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Ganl. Bull. Singapore 70 (Suppl. 1) 2018 
Fig. 17. Distribution and abundance of order Mesogastropoda in the Nee Soon drainage. Solid 
circles indicate sites where representatives were collected and circle sizes are proportionate to 
abundance/numbers of individuals captured as reflected in the accompanying legend. 
Class BIVALVIA 
The bivalves are another group of molluscs, named after the two hinged shells all 
members of the class possess. Most bivalves are filter feeders (though some are 
deposit feeders), extracting food particles from the water they are submerged in using 
modified gills (Ruppert & Barnes, 1994). When startled, bivalves close their shells 
tightly and can hold them closed with surprising force. Bivalves are most abundant in 
the marine environment, but many families have adapted to a freshwater environment. 
In fact, some freshwater bivalves are among the most invasive species in the world, 
such as the Ponto-Caspian zebra mussel (Dreissena polymorpha), introduced into 
North America (Lowe et al., 2004). Ironically, native bivalves in North America are 
also among the most imperilled freshwater fauna (Ricciardi et al., 1998). 
