Do Singapore ’ s seawalls host non-native marine molluscs ?

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 IndoPacific, 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.


Introduction
Man-made coastal structures have been associated with the proliferation and establishment of marine non-native species (e.g., Amat et al. 2008;Mineur et al. 2012;Airoldi et al. 2015).They represent novel and "invasible" habitats that tend to display traits such as high levels of disturbance and lack of natural predators, which can facilitate colonization by opportunistic species (Lodge 1993;Mineur et al. 2012).For instance, the pantropical seaweed, Caulerpa webbiana, rapidly colonized Horta harbour in Faial in the North-Eastern Atlantic Azores Islands, occurring at highest densities along the seawall of the harbour (Amat et al. 2008).In the North Adriatic Sea, Airoldi et al. (2015) showed that some artificial marine infrastructure, including seawalls and breakwaters, significantly favoured non-native over native marine species as the assemblages were dominated by nonnative and cryptogenic species.
As the number of people living within 100 km of the sea increases (Martínez et al. 2007), huge stretches of coastal landscapes are being developed to facilitate a range of economic and social activities (Yeung 2001;Masselink and Gehrels 2014).This process has been exacerbated by the combined threats of sea-level rise and more frequent and intense storms associated with global warming (Bulleri and Chapman 2015) that necessitate the construction of defences (e.g., seawalls and breakwaters) to provide shoreline protection (Masselink and Gehrels 2014).Seawalls, in particular, are becoming one of the most prevalent man-made features along urban coastlines (Chapman and Bulleri 2003).For instance, seawalls comprise ~ 95% of the coastline in Victoria Harbour in Hong Kong (Lam et al. 2009), while in Sydney Harbour, they represent approximately half of the entire foreshore (Chapman and Bulleri 2003).In Singapore, 63% of the island-nation's coastline is seawall and this is predicted to reach up to 74% in the next 50 years (Lai et al. 2015).
Non-native species can exert wide-ranging impacts in their newly-established habitats; however, it is only when they generate ecological and economic costs, that they are considered invasive (International Union for the Conservation of Nature 2000).For example, serpulid polychaetes such as Hydroides elegans (Haswell, 1883) have become a major biofoulant on artificial surfaces throughout the Mediterranean and their control incurs significant costs (Kocak et al. 1999;Streftaris and Zenetos 2006).In Hawaii, the invasive algae Hypnea musciformis (Wulfen) J.V.Lamouroux, 1813, forms massive algae blooms with detrimental impacts to coral reefs as well as to tourism and property value (Cesar et al. 2002).It is difficult to predict whether the impact of a non-native species will be neutral, positive or negative, and therefore it is prudent to treat all of them with caution.
Globalization has facilitated the transport of species via a multitude of introduction pathways (Occhipinti-Ambrogi and Savini 2003;Mineur et al. 2012).Shipping, aquaculture, canal construction and the aquarium trade are some of the most common anthropogenic routes of introduction for non-native marine species (Molnar et al. 2008).The intensification of maritime trade, in particular, has played a principal role in accelerating their proliferation, accounting for more than half of global marine bioinvasions (Molnar et al. 2008;Hulme 2009).In light of the importance of shipping activities in the spread of marine species, Seebens et al. (2013) incorporated data on global cargo ship networks to model the risks of marine invasions via release of ballast water.The study identified Singapore as the number one bio-invasion hot spot-facing the highest risk of introductions via shipping ballast (Seebens et al. 2013).However, Jaafar et al. (2012) reported only 17 non-native marine species established in Singapore, with just six of these thought to have been introduced via shipping-mediated pathways.Out of the 17 non-native species reported, two were molluscs, namely Mytilopsis sallei and Brachidontes striatulus.
One additional non-native mussel, Mytella strigata, was detected in 2016 and subsequently ascertained to be established in Singapore (Lim et al. 2018).Hence, despite being potentially exposed to high propagule pressure due to shipping activity, the available literature indicates that Singapore hosts a surprisingly low number of non-native marine species, seemingly contradicting Seebens et al.'s (2013) model predictions.
Singapore, nevertheless, remains a major shipping hub that is surrounded by seawalls and other artificial marine structures, thus a targeted investigation into the presence/absence of non-native species is warranted.Based on tonnage, Singapore is currently the second largest port in the world, with over 130,000 ship calls every year (Singapore Department of Statistics 2016).Almost two thirds of Singapore's coastline is seawall: generally grouted or un-grouted granite "rip-rap" armour, i.e. boulders stacked along the coastline at a ~30º slope (Lai et al. 2015).While there have been some surveys of the biodiversity found on local seawalls (Lee and Sin 2009;Lee et al. 2009a;Lee et al. 2009b), the native/non-native status of these inhabitants has never been documented.If the origin of the species remains unknown, it is impossible to assess the extent of any biological invasion and hence devise appropriate management strategies.The present study aimed to document as many non-native species as possible.We focused on molluscs as they are dominant fauna on Singapore's seawalls (Lee et al. 2009a;Loke et al. 2016;Loke and Todd 2016;Loke et al. 2017) and local taxonomic expertise is available.Specific objectives were to add to the number of seawalls surveyed and to produce a comprehensive list of native, non-native, and cryptogenic molluscs on seawalls in Singapore.The findings from this study will provide a much-needed baseline for monitoring of potentially invasive molluscs.The results will also have implications for management strategies and habitat reconciliation efforts for seawalls in Singapore.

Material and methods
Located just over one degree north of the equator, Singapore is a tropical island city-state separated from Peninsular Malaysia by the Straits of Johor in the north, and from Indonesia by the Straits of Singapore in the south.The total land area of Singapore (approximately 719.1 km 2 ) comprises the main island and over 60 smaller natural and manmade islands (Singapore Department of Statistics 2016).Almost 20% of this area has been reclaimed from the sea during the last 50 years and erosion of this new land is controlled using artificial coastal defences (Singapore Land Authority 2017).

Sampling sites, survey protocol and species identification
Surveys were conducted at seven seawall sites, two on the western coast of mainland Singapore and five among the Southern Islands of Singapore (Figure 1; Supplementary material Table S1).None of these seven sites had been surveyed before.Results were compared to data from previous seawall studies (i.e. Lee et al. 2009a;Loke and Todd 2016;Loke et al. 2016) conducted at Pasir Ris, Marine Parade, Fort road, Marina South, Labrador park, Tuas, St John's Island, Pulau Tekukor, Sister's Islands, Pulau Hantu, Pulau Satumu and Sultan Shoal.
Temperature, light, slope angle and surface rugosity were measured at all seawalls surveyed to characterise each site (Table S1).Temperature and light were measured using data loggers (HOBO Pendant ® ), two of which were attached to the substrate at each site for two weeks.Maximum, minimum and average temperature and light readings were obtained for all sites.Slope angle was calculated by measuring the horizontal distance from a virtual point 1.8 m above the base of the seawall to the slope of the seawall (Figure S1).Surface rugosity was measured using the "chain link method" (Luckhurst and Luckhurst 1978), where a steel chain with fixed link length was laid over the substrate surface along a straight transect length perpendicular to the base of the seawall and was made to adhere as closely as possible to the contours and crevices of the seawall.Linear distance of the same transect length was also measured to derive the ratio of surface distance to linear distance as a measure of surface rugosity.
All surveys were conducted during evening spring low tides from November 2014 to January 2015 when the entire seawall is exposed, i.e. at tidal heights of 0.4-0.7 m.At each site, a single 50 m length of wall was demarcated parallel to the shoreline.The entire seawall, comprising a flattened top, a middle sloping surface, a base and in some cases, a horizontal revetment prior to the base, within this 50 m stretch was surveyed for 90 mins by one person (WTT).The survey method was based on the Rapid Assessment Survey (RAS) protocol, one of the five most common survey methods utilised to detect introduced marine species (Campbell et al. 2007).The advantage of the RAS protocol is its inclusion of qualitative sampling methods which are not constrained to any particular sample area or do not collect all material from an area (i.e.complete census) but, rather, are largely based on time-restricted haphazard visual searches standardized by catch effort (e.g., all species collected within one hour at a specific site; Campbell et al. 2007).In the present study the protocol was applied to one taxon only: molluscs.Samples were preserved in 75% ethanol and identified in the laboratory to species level using the following guides and identification keys: Mollusca (Tan and Ng 1988;Poutiers 1998a, b;Tan and Chou 2000), Cerithiidae (Houbrick 1985;Houbrick 1992), Littorinidae (Reid 1984), Muricidae (Tan and Sigurdsson 1990;Tan and Sigurdsson1996;Tan 2000) and Neritidae (Tan and Clements 2008).The species list arising from the current study was merged with previous lists from Lee et al. (2009a) and Loke and Todd (2016) to create the most complete catalogue of mollusc species found on seawalls in Singapore to date.
Determining the status (native/non-native/cryptogenic) of species Native (or indigenous) species refers to species that occur within their natural range and dispersal potential, i.e. they occur in Singapore naturally without direct or indirect anthropogenic introduction or maintenance (International Union for the Conservation of Nature 2000).Non-native (alien, introduced, or non-indigenous) species are those that occur outside their natural range and dispersal potential, i.e. they are unlikely to occur in Singapore naturally without direct or indirect anthropogenic introduction or maintenance (International Union for the Conservation of Nature 2000).Cryptogenic species are species that cannot be demonstrated to be native or introduced (Carlton 1996).
Criteria used to determine whether a species was native, non-native or cryptogenic to Singapore were adapted from Chapman and Carlton (1991), whereby 10 attributes for introduced species were proposed in the form of testable hypotheses to be validated or disproved.Under an "introduced species hypothesis", positive evidence of an attribute indicates that a species may be introduced to a locale whereas negative evidence implies that a species is native (Chapman and Carlton 1991).A species for which there is insufficient evidence to demonstrate it to be either introduced or native status is regarded as cryptogenic, following the definition by Carlton (1996).The overall correspondence of all attributes of a species being examined is evaluated to determine its introduced status.The specific attributes that were considered following Chapman and Carlton (1991) and adapted to Singapore in the present study are summarised in Table 1.
Attributes 1, 6 and 7 (Appearance in local regions where not found previously, Discontinuous regional distribution, and Discontinuous global distribution, respectively) were first evaluated for all species using a variety of published material, including historical literature, regional checklists of molluscs, taxonomic monographs and "grey literature", to determine the local, regional and global distribution of the species and thereby narrow down the species that are potentially non-native to Singapore.Attribute 5 (Prevalence on or restriction to new or artificial environment(s)) was also examined to assess the prevalence of all species on seawalls in Singapore.Attribute 3 (Association with human mechanism(s) of dispersal) was positive for all the species identified in this study owing to Singapore being a major shipping hub.The remaining attributes were then applied to "potentially introduced" species using species-specific studies where available.

Results
A total of 42 species of mollusc were recorded from the seven sites sampled in this study (Table 2).Thirty-five of these had been recorded previously on Singapore's seawalls (Lee et al. 2009a;Loke and Todd 2016;Loke et al. 2016), while seven species are new records (i.e.previously unrecorded) from the seawalls.Adding these seven species to the previous lists in Lee et al. (2009a) and Loke and Todd (2016) results in a combined total of 65 species.Based on our survey, species richness among the sites ranged from six to 23 (Table 2; Figure 2).West Coast was particularly species-poor, with only six species of molluscs identified; the seawall with the second lowest species richness was Kusu Island with 16 mollusc species.Seringat Island had the highest mollusc species richness with 23 species identified.Not all of the species were common across the sites.Only one species, Drupella margariticola (Broderip, 1833) was found at all the seven surveyed seawalls, while four species (i.e.Siphonaria guamensis Quoy andGaimard, 1833, Monodonta labio (Linnaeus, 1758), Nerita undata Linnaeus, 1758 and Reishia bitubercularis (Lamarck, 1822)) were found at six of the seven seawall sites.There were 16 species that were only recorded at a single seawall site (Table 2).

Status of native, non-native and cryptogenic species on Singapore's seawalls
A literature review on the distribution of the mollusc species identified in the present study and those listed in Lee et al. (2009a) and Loke and Todd (2016), revealed that all of the 65 species found on seawalls in Singapore to date occur within the larger realm of the Indo-Pacific (Figure 3).
Variation in the terms used to describe species distribution includes the regions: Indo-West Pacific, Western Pacific, Western-Central Pacific and Indian Ocean (Table 3).As such, there were no obvious non-native mollusc species from the East Pacific, Atlantic coasts or the Mediterranean, for example.However, the Indo-Pacific realm encompasses a vast region of great diversity and species are not necessarily spread homogeneously, i.e. organisms can occur in a discontinuous manner within the Indo-Pacific and be considered native only to a sub-region of the larger Indo-Pacific.Thus, based on the broadly-defined Indo-Pacific distribution of species alone, it is difficult to determine whether there are non-native species from other parts of the Indo-Pacific region occurring on Singapore's seawalls.
Application of the attributes described in Table 1 to all 65 mollusc species revealed that only one species Table 1.Specific attributes to assess the native/non-native/cryptogenic status of species in the present study.Adapted from Chapman and Carlton (1991) to Singapore's context.

No
Attributes by Chapman and Carlton (1991) Adaptation to Singapore in the present study 1

Previously unrecorded in local region
This attribute can be assessed by referring to reliable historical surveys of local fauna.In this study, the checklist of Singapore molluscs by Tan and Woo (2010) was the primary reference for this purpose.

Range expansion following introduction
This attribute can be assessed only if information regarding the exact location of the first appearance of the species in question and its subsequent range expansion is available.In this study, the location of a species' first record within Singapore was often not available or ambiguous.Therefore, this attribute could not be adequately evaluated.
3 Access to human mechanism(s) of dispersal Since populations of introduced species usually remain associated with the dispersal mechanisms by which they arrived (Chapman and Carlton 1991), this attribute can be assessed by identifying the possible routes of introduction into a location.Some of the most common human-assisted pathways include shipping, aquaculture and aquarium trade (Molnar et al. 2008).Singapore has been a major centre for global trade ever since its establishment (Buckley 1902).The long maritime history coupled with the voluminous level of shipping activities of Singapore would provide considerable human mechanisms of dispersal of organisms to Singapore (Yeo et al. 2011;Jaafar et al. 2012).It is likely that shipping is a major introduction pathway for any non-native species found on the seawall.Other possible human mechanisms of dispersal include commercial fisheries and the ornamental trade (Yeo et al. 2011;Jaafar et al. 2012).

Association with other introduced species
Introduced species tend to co-exist predominantly with other introduced taxa (Chapman and Carlton 1991).This attribute can only be assessed if species-specific ecological studies are available.Also, since the non-native status of many taxa on seawalls in Singapore have yet to be verified prior to this study, it is difficult to adequately evaluate this attribute in this study.

5
Prevalence on or restriction to new or artificial environment(s) Introduced species often dominate on or are limited to human-created substrates such as coastal defences, rock jetties and buoys (Chapman and Carlton 1991;Mineur et al. 2012).In this study, this attribute was assessed by evaluating the prevalence of the species in question across all the seawalls sites surveyed in this present study as well as in Lee et al. (2009a).
6 Discontinuous or restricted regional distribution Introduced species tend to have a restricted distribution along a continuous continental margin (Chapman and Carlton 1991).When applied by Chapman and Carlton (1991) and subsequent authors (e.g., Wasson et al. 2000;Toft et al. 2002), the regional distribution of species was evaluated along a continuous coastline, e.g., the North American Atlantic coastline.However, Singapore is a small island and a "continuous continental margin" is limited to that of the island.Therefore, in the present study, the attribute was assessed by determining the presence of the species in Singapore and Peninsular Malaysia, since both Singapore and Peninsular Malaysia belong to the same marine ecoregion (Spalding et al. 2007) so it can be expected that both locales may share similar marine species.

Disjunct global distribution
It is rare for marine invertebrates of the northern hemisphere, which have been extensively studied, to have naturally isolated intercontinental or interoceanic populations (Chapman and Carlton 1991).In the present study, this attribute was assessed by looking at the overall distribution of the species in question beyond Singapore and Peninsular Malaysia, and determining whether the known populations form a discontinuous distribution globally.

8, 9
Insufficient (active and passive) dispersal capabilities to account for observed distribution This attribute can be assessed only if information regarding the life history stages and adaptations for dispersal of the species in question is available.
10 Exotic evolutionary origin Most introduced species share the closest genetic affinity to species groups occurring elsewhere in the world (Chapman and Carlton 1991).This attribute can be assessed only if there are rigorous phylogenetic analyses of all species in a genus.is likely to be non-native to Singapore, Siphonaria guamensis.Forty-one species were found to be native and the remaining 23 species were classed as cryptogenic (Table 3).Of the species classified as cryptogenic, 22 have two or less attributes with positive evidence that they may be introduced.One other, Semiricinula fusca (Küster, 1862), had three positive attributes supporting a non-native origin, however, as its presence in Singapore was recorded in the early 1970s (Chuang 1973)-suggesting it is native, its status remains ambiguous.

Discussion
This study is the first to examine the origin status of marine molluscs on Singapore's seawalls.Our present surveys produced seven new records for Singapore's seawalls, i.e. these seven taxa were not recorded in previous studies of Singapore seawalls by Lee et al. (2009a) and Loke and Todd (2016).Combining our survey results with the lists in Lee et al. (2009a) and Loke and Todd (2016) yielded a baseline total of 65  species of molluscs now known from Singapore's seawalls.When attributes used by Chapman and Carlton (1991) to assess whether an introduction has occurred were applied to these 65 species, only one was identified as being possibly introduced, Siphonaria guamensis.This is an unexpectedly low number when considering that Seebens et al. (2013) proposed Singapore as the top marine bio-invasion risk hotspot, and given the hypothesized association of marine alien species with seawalls (Mineur et al. 2012).Below we expand on how Siphonaria guamensis was determined to be "possibly introduced".Attribute 1: Appearance in local regions where not found previously.-The earliest published record of Siphonaria guamensis in Singapore is relatively recent (e.g., Tan and Chou 2000;Goh et al. 2002).
Local museum records of the species date back only as far as the late 1980s.The species is absent from earlier historical checklists of molluscs in Singapore (e.g., Chuang 1973;Chou et al. 1994;Way and Purchon 1981), which suggests a possible recent introduction.It is worth noting that S. guamensis is very common and abundant on Singapore's seawalls (pers.obs.; Chim and Tan 2009;Lee et al. 2009a).Hence, it is unlikely to have gone unnoticed if it was present historically.Nevertheless, the taxonomy of this family remains problematic and we do not rule out the possibility that the absence of earlier records of this species could be due to misidentification or a lack of means of identification.

Sources
Attribute 3: Association with human mechanism(s) of dispersal.-Singapore has been a major centre for global trade since its establishment as a British port in 1819 (Buckley 1902).This long maritime history coupled with the voluminous level of shipping activities represents major human pathways of translocation of organisms to Singapore (Jaafar et al. 2012).Although it is not known if S. guamensis is transported by ships, some studies have evoked this vector to account for similar species being found outside of their known geographical occurrence (Zenetos et al. 2005).Siphonaria guamensis is neither a typical fouling species or species imported for aquaculture or the aquarium trade in Singapore (Jaafar et al. 2012), but it is possible that it is dispersed via ship ballast water.
Attribute 4: Association with or dependency on other introduced species.-This attribute was undetermined for S. guamensis, since the native/nonnative status of many coastal marine taxa in Singapore has yet to be documented.There is also a lack of species-specific ecological studies for marine taxa in Singapore's coastal environment.
Attribute 5: Prevalence on or restriction to new or artificial environment(s).-Siphonaria guamensis was one of the most commonly-occurring species on the seawalls surveyed, appearing on 18 out of the 19 seawall sites surveyed to date (Lee et al. 2009a;Loke and Todd 2016).In addition, Chim and Tan (2009) found that S. guamensis exclusively inhabited seawalls at Sungei Api Api (an estuary in the north-east of Singapore).Hence, S. guamensis appears to be prevalent on artificial structures, a characteristic of non-native species (Chapman and Carlton 1991;Mineur et al. 2012).
Attribute 6: Relatively restricted distribution on a continent compared to distributions of native species (i.e.discontinuous regional distribution).-Siphonaria guamensis is not observed in Peninsular Malaysia (Way and Purchon 1981;Wong and Arshad 2011) which suggests it has a discontinuous regional distribution.
Attribute 7: Isolated populations on different continents or in isolated oceans (i.e.discontinuous global distribution).-Since its first record on the island of Guam in 1833 (Quoy and Gaimard 1833), other early records of S. guamensis have been limited to the islands of Micronesia on Guam and Palau (Vermeij 1971), and in the Java Sea (Chim and Tan 2009).Records of the species outside of Micronesia are recent and discontinuous, e.g., Singapore (Tan and Chou 2000), Natuna Islands, Indonesia (Tan and Kastoro 2004) and India (Murty et al. 2013) (Table 4).
Attributes 8, 9: Insufficient active/passive dispersal capabilities to account for the observed distribution of the species.-Information regarding the dispersal capability of S. guamensis remains limited.Siphonaria species generally spawn by producing strings of egg masses attached to a substrate, which may hatch within 4 to 20 days depending on the species (Abe 1940;Huang and Chan 2000).Survival period of the larval stage remains virtually unknown.In Singapore, Chim and Tan (2009) noted that veligers of S. guamensis are released within 18 days upon production of the egg masses, so long distance dispersal via natural mechanisms such as rafting would be constrained, but still possible.
Attribute 10: Exotic evolutionary origin.(Most introduced species populations have the closest morphologic and genetic affinities to species groups occurring elsewhere in the world).-Phylogenetic studies of Siphonaria are still lacking, possibly since the delineation of Siphonaria species based on shell morphology remains challenging and the use of molecular data in Siphonaria taxonomy is still in its infancy (Dayrat et al. 2014).

Latitudinal trends in marine invasions and possible contributing factors
Establishing the biogeographic and thereby native status of many tropical marine species remains a challenge due to the paucity of historical biodiversity records as well as the poorly resolved systematics of several tropical taxa (Yeo et al. 2011).Records of species' presence are therefore more robust than records of absence, since the latter may be compounded by differences in survey effort across geographical areas and taxonomic groups (Campbell et al. 2007).Consequently, present-day new records of species in a particular locale may represent either introductions via anthropogenic means of dispersal or discoveries of native species that have escaped detection, and Yeo et al. (2011) suggested that a number of new species found during marine surveys in recent years (referring to the 2000s) "may in fact be considered cryptogenic", pg 405.In this study, 23 species were categorised as cryptogenic because there was insufficient information to suggest they were native to Singapore or potentially non-native.Most of these species have two or less attributes with positive evidence lending support that it may be introduced, with the exception of Semiricinula fusca which had three attributes with positive evidence.The attributes (generally attributes 1 and 6) with positive evidence for the cryptogenic species were mainly concerned with historical records in Singapore and regional distribution.
The low occurrence of non-native marine species currently observed in Singapore appears to be part of an emerging pattern in global marine invasions whereby there are fewer successful invasion events in the tropics than in temperate zones (Knops et al. 1999;Freestone et al. 2013).Even though most introduced marine species have been reported in temperate areas, and the lower incidence of invasion events in the tropics may simply be an artefact of less research conducted in the region, recent tropical invasion studies are lending support to the observation that ecosystems in lower latitudes may indeed be more resistant to marine invasion than those in temperate zones (Hutchings et al. 2002;Astudillo et al. 2014).
A combination of abiotic and biotic factors has been proposed to explain the observed latitudinal trend in marine invasions.In order for a non-native species to successfully invade a new locale, it must first overcome the initial "abiotic filter" which affects the extent of compatibility between the native distribution of the species and the new environment in which they find themselves (Hutchings et al. 2002;Olyarnik et al. 2009).Frequently, the environmental conditions in tropical ecosystems are too harsh for non-natives to establish (e.g., Hutchings et al. 2002;Astudillo et al. 2014).In a study comparing temperate and tropical ports in Australia, Hutchings et al. (2002) proposed that tropical ports in the subequatorial latitudes of Australia experienced stronger physical stresses such as irregular fluctuations in temperature, salinity, emersion periods and rainfall.Therefore, tropical ports in Australia appear to have abiotic conditions unsuitable for their temperate counterparts, which could hinder the establishment of non-native species which are not accustomed to the local environment.This lends support to the observation that the 'possibly introduced' species (S. guamensis) identified on Singapore's seawalls may be from within the tropical parts of the Indo-Pacific, whereby there is likely to be a greater match in habitats between the source and recipient locale.This is opposed to the scenario in which potentially non-native species were to originate from a source locale outside of the Indo-Pacific where there could be greater disparity in abiotic and biotic conditions.
Unsuitable environmental conditions are a key factor that prevents the establishment of non-native species outside their native range (Lodge 1993).In a review of non-native marine species in Hong Kong, Astudillo et al. (2014) re-examined the status of six non-native species first identified by Morton (1987) with the expectation that they would become more abundant along the coasts of Hong Kong.However, only four of the original six non-native marine species were found, three of which were uncommon (Astudillo et al. 2014).In Singapore, when the tide is low, rock surface temperatures on seawalls regularly exceed 50 °C, which may be beyond the tolerance of non-native species (Table S1; Loke 2015).Limited resource heterogeneity, particularly on artificial seawalls which are less complex than natural rocky shores, is another possible cause of potential failure for establishment as topographically "simple" seawalls provide less available niches for species to utilize (Lodge 1993;Loke and Todd 2016).An alternative, or complementary, hypothesis to explain the latitudinal trend in marine bio-invasions is biotic resistance (Stachowicz et al. 1999(Stachowicz et al. , 2002;;Freestone et al. 2013).Biotic resistance predicts that speciesrich communities typical in the tropics provide stronger community resistance to invasions via a range of potential mechanisms such as the limiting of space as a resource.However, given that the species richness of conventional (unenhanced) seawalls in Singapore is, to begin with, lower than natural rocky shores (i.e.105 species on seawalls versus 138 species on rocky shores; Lai et al. 2018), biotic resistance may be a less significant factor than abiotic environmental conditions in influencing the type of organisms that inhabit seawalls.

Conclusion
The present study has increased the number of seawalls surveyed in Singapore, expanded the species list of molluscs on Singapore's seawalls with seven new records/previously unrecorded taxa, and highlighted one potentially non-native mollusc species.Our findings contribute to the growing body of literature indicating the occurrence of non-native marine species in Singapore is, in fact, very low.This trend directly contradicts Seebens et al.'s (2013) model predictions about Singapore.A combination of abiotic and biotic factors could prevent the establishment of non-native species on Singapore's seawalls and explain this dichotomy.Even though the results from the present study suggest that there is little in the way of invasion events along Singapore's seawalls, the high shipping volume and connectivity of its port cannot be ignored, and regular monitoring must therefore be instituted as a preventative management strategy for biosecurity.

Figure 2 .
Figure 2. Mollusc richness and proportion of native, cryptogenic and possible introduced species at each of the seven sites surveyed in this study.Abbreviations used: KU, Kusu Island; LA, Lazarus Island; PH, Pulau Hantu; SE, Sentosa; SR, Seringat Island; TU, Tuas; WC, West Coast.

Figure 3 .
Figure 3. Location of Singapore within the Indo-Pacific region (outlines in green) as defined in Spalding et al. (2007).

Table 2 .
List of taxa collected and identified from all seven sites.Presence () of taxa found at each site is indicated.Please refer to Figure1for abbreviations of sites surveyed.