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Photo credit: Amy Benson, U.S. Geological Survey, Bugwood.org
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Species at a Glance
The Asian clam, also called the Asiatic clam, pygmy clam, or gold clam, is a small freshwater bivalve with two thick-hinged shells that rarely exceeds the size of a quarter. It was first introduced to the west coast of the United States in 1924, possibly as a food item. By the 1970s, it occupied most of the Mississippi River Basin, the Gulf Coast, and the eastern United States.
The shell is typically yellow-green to brown; however, darker morphs exist, usually in the southwestern United States. While the small shell averages 2.5 cm (1 in), it can reach up to 6.5 cm (2.6 in) long. The shell is thick, rounded to slightly triangular in shape, and displays elevated, concentric growth rings. The inside of the shell is layered with white to light purple polished nacre and the teeth are finely serrated. Microscopic juveniles called veligers appear under a microscope in a “D”-shape less than 1 mm (0.04 in) in length.
Fingernail and pea clams have smooth, instead of serrated, teeth and are generally smaller, with thinner shells and less prominent growth rings. There is also evidence that there may be other invasive species of Corbicula (C. largillierti, and C. species unknown) in Illinois; however, it is unknown whether these species are present in the Mid-Atlantic, and suspected populations should be reported. C. largillierti is dark olive-brown, has as a pyramidal-shaped shell, lateral purple teeth, and the ridges are tighter and less pronounced than C. fluminea. The unknown Asian clam species has a yellow, triangular shell with fine pinkish-rust colored rays radiating from the umbo.
The Asian clam prefers running water with a sand or gravel substrate. It can be found in streams, rivers, ponds, lakes, and man-made canals. Although it is a freshwater species, it can withstand slightly brackish waters and is also tolerant of degraded waters.
Because it is hermaphroditic, the Asian clam is capable of self-fertilization. In warmer waters, it can spawn year round, and a single clam can release hundreds to thousands of free-floating, microscopic veligers per day. Juveniles are then spread by water currents and human activity. The Asian clam attaches to boating, fishing, and scuba diving equipment, and veligers can be transferred in bait buckets or live wells.
While native to the temperate and tropical regions of Asia and Africa, the Asian clam is widespread in the United States and spans every state in the Mid-Atlantic region.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
The Asian clam is a known biofouler that blocks water flow to power plants and industrial water systems, and causes problems in irrigation canals and pipes. It also increases clarity in the water column by filtering suspended matter, leading to excessive plant growth and altered nutrient and water quality. This clam may also compete with native mollusks for food and habitat.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).