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 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.