Dwarf Wedgemussel

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Dwarf Wedgemussel

Alasmidonta heterodon

ListedMarch 14, 1990
DescriptionFreshwater mussel with two lateral teeth on the right valve and one on the left.
HabitatCreek and river areas with a slow to moderate current and a sand, gravel, or muddy bottom.
ReproductionEggs are fertilized in the female as sperm passes through its gills; the resulting larvae than attaches to a fish host.
FoodFilter feeder.
ThreatsWater pollution and the construction of impoundments.
RangeDelaware, New York, North Carolina, Virginia


The dwarf wedgemussel's shell rarely exceeds 1.5 in (3.7 cm) in length. It is also the only North American freshwater mussel that has two lateral teeth on the right valve, but only one on the left. The female's shell is inflated in the back where the marsupial gills are located.


Little is known about the species' life history and reproductive cycle. Gravid females have been observed from late August until June. Like other freshwater mussels, this species' eggs are fertilized in the female as sperm passes through its gills; the resulting larvae than attaches to a fish host. Although this host is still unknown, strong evidence suggests that it is an anadromous fish which migrates from the ocean into freshwater to spawn.


The dwarf wedgemussel inhabits creek and river areas with a slow to moderate current and a sand, gravel, or muddy bottom. These areas must be nearly silt free.


Once known from about 70 Atlantic Slope river systems, the dwarf wedgemussel is now known from only 12 sites. Two of these populations have recently been discoveredone in Nottoway River, Virginia, and one on Neversink Creek (Delaware River drainage), New York. Four of the existing populations are located in North Carolina one in Little River (Johnston County); another on the Tar River (Granville County); and one each in two of the Tar River Tributaries (Franklin County). The remaining populations occur in Maryland, New Hanpshire, and Vermont. These locations are the Ashuelot River, Chesire County, New Hampshire; two Connecticut River reaches in Sullivan County, New Hampshire, and Windsor County, Vermont; McIntosh Run in St. Mary's County, Maryland; and two Tuckahoe Creek tributaries in Talbot, Queen Annes, and Caroline Counties, Maryland.

Historically, this mussel occurred in 11 States and one Canadian province. It ranged from the Petitcodiac River system in New Brunswick, Canada, south to the Neuse River System in North Carolina. The dwarf wedge mussel is extirpated from both river systems. Other former Southeastern river system sites include the Choptank River; the Rappahannock River; and the James River. In the Middle Atlantic States, the dwarf wedgemussel inhabited the Hackensack River; the Delaware River; and the Susquehanna River systems. New England habitat sites included the Taunton River, the Agawam River, the Merrimac River, the Connecticut River, and the Quinnipiac River systems. One other population from the Fort River in Hampshire County, Massachussetts, also appears extinct.


Water pollution and the construction of impoundments are the primary threats to this mussel's survival. Increased acidity, caused by the mobilization of toxic metals by acid rain, is thought to be one of the chief causes of the species' extirpation from the Fort River in Massachusetts. One of the largest remaining populations has declined dramatically in the Ashuelot River, downstream of a golf course. This population probably has been affected by fungicides, herbicides, insecticides, and fertilizers which have been applied to the golf course. Agricultural runoff from adjacent corn fields and pastures also is contributing to this population's decline. Freshwater mussels, including the dwarf wedgemussel, are sensitive to potassium, zinc, copper, cadmium, and other elements associated with industrial pollution. Industrial, agricultural, and domestic pollution is responsible for the dwarf wedge's disappearance from much of its historic range. To survive, the dwarf wedgemussel needs an almost silt free environment with a slow to moderate current. The construction of dams alters these conditions. For example, most of the Connecticut River's main stem is now a series of impoundments. Upstream from each dam, heavy silt disposition, combined with low oxygen levels, has made the area unsuitable for mussels. Downstream of the dams, water level and temperature fluctuations, caused by hypolimnetic discharges and intermit-tent power generation, have been stressful to the mussels. In some areas below the dams, the river banks have stabilized and the dwarf wedge's required substrate (sandy, gravel, or muddy) no longer exists.

Another reason the species is declining is because its anadromous fish host has been blocked from some habitat areas. For example, the Petitcodiac River system in Canada still hosts several rare mussels, but the dwarf wedge has disappeared. Apparently a downstream water causeway, constructed since the species was last seen, has denied access to the fish host. Populations in the species' remaining range are suffering a decline in reproductive capacity because of its low numbers and isolated population distribution.

Conservation and Recovery

The Maryland Natural Heritage Program has started a program to surround creeks with natural vegetated buffer strips. These strips, which are being established through voluntary landowner cooperation, will protect dwarf wedgemussel habitat by filtering out sediment, excess nutrients, and pollutants. Planned recovery efforts throughout the species' range include encouraging the development of mussel sanctuaries, and reintroducing the species into suitable historic habitats. Buffer strips, conservation easements, and other protective measures should be negotiated through management agreements with local, State and Federal government authorities and private landowners. Dwarf wedgemussel ecology and life history should be studied, and periodic population surveys conducted at historic and existing sites. It's also essential to determine the identity of the species' fish host(s).


U.S. Fish and Wildlife Service
Division of Endangered Species
1875 Century Boulevard, Suite 200
Atlanta, Georgia 30345-3319
Telephone: (404) 679-4159
Fax: (404) 679-1111

U.S. Fish and Wildlife Service
Office of the Regional Director
300 Westgate Center Drive
Hadley, Massachusetts 01035-9589
Telephone: (413) 253-8308
Fax: (413) 253-8308

U.S. Fish and Wildlife Service
Chesapeake Bay Ecological Services Field Office
177 Admiral Cochrane Drive
Annapolis, Maryland 21401-7307
Telephone: (410) 573-4500
Fax: (410) 263-2608
E-mail: [email protected]

U.S. Fish and Wildlife Service
1825 Virginia Street
Annapolis, Maryland 21401


Clark, A.H. 1981. "The Tribe Alasmidontini (Unionidae, Anodontinae), Part 1 Pegias, Alasmidonta, and Arcidens." Smithsonian Contributions to Zoology, No. 326.

Fuller, S.L.H. "Freshwater and Terrestrial Mollusks." In J.E. Cooper et al, eds. Endangered and Threatened Plants and Animals of North Carolina. North Carolina State Museum of Natural History, Raleigh, pp. 143-194.

Havlik, M.E., and L.L. Marking. 1987. "Effects of Contaminants on Naiad Mollusks (Unionidae) A Review." Fish and Wildlife Service, Resource Publication 164. Washington, D.C.

Mcknight, Jonathan. May 1989. "The Dwarf Wedge Mussel." Bulletin of the Maryland Natural Heritage Program (Department of Natural Resources).

U.S. Fish and Wildlife Service. 14 March 1990. "De-termination of Endangered Status for the Dwarf Wedge Mussel." Federal Register, 55(50):9447-9450.