Montana Field Guide

The Asiatic clam is a small (<60 mm) light-colored (yellow or tan) bivalve with shell ornamented by distinct, concentric sulcations (ridges), anterior and posterior lateral teeth with many fine serrations. Dark shell morphs exist but are limited to the southwestern United States. The shells are very hard and will not break under finger-strength pressure like our native pea-clams will.

A small light-colored bivalve with shell ornamented by distinct, concentric sulcations, anterior and posterior lateral teeth with many fine serrations. Dark shell morphs exist but are limited to the southwestern United States. The light-colored shell morph has a yellow-green to light brown periostracum and white to light blue or light purple nacre while the darker shell morph has a dark olive green to black periostracum and deep royal blue nacre (McMahon 1991). The shells of the yellow morphs were straw yellow on the outside and white on the inside; those of brown morphs were dark brown and purple, respectively. Further analyses revealed that the yellow and brown morphs are triploid and tetraploid, respectively.
The taxonomy of Corbicula species needs further work. Until then, all species of the genus Corbicula collected in the United States are compiled under one name, Corbicula fluminea.

This species could be confused with our native pea-clams, but the Asian clam shells are very hard and will not break under finger-strength pressure like our native pea-clams will.

This species does not migrate as adults. The Asian clams have a dispersal stage (veligers) that can float in water currents, or adhere to surfaces. The primary means of dispersion of the Asiatic clam is through human transport, by way of water transfer through recreational activities, accidental transfer with imported aquaculture, and intentional introduction to provide a food item.

Corbicula fluminea prefers sand or gravel substrates in areas with running water. These filter feeders can be found at the sediment surface or slightly buried beneath (Morton 1986). Widespread throughout freshwater streams, lakes, reserviors and some estuarine habitats in the U.S.

The Asian clam is a filter feeder that removes particles from the water column or within the interstitial spaces in the gravels where they may be buried.

It can be found at the sediment surface or slightly buried. Its ability to reproduce rapidly, coupled with low tolerance of cold temperatures (2-30°C), can produce wild swings in population sizes from year to year in northern water bodies. Both yellow and brown morphs are simultaneous hermaphrodites and brood their larvae in the inner demibranchs (Qiu et al. 2001). Furthermore, C. fluminea is able to reproduce by self-fertilization at different ploidy levels. The life span is about one to seven years. C. fluminea is consumed mainly by fish and crayfish. An account of the different species which prey on C. fluminea in the USA is given by McMahon (1983).

While self-fertilization occurs, they will also release sperm into the water that can be captured by other clams for fertilization of eggs. One small clam can produce as many as 400 larvae per day or up to 70,000 per year (McMahon 2000). The Asian clam can reach densities of 10,000 to 20,000 clams per square meter in a very short time. The Asian clam is able to reproduce by self-fertilization at different ploidy levels.

Since man has been demonstrated to be the primary agent of dispersal, no large-scale geographic features function as dispersal barriers (Counts 1986, Isom 1986). Current methods of introduction include bait bucket introductions (Counts 1986), accidental introductions associated with imported aquaculture species (Counts 1986). The only other significant dispersal agent is thought to be passive movement via water currents (Isom 1986); fish and birds are not considered to be significant distribution vectors (Counts 1986, Isom 1986).

The most prominent effect of the introduction of the Asian clam into the United States has been biofouling, especially of complex power plant and industrial water systems (Isom et al. 1986; Williams and McMahon 1986). It has also been documented to cause problems in irrigation canals and pipes (Prokopovich and Hebert 1965; Devick 1991) and drinking water supplies (Smith et al. 1979). It also alters benthic substrates (Sickel 1986), and competes with native species for limited resources (Devick 1991). More recent studies have implicated the Asiatic clam with the decline of native mussels for reasons that include: competition for food resources and/or physical habitat space which may result in native mussel displacements (Ferreira-Rodriguez et al 2018).

Contact information for Aquatic Invasive Species personnel:
Montana Fish, Wildlife, and Parks Aquatic Invasive Species staff
Montana Department of Natural Resources and Conservation's Aquatic Invasive Species Grant Program
Montana Invasive Species Council (MISC)
Upper Columbia Conservation Commission (UC3)

Limiting Factors that may affect population density and distribution of Asian clams include excessively high or low temperatures, salinity, drying out, low pH, silt, hypoxia, pollution, bacterial, viral and parasitic infections, inter- and intraspecific competition, predators, and genetic changes