Dwarf wedgemussel (Alasmidonta heterodon)
Federal Status: Endangered, Listed March 14, 1990
Description: The dwarf wedgemussel is a small bivalve, rarely exceeding 45 mm in length. Clean young shells are usually greenish-brown with green rays. As the animal ages, the shell color becomes obscured by diatoms or mineral deposits and appears black or brown. The shell is thin but does thicken somewhat with age, especially toward the anterior end. The anterior end is rounded while the posterior end is angular forming a point near the posterio-ventral margin. The ventral margin is only slightly curved. The nacre is bluish-white, appearing whiter in the thicker anterior end. The most distinctive shell character of the dwarf wedgemussel is the arrangement of the lateral teeth. There are two lateral teeth in the right valve and one in the left valve. The typical arrangement for most freshwater mussel species consists of two lateral teeth in the left valve and one in the right valve. The incurrent and excurrent apertures and their associated papillae are usually white. The foot and other organs are also white. Maximum age for the dwarf wedgemussel is around twelve years. The species is a bradytictic breeder, meaning that females become gravid in the early fall and glochidia are released by mid-spring. The tessellated darter (Etheostoma olmstedi), johnny darter (Etheostoma nigrum), and mottled sulpin (Cottus bairdi) have been identified as hosts for the dwarf wedgemussel. An anadromous fish may also serve as a host species but this has not been documented for the dwarf wedgemussel in the southern portion of its range.
Habitat: The dwarf wedgemussel appears to be a generalist in terms of its preference for stream size, substrate and flow conditions – it inhabits small streams less than five meters wide to large rivers more than 100 meters wide; it is found in a variety of substrate types including clay, sand, gravel and pebble, and sometimes in silt depositional areas near banks; and it usually inhabits hydrologically stable areas, including very shallow water along streambanks and under root mats, but it has also been found at depths of 25 feet in the Connecticut River. Dwarf wedgemussels are often patchily distributed in rivers.
Distribution: Historically, the dwarf wedgemussel was found from the Petitcodiac River in New Brunswick, Canada to the Neuse River in North Carolina, and was found in 15 major Atlantic slope river systems. It is now extinct in Canada, extirpated in the Neuse River, and present in low densities through-out much of its former range. It is known from 54 locations in 15 major watersheds, with the largest populations in the Connecticut River watershed. North Carolina supports the greatest number of known sites: Neuse River Basin: Orange County, Wake County, Johnston County, Wilson County, and Nash County; Tar River Basin: Person County, Granville County, Vance County, Franklin County, Warren County, Halifax County, and Nash County. Unfortunately, most of these populations are very small and isolated.
Threats: Impacts including riparian disturbance, pollution, sedimentation, impoundments, artificial flow regimes, and stream fragmentation disrupt mussel life cycles, prevent host fish migration, block gene flow, and prohibit recolonization, resulting in reduced recruitment rates, decreased population densities and increased probability of local extinctions. Toxic effects from industrial, domestic and agricultural pollution 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.
Short life spans, low fecundity, high degree of host specificity, limited dispersal ability of its primary host, low population densities, coupled with the threats facing the species, likely all contribute to the endangered status of the dwarf wedgemussel.
N.C. Natural Heritage Program. 2001. Guide to Federally Listed Endangered and Threatened Species of North Carolina: Dwarf wedgemussel. NC Department of Environment and Natural Resources, Raleigh, NC. Page 61.
U.S. Fish and Wildlife Service. 1993. Dwarf wedgemussel Recovery Plan. Hadley, MA. 48 pp.
Massachusetts Natural Heritage and Endangered Species Program. 2009. Dwarf wedgemussel (Alasmidonta heterodon). Westborough, MA.
New Hampshire Fish and Game. 2006. New Hampshire Wildlife Action Plan: Appendix A: Species Profiles – Invertebrates. Dwarf Wedgemussel. Concord, NH.
Sarah McRae, Fish and Wildlife Biologist, 919-856-4520 ext. 16