The changing natural world at our doorsteps | Illustration and text by Patterson Clark
August 7, 2012
The Asiatic clam: A double-edged guest
Sandbars on the Potomac River are often topped with heavy deposits of Asiatic clam shells, which are dime- to silver-dollar-size with concentric ridges and an olive-yellow coating that turns black and flakes off after the clam dies.
Although the exotic mollusk creates environmental problems in some parts of the country, it appears to improve aquatic habitats in some local waters without displacing native clams or mussels.
Asiatic clams are very effective at removing sediment and algae from the water, allowing deeper penetration of light. That encourages the return of submerged aquatic vegetation, which provides cover for young fish and other organisms.
Birds, fish and river otters feast on the little clams, which people can also eat, provided the clams are living in unpolluted water. In some parts of the world, the "good luck clam" is a major food source.
"It has a high glycogen content, like oysters," says Harriette L. Phelps, a University of the District of Columbia emeritus professor who has been studying the clam for decades. Although the clams are found in areas closed to shellfish harvest, Phelps says that Asians have been seen taking the clams from shallow Potomac waters using sieves.
The clams rest on or burrow slightly into silt, sand or gravel. When scooped up with river gravel used as aggregate at concrete plants, the tough little clams can wriggle their way to the surface of freshly poured cement, ruining concrete projects.
Although the Washington Aqueduct, which supplies water to the District and parts of Virginia, has never had problems with the clam, tiny juvenile Asiatic clams have been reported to clog industrial water-intake pipes farther downstream.
When they're not annoying industry, Asiatic clams can be used to monitor it. Phelps, who revealed the clam's environmental benefits to the Potomac, has for almost 20 years used Asiatic clams to measure pollutants in the Anacostia River and its tributaries. The clams, suspended in mesh bags for two weeks, concentrate toxins in their tissue, which is then tested in the laboratory to pinpoint contaminated areas in the troubled watershed. "In the Anacostia, many sites had high toxics and little life, but Corbicula fluminea can survive over a month," says Phelps. "We are fortunate in having local Corbicula available."
Sources: Estuaries, Army Corps of Engineers, Water Resources Research Institute, National Exotic Marine and Estuarine Species Information System, USGS, Potomac River Fisheries Commission