PhD Defense Seminar cum Oral Examination
Department of Biological Sciences, NUS
“Introduction and Impacts of Freshwater Gastropods In Singapore”
Ng Ting Hui (Graduate Student, NUS DBS)
Mon 21 Nov 2016: 2.00pm @ Seminar Room 1 (S2 Level 4)
Supervisor: Asst Prof Darren Yeo Chong Jinn; Co-supervisor Dr. Tan Heok Hui
Abstract – Freshwater snails (Gastropoda) in Southeast Asia are among the most diverse aquatic groups. Like freshwater molluscs globally, they are threatened by, among other things, aquatic invasive species. Fourteen gastropod species have been introduced in Southeast Asia, including globally-invasive and cryptogenic species (of unknown origin), with Singapore having the highest number (23 species). Freshwater gastropods in Singapore are almost exclusively found in urban habitats, including human commensal native species suspected to have been introduced. Clarifying the status of these and other poorly-known freshwater gastropods can help to prioritise conservation and management efforts of native and invasive species, respectively. My thesis therefore aims to examine the origins and potential impacts of introduced freshwater gastropods in Singapore.
I began with a morphological and molecular study of freshwater mollusc diversity in the ornamental pet trade, the key introduction pathway for aquatic organisms in Singapore. Fifty-nine species (most from the Oriental region) were recorded in the trade, accounting for almost half of known introduced freshwater molluscs in Singapore, including the globally-invasive New World apple snails, Pomacea spp. (Ampullariidae). This led me to investigate the Ampullariidae in Singapore, and I confirmed the establishment of two superficially similar South American species, Pomacea canaliculata and Pomacea maculata. Surprisingly, the purportedly native Pila scutata, was found to be a human commensal, and probably introduced to Singapore instead (further reinforced by demonstrated resistance to anthropogenic habitat disturbance in subsequent stable isotope studies).
To resolve this, I then surveyed Pila scutata in Peninsular Malaysia and Singapore to investigate its distribution and genetic diversity. I found that Pila had been displaced by Pomacea at most historical sites. Only one Pila scutata population in Malaysia and six in Singapore were found, with all individuals sharing a single haplotype—further strong evidence that these populations were introduced by humans.
Pila nevertheless remains a native Southeast Asian genus, so I investigated the potential impacts of South American Pomacea on Pila in terms of feeding ecology. Stable isotope analysis showed no resource overlap between these taxa in habitats where they co-occur, while ex-situ experiments comparing feeding rates indicated that Pomacea feeds more quickly than Pila. My results indicate that Pila may be able to persist at sites where it is syntopic with Pomacea, but further studies are required to understand the interactions between the taxa.
Though my study mostly focussed on apple snails, non-ampullariids, constituting > 80% of Singapore’s introduced gastropods, are also important components of the ecosystem that should not be overlooked. In the course of this study, I clarified the confused taxonomy of two long-established species, Sinotaia guangdungensis (Viviparidae) and Physa acuta (Physidae), and reported two new records of introduced species, Pyrgophorus platyrachis (Cochliopidae) and Anentome helena (Nassariidae). Based on my study, the number of freshwater mollusc species in Singapore is now updated to 4 native, 15 introduced, and 10 cryptogenic.
Overall, this thesis provides, for the first time, important baseline information and further research directions on various aspects of introduced freshwater molluscs in Southeast Asia, which are more widespread than previously thought.