2006), where the species have been documented to cause damage to rice fields,Pomacea species have not caused similar problems in highly-urbanised Singapore, which does not have large-scale agriculture. Nonetheless, the introduced snails may be impacting the confamilial species Pila scutata (Fig. 1). This article reviews the introduction of Pomacea to Singapore, its current status and distribution, its possible environmental impacts, especially on native biodiversity, and existing management approaches to control the species. Fig. 2. Approximate localities where Pomacea specieshave been recorded in Singapore. 1, Kranji Reservoir and tributaries (Kranji marshes, Sungei Tengah); 2, Lower Peirce Reservoir; 3, Western Catchment Reservoirs; 4, Jurong Lake and tributaries (Sungei Jurong; Sungei Lanchar); 5, formerly Marina City Park, now Gardens by the Bay; 6, Upper Thomson Road; 7, Lower Seletar Reservoir and tributaries (Sungei Seletar, Sungei Seletar Simpang Kanan); 8, Bedok Reservoir; 9, Punggol Reservoir and tributaries (Seletar Wet Gap); 10, Bishan-Ang Mo Kio park; 11, Pang Sua pond; 12, Singapore Quarry; 13, Serangoon Reservoir; 14, Tampines Eco Green; 15, Bukit Batok Nature Park; 16, Botanic Gardens. SOUTH AMERICAN APPLE SNAILS, POMACEA SPP. (AMPULLARIIDAE), IN SINGAPORE 223 Status and distribution Pomacea has successfully spread throughout Singapore over the past two decades (Fig. 2). Until recently, it was assumed that all Pomacea in Singapore were Pomacea canaliculata. The presence of both Pomacea canaliculata and Pomacea maculata among established populations in Singapore has since been confirmed by sequencing the mitochondrial CO1 gene of a few individuals (Ng, unpublished). The results also indicated that both species were present in at least one locality, and it is likely that many other Pomacea populations consist of mixed species. Without extensive molecular sequencing of all populations in Singapore, it is currently impossible to determine the species or species composition at localities where the snails are established (see also Ng et al., 2014). Pomacea are commonly found grazing at the littoral zone of reservoirs and in shallow water along the edges of canals. Females leave the water to lay eggs, and clusters of these bright pink eggs can often be found on the surfaces of rocks or concrete banks, and on Fig. 3. Habitats where Pomacea are found in Singapore. A,B, canals; C, banks of reservoirs; D, freshwater marshes.(Photos: H.H. Tan [A], S.K. Tan [B andC],T.H. Ng [D]). 224 BIOLOGY AND MANAGEMENT OF INVASIVE APPLE SNAILS emergent vegetation (Fig. 3). Various animals have been reported to prey on Pomacea species, including numerous birds, e.g. crows and ducks (Yusa, 2006). In Singapore, the snails were recently reported to be preyed on by the Asian openbill, Anastomus oscitans, a species of stork that specialises on molluscan prey and that has apparently spread into the Malay Peninsula from Thailand, thanks in part to the success of introduced Pomacea in the region (Low et al., 2013). Although Pomacea eggs contain anti-predatory chemicals (Dreon et al., 2010), they are also eaten by insects (Yusa, 2001, 2006). A terrestrial snail, Bradybaena similaris, was observed apparently feeding on a cluster of Pomacea eggs on the banks of a reservoir in Singapore, and another snail, Quantula striata, was subsequently recorded in the laboratory to be able to feed on the eggs (Ng & Tan, 2011). In spite of predation pressure, populations are widespread and mainly established in modified, urban freshwater habitats.Pomacea are now found in at least 13 of the 17 reservoirs in Singapore, their associated tributaries and canals, and in various artificial ponds in parks and golf courses around the island (Ng et al., 2014; Ng, unpublished; Fig. 3). These apple snails do not appear to have any particular preference for type of habitat; they thrive at sites regardless of bank type-granite rock walls or more natural vegetated shorelines (e.g. Lower Seletar and Kranji Reservoirs) as well as steep concrete banks (e.g. canals and storm-water drains [Pang Sua pond]), or bottom substrate type-soft muddy (e.g. Jurong Lake), sandy (e.g. Sungei Seletar Simpang Kanan) or concrete (e.g. canals). Records from the Zoological Reference Collection (ZRC) of the Lee Kong Chian Natural History Museum at the National University of Singapore, published literature and unpublished data (including personal communications) were reviewed, as summarised below. Museum specimens examined Sungei Kranji, upper stream: ZRC1989.2295–1989.2296, 2 Sep 1988; Kranji Reservoir, upstream: ZRC1990.843–1990.851, 2 Sep 1988; Sungei Tengah Road: ZRC1990.17328–1990.17331, 24 Feb 1989; Kranji Reservoir: ZRC1990.17012– 1990.17039, ZRC1990.17049–1990.17054, ZRC1992.3111, 5 Oct 1989; Jurong Lake: ZRC1991.19276–1991.19280, Apr 1991; Marina City Park, pond: ZRC1995.676, 28 May 1995; Chinese Gardens: ZRC.MOL.8, 2 May 2002; Lower Seletar Reservoir: ZRC.MOL.9, 24 May 2002; Upper Thomson Road: ZRC.MOL.12, 29 Jun 2002; Kranji SOUTH AMERICAN APPLE SNAILS, POMACEA SPP. (AMPULLARIIDAE), IN SINGAPORE 225 Reservoir: ZRC.MOL.7, 7 Jul 2002; Jurong Lake: ZRC.MOL.11, 11 Jul 2002; Lower Peirce Reservoir: ZRC.MOL.10, 8 Aug 2002. Literature records Ng, 1991: 130 (remarks on the species or local distribution; sold in aquarium trade); Ng et al., 1993 (prefers open areas); Chan, 1996 (shares same habitat as Pila scutata); Maassen, 2001 (as Pomacea lineata); Goh et al., 2002 (able to survive harsher environments than Pila scutata); Clements et al., 2006 (in reservoirs and rural streams); Hayes et al., 2008 (as Pomacea insularum, Singapore specimens obtained from aquarium trade [R.C. Joshi, pers. comm.]); Tan et al., 2012 (found in most reservoirs and tributaries); Ng et al., 2014 (a review of the family Ampullariidae