Aquatic Invasions (2018) Volume 13, Issue 3: 365–378 DOI: https://doi.org/10.3391/ai.2018.13.3.05 Open Access © 2018 The Author(s). Journal compilation © 2018 REABIC Research Article Do Singapore’s seawalls host non-native marine molluscs? Wen Ting Tan1, Lynette H.L. Loke1, Darren C.J. Yeo2, Siong Kiat Tan3 and Peter A. Todd1,* 1Experimental Marine Ecology Laboratory, Department of Biological Sciences, National University of Singapore, 16 Science Drive 4, Block S3, #02-05, Singapore 117543 2Freshwater & Invasion Biology Laboratory, Department of Biological Sciences, National University of Singapore, 16 Science Drive 4, Block S3, #02-05, Singapore 117543 3Lee Kong Chian Natural History Museum, Faculty of Science, National University of Singapore, 2 Conservatory Drive, Singapore 117377 *Corresponding author E-mail: [email protected] Received: 9 March 2018 / Accepted: 8 August 2018 / Published online: 17 September 2018 Handling editor: Cynthia McKenzie Abstract Marine urbanization and the construction of artificial coastal structures such as seawalls have been implicated in the spread of non-native marine species for a variety of reasons, the most common being that seawalls provide unoccupied niches for alien colonisation. If urbanisation is accompanied by a concomitant increase in shipping then this may also be a factor, i.e. increased propagule pressure of non-native species due to translocation beyond their native range via the hulls of ships and/or in ballast water. Singapore is potentially highly vulnerable to invasion by non-native marine species as its coastline comprises over 60% seawall and it is one of the world’s busiest ports. The aim of this study is to investigate the native, non-native, and cryptogenic molluscs found on Singapore’s seawalls. Seven seawall sites around Singapore were surveyed and all specimens found were either Indo-Pacific species or of unknown origin. To determine whether there were potential non-natives from within the Indo- Pacific, a set of attributes concerning the history, biogeography, detectability, human affinity, invasion pathway, biology, ecology, life-history, pre-history, evolution and genetics of mollusc species was collected from available literature. Only one “possibly introduced” species, Siphonaria guamensis Quoy and Gaimard, 1833 (Gastropoda), was identified. The remaining species consisted of 41 native to Singapore and 23 cryptogenic species. The results from this study add to the increasing pool of literature showing that, contrary to widespread assumption, there is a very low occurrence of non-native marine species in Singapore. Key words: intertidal, ecological engineering, urban ecology, invasive species, reconciliation Introduction (Amat et al. 2008). In the North Adriatic Sea, Airoldi et al. (2015) showed that some artificial marine Man-made coastal structures have been associated infrastructure, including seawalls and breakwaters, with the proliferation and establishment of marine significantly favoured non-native over native marine non-native species (e.g., Amat et al. 2008; Mineur et species as the assemblages were dominated by non- al. 2012; Airoldi et al. 2015). They represent novel native and cryptogenic species. and “invasible” habitats that tend to display traits As the number of people living within 100 km of such as high levels of disturbance and lack of natural the sea increases (Martínez et al. 2007), huge stretches predators, which can facilitate colonization by oppor- of coastal landscapes are being developed to facilitate tunistic species (Lodge 1993; Mineur et al. 2012). a range of economic and social activities (Yeung For instance, the pantropical seaweed, Caulerpa 2001; Masselink and Gehrels 2014). This process has webbiana, rapidly colonized Horta harbour in Faial been exacerbated by the combined threats of sea-level in the North-Eastern Atlantic Azores Islands, occurring rise and more frequent and intense storms associated at highest densities along the seawall of the harbour with global warming (Bulleri and Chapman 2015) 365 W.T. Tan et al. that necessitate the construction of defences (e.g., One additional non-native mussel, Mytella strigata, seawalls and breakwaters) to provide shoreline was detected in 2016 and subsequently ascertained protection (Masselink and Gehrels 2014). Seawalls, to be established in Singapore (Lim et al. 2018). in particular, are becoming one of the most prevalent Hence, despite being potentially exposed to high propa- man-made features along urban coastlines (Chapman gule pressure due to shipping activity, the available and Bulleri 2003). For instance, seawalls comprise literature indicates that Singapore hosts a surprisingly ~ 95% of the coastline in Victoria Harbour in Hong low number of non-native marine species, seemingly Kong (Lam et al. 2009), while in Sydney Harbour, they contradicting Seebens et al.’s (2013) model predictions. represent approximately half of the entire foreshore Singapore, nevertheless, remains a major shipping (Chapman and Bulleri 2003). In Singapore, 63% of hub that is surrounded by seawalls and other artificial the island-nation’s coastline is seawall and this is marine structures, thus a targeted investigation into predicted to reach up to 74% in the next 50 years the presence/absence of non-native species is warran- (Lai et al. 2015). ted. Based on tonnage, Singapore is currently the Non-native species can exert wide-ranging impacts second largest port in the world, with over 130,000 in their newly-established habitats; however, it is ship calls every year (Singapore Department of only when they generate ecological and economic Statistics 2016). Almost two thirds of Singapore’s costs, that they are considered invasive (International coastline is seawall: generally grouted or un-grouted Union for the Conservation of Nature 2000). For granite “rip-rap” armour, i.e. boulders stacked along example, serpulid polychaetes such as Hydroides the coastline at a ~30º slope (Lai et al. 2015). While elegans (Haswell, 1883) have become a major there have been some surveys of the biodiversity biofoulant on artificial surfaces throughout the Medi- found on local seawalls (Lee and Sin 2009; Lee et al. terranean and their control incurs significant costs 2009a; Lee et al. 2009b), the native/non-native status (Kocak et al. 1999; Streftaris and Zenetos 2006). In of these inhabitants has never been documented. If Hawaii, the invasive algae Hypnea musciformis the origin of the species remains unknown, it is (Wulfen) J.V.Lamouroux, 1813, forms massive algae impossible to assess the extent of any biological blooms with detrimental impacts to coral reefs as invasion and hence devise appropriate management well as to tourism and property value (Cesar et al. strategies. The present study aimed to document as 2002). It is difficult to predict whether the impact of many non-native species as possible. We focused on a non-native species will be neutral, positive or molluscs as they are dominant fauna on Singapore’s negative, and therefore it is prudent to treat all of them seawalls (Lee et al. 2009a; Loke et al. 2016; Loke and with caution. Todd 2016; Loke et al. 2017) and local taxonomic Globalization has facilitated the transport of expertise is available. Specific objectives were to species via a multitude of introduction pathways add to the number of seawalls surveyed and to produce (Occhipinti-Ambrogi and Savini 2003; Mineur et al. a comprehensive list of native, non-native, and 2012). Shipping, aquaculture, canal construction and cryptogenic molluscs on seawalls in Singapore. The the aquarium trade are some of the most common findings from this study will provide a much-needed anthropogenic routes of introduction for non-native baseline for monitoring of potentially invasive marine species (Molnar et al. 2008). The intensifi- molluscs. The results will also have implications for cation of maritime trade, in particular, has played a management strategies and habitat reconciliation principal role in accelerating their proliferation, efforts for seawalls in Singapore. accounting for more than half of global marine bio- invasions (Molnar et al. 2008; Hulme 2009). In light Material and methods of the importance of shipping activities in the spread of marine species, Seebens et al. (2013) incorporated Located just over one degree north of the equator, data on global cargo ship networks to model the Singapore is a tropical island city-state separated risks of marine invasions via release of ballast water. from Peninsular Malaysia by the Straits of Johor in The study identified Singapore as the number one the north, and from Indonesia by the Straits of bio-invasion hot spot—facing the highest risk of Singapore in the south. The total land area of introductions via shipping ballast (Seebens et al. Singapore (approximately 719.1 km2) comprises the 2013). However, Jaafar et al. (2012) reported only main island and over 60 smaller natural and man- 17 non-native marine species established in Singapore, made islands (Singapore Department of Statistics with just six of these thought to have been intro- 2016). Almost 20% of this area has been reclaimed duced via shipping-mediated pathways. Out of the from the sea during the last 50 years and erosion of 17 non-native species reported, two were molluscs, this new land is controlled using artificial coastal namely Mytilopsis sallei and Brachidontes striatulus. defences (Singapore Land Authority 2017). 366 Do Singapore’s seawalls host non-native marine molluscs? Figure 1. Survey sites in present study. Abbreviations used: KU, Kusu Island; LA, Lazarus Island; PH, Pulau Hantu; SE, Sentosa; SR, Seringat Island; TU, Tuas; WC, West Coast. Map of seawalls in Singapore adapted from Lai et al. (2015). Sampling sites, survey protocol and species All surveys were conducted during evening spring identification low tides from November 2014 to January 2015 when the entire seawall is exposed, i.e. at tidal heights Surveys were conducted at seven seawall sites, two of 0.4–0.7 m. At each site, a single 50 m length of on the western coast of mainland Singapore and five wall was demarcated parallel to the shoreline.