The ornamental pet trade has long been considered the main culprit in introducing non-native species into the wild.
Organisms like snails may seem innocuous next to non-native fish or frogs, but they, too, can make their way into local habitats once they are imported and cause harm to native species living here.
In a recently published paper in science journal PLoS One, local researchers found an "unexpectedly high" number of snails and bivalves - a total of 59 species - that had been brought into Singapore via pet shops and ornamental exporters from 2008 to 2014.
A handful of the species were "hitchhikers" in tanks holding other mollusc or fish species. Some were also found to have hitched a ride on aquatic plants in nurseries.
"The number is considered high for snails and clams because these are not what people would usually buy," said Ms Ng Ting Hui, who did the study as part of her doctoral thesis at the National University of Singapore's (NUS) Department of Biological Sciences. "When we talk about the ornamental trade, most would think of fish but some of these species (snails and bivalves) have been known to cause damage in native habitats as well."
In order to gather data for her study, Ms Ng surveyed exporters and pet retail shops in Singapore. As for the records of inadvertent imports, she observed those found in tanks and aquatic plants.
Among the 59 species was the notorious golden apple snail of the genus Pomacea native to South America. In 2012, the European Union banned all imports of snails belonging to the genus because of their destructive impact on agriculture.
The snail, which is usually identified by its pink eggs laid in clusters of hundreds up to a thousand, has been documented to cause widespread damage in rice crops all over the world, said Assistant Professor Darren Yeo from the NUS Department of Biological Sciences, Ms Ng's thesis supervisor.
Native species of apple snails can lay eggs only in the hundreds, which could be a reason why they are being crowded out.
"In Singapore, instead of rice plants, they may cause problems by feeding on or damaging ornamental plants in landscape ponds," Prof Yeo explained.
"They are also a problem as they pose a potential threat to native apple snails in South-east Asia, including Singapore."
Ms Ng said: "Pomacea were originally brought into Asia for food, when their impacts were largely unknown, and were likely first brought into Singapore in the 1980s through the aquarium trade."
While there is no direct evidence to prove that Pila scutata - an apple snail species native to South-east Asia - is on the decline because of the introduced golden apple snail, Ms Ng said an earlier study she did in 2013 seems to show such an association.
Although not all non-native species such as plants and animals are harmful, those which are invasive can become established and out-compete native species for food and shelter.
Today, the golden apple snail can be found in many ponds and reservoirs here, including the Bukit Batok Town Park, Bishan-Ang Mo Kio Park and Singapore Quarry, said Dr Lena Chan, group director of the National Parks Board's (NParks) National Biodiversity Centre.
She said that NParks regularly removes golden apple snails from the Singapore Quarry. This is carried out together with volunteers and schools as part of the agency's invasive species management programme.
Volunteers look for the snails, which have a shell height of at least 5cm to 6cm, as well as their eggs.
Another species that came up during Ms Ng's study was the assassin snail, or Anentome helena.
While it was deliberately brought in for the ornamental trade because it feeds on other snails that are considered pests in aquarium tanks, established populations have recently been found in Kranji Reservoir.
"Its presence is the clearest link to the aquarium trade being a source of introduced snails," said Ms Ng.
"It has the potential to impact other snail species and aquatic invertebrates."
Her study on freshwater snails was funded by NUS and Wildlife Reserves Singapore.