Alasmidonta heterodon Kathryn J. Schneider

Alasmidonta heterodon
Kathryn J. Schneider

Bivalvia (Bivalves)
Unionidae (Unionid Mussels)
State Protection
Federal Protection
State Conservation Status Rank
Global Conservation Status Rank


Did you know?

The Neversink River population, estimated at 20,000 individuals is one of the largest known populations in the U.S. (Strayer and Jirka, 1997).

State Ranking Justification

The extent of this species in New York is limited to a small area within the Delaware River watershed. The Neversink River population, a tributary of the Delaware, dropped by an estimated 60,000 individuals between 1990 to 1994 and continues to be vulnerable to a myraid of stressors that affect aquatic systems, including non-point pollution and existing impoundments (Strayer et al. 1996).

Short-term Trends

The Neversink River population has apparently declined by 75% since it was first located in 1990 when it dropped from an estimated 80,000 individuals to 50,000 in 1991, then to 20,000 in 1994 (Strayer et al., 1996). It is not known if the population still numbers in the tens of thousands since the last survey over a decade ago. Since these population estimates are based on the direct capture of only a small number of individuals and standardized monitoring methods have only recently been adopted for Unionids (Strayer and Smith 2003) the estimates may not be accurate, nor directly comparable. The short-term trend for the Delaware River (meta)population is not known at this time because the sites have not been monitored since they were first located by U.S. Geological Survey researchers in 2000 (Lellis, 2001).

Long-term Trends

All 13 major populations studied rangewide, including the Neversink River in New York, had low densities, similar to earlier 20th century observations. About 100 years ago the species was known from about 70 Atlantic seaboard river systems, but now from only 25-30 (USFWS, 1993). Similar declines in the distribution of dwarf wedgemussels have undoubtedly occurred in New York. However, since the Recovery Plan was published in 1993, increased survey effort has led to the discovery of 40 new locations where the species had been presumed extirpated, or in rivers where it had never been found (Nedeau 2005). Of the 70- 80 known locales in 2004 only 16 were believed to support reprodcuing populations, while 31 were based on observations of of five or fewer individuals, or solely on spent shells.

Conservation and Management


Water pollution and impoundments are the primary threats. This species requires a low silt environment with a slow to moderate current, a situation that dams alter both upstream and downstream of the impoundment. A wide array of industrial, agricultural and domestic pollutants have been responsible for the dwarf wedgemussel's disappearance from much of its historical range and continues to be a problem in most aquatic systems (USFWS 1993). The darter and sculpin glocidial host fish species are generally pollutant sensitive taxa and a healthy fish assemblage is critical to viable mussel populations (Pinkney et al. 1997). The low densities (< 0.5 per square meter) in which this species occurs is problematical since successful reproduction is density dependent. Females need to be in close proximity to a sperm- releasing male to be successfully fertilized (Strayer et al. 1996). Competition with exotic bivalves, both the Asian clam (Corbicula fluminea) and zebra mussel (Dreissena polymorpha) could pose a threat because they are expected to eventually invade all of New York's watersheds, although neither has yet invaded the upper Delaware system (Strayer and Ralley 1991). The majority of 45 individuals collected from the Neversink population were 4 and 5 year old specimens, and none were older than 8 years (Michaelson and Neves 1995). This indicates recent successful reproduction. However, since this species can live at least 20 years, and perhaps much longer, the lack of older individuals in the popualtion could indicate high rates of mortality of older animals, possibly from pollution. Ageing Unionids by counting growth rings has recently been shown to vastly underestimate the true age of individuals, so the ages cited above may be too low by up to an order of magnitude (Strayer 2004).

Conservation Strategies and Management Practices

When feasible, the removal of impoundments in order to restore rivers to their natural flow would be benefical. The Nature Conservancy negotiated with the Army Corps of Engineers for the removal of the Cuddebackville Dam in the summer of 2003 in order to restore natural flow patterns to the lower Neversink and is working to reduce alterations to the natural flow caused by the upstream Neversink Reservoir Dam. The outcome of this management strategy on the mussel populations has not been evaluated. The U.S. Fish and Wildlife Service has suggested that reintroductions may need to be undertaken to bring low- density populations back up to viable levels and re-establish populations extirpated from certain rivers (USFWS 1993).

Research Needs

There is still much to learn about this species, including confirmation of host fish(es) in the Delaware and the Neversink rivers, diet, age and growth, and mortality factors. Details about habitat requirements (current speed, water depth, substrate grain size, substrate stability, water temperature, and water quality factors) also need work. However, Strayer and Ralley (1993) found that the distribution of this species was not related to these typical physical habitat qualities, but instead to long term stability of the substrate (i.e., flow refuges). Both large and smaller scale forces promoting the patchy occurrence of Unionid mussel beds is an active area of research (Strayer 2004).



In New York, dwarf wedgemussels live embedded in the fine sediment that has accumulated between cobbles in slow to moderate current and relatively shallow water (40 cm) in small cool water rivers and similar habitat in larger rivers (Strayer and Jirka 1997).

Associated Ecological Communities

  • Confined river (guide)
  • Deepwater river*

Associated Species

  • Triangle Floater (Alasmidonta undulata)
  • Brook Floater (Alasmidonta varicosa) (guide)
  • Alewife Floater (Anodonta implicata) (guide)
  • Eastern Elliptio (Elliptio complanata)


New York State Distribution

The only two known populations occur in the upper Delaware River in Sullivan and Delaware Counties and one of its major downstream tributaries, the lower Neversink River in Orange County.

Global Distribution

Today dwarf wedgemussels are discontinuously distributed along Atlantic seaboard drainages from New Hampshire to North Carolina (New Hampshire, Vermont, Connecticut, New York, New Jersey, Pennsylvania, Massachusetts, Maryland, Virginia, North Carolina), but in 1999 it was declared extirpated from New Brunswick. The dwarf wedgemussel was first described from the Schuylkill River in Pennsylvania although it has not been seen in that river since 1919. Historically known from about 70 locations in 15 major drainages, it was listed as a federally endangered species in the United States in 1990. When the federal recovery plan was published three years later, the dwarf wedgemussel was known to exist in only 20 of 70 locations where it had originally been known to occur. Most of the populations were thought to be small and declining. Since the federal recovery plan was published in 1993, surveyors have found nearly 40 new populations in locations where the species had been presumed extirpated or in rivers where the species had never been found (Nedeau 2005).

Best Places to See

  • Neversink River (Orange County)

Identification Comments

General Description

Alasmidonta heterodon is a small freshwater mussel, usually less than 55 mm long. Its shell is subrhomboidal to suntriangular, with a prominent posterior ridge. The beak sculpture consists of two concentric ridges surrounded by two to three trapezoidal ridges along the posterior slope. The periostracum is greenish to brownish, usually with many fine green color rays. Pseudocardinal and lateral teeth are both present and the nacre is white (Strayer and Jirka 1997).

Identifying Characteristics

The small size, roundly pointed posterio-basal margin, and reversed lateral hinge teeth readily distinguish this species (Strayer and Jirka 1997).

Best Life Stage for Proper Identification



Adults of this species are sessile with only limited movement in the substrate. Passive downstream movement may occur when they are displaced from the substrate during floods. More major dispersal occurs while glochidia are encysted on their darter and sculpin hosts, although McLain and Ross (2005) showed that tesselated darters move very little which promotes a patchy distribution of mussel beds. Being ectothermic, activity levels are reduced greatly during colder months of the year. This is a long- term brooder that spawns in late summer, becomes gravid in the fall and the larvae become active the following spring (Michaelson and Neves, 1995).

Best Time to See

Little is known about the activity periods of Unionid mussels but they are presumed to be greatly reduced during cold times of the year. Freshwater mussels are most often easiest to locate during late summer when water levels are lowest. This species is a long-term breeder (winter- early spring) (Clarke 1981) and larvae (glochidia) are released into the water from early March to June (Wicklow 2004).

  • Present
  • Active
  • Reproducing
  • Larvae present and active

The time of year you would expect to find Dwarf Wedgemussel present, active, reproducing, and larvae present and active in New York.

Similar Species

  • Brook Floater (Alasmidonta varicosa) (guide)

Dwarf Wedgemussel Images


Dwarf Wedgemussel
Alasmidonta heterodon (I. Lea, 1830)

  • Kingdom Animalia
    • Phylum Mollusca (Mollusks)
      • Class Bivalvia (Bivalves)
        • Order Unionoida (Freshwater Mussels)
          • Family Unionidae (Unionid Mussels)

Additional Resources


Burch, J.B. 1975a. Freshwater unionacean clams (Mollusca: Pelecypoda) of North America. Malacological Publications: Hamburg, Michigan. 204 pp.

Clarke, A.H. 1981a. The freshwater mollusks of Canada. National Museum of Natural Sciences, National Museums of Canada, D. W. Friesen and Sons, Ltd.: Ottawa, Canada. 446 pp.

Gordon, M.E. and J.B. Layzer. 1989. Mussels (Bivalvia: Unionoidea) of the Cumberland River review of life histories and ecological relationships. U.S. Fish and Wildlife Service Biological Report, 89(15): 1-99.

Lellis, W.A. 2001. Freshwater mussel survey of the Upper Delaware Scenic and Recreational River: Qualitative Survey 2000. Report to the National Park Service. New York Natural Heritage Program, Albany, NY.

Letson, E. J. 1905. Checklist of the Mollusca of New York. Bulletin. No. 88. New York State Museum, Albany, NY.

Marshall, W.B. 1895. Geographical distribution of New York Unionidae. Annual Report. New York State Museum. 48: 47-99.

Master, L. 1986. Alasmidonta heterodon; results of a global status survey and proposal to list as an endangered species. A report submitted to Region 5 of the USFWS, Hadley, Massachusetts. 10 pp.

McLaine, D.C., and M.R. Ross. 2005. Reproduction based on local patch size of Alasmidonta heterodon and dispersal by its darter host in the Mill River, Massachusetts, USA. Journal of the North American Benthological Society 24:138-147.

Michaelson, D.L. and R.J. Neves. 1995. Life history and habitat of the endangered dwarf wedgemussel Alasmodonta heterodon (Bivalvia: Unionidae). Journal of the North American Benthological Society, 14(2): 324-340.

Nedeau, E. 2005. In hopes the dwarf wedgemussel survives: presumed extirpated, surveyors have discovered new populations. Gulf of Maine Times. Winter 2005.

New York Natural Heritage Program. 2019. New York Natural Heritage Program Databases. Albany, NY.

Pinkney, A.E., D.R. Murphy, and P.C. McGowan, preparers. 1997. Characterization of endangered dwarf wedgemussel (Alasmidonta heterodon) habitats in Maryland. Branch of Water Quality and Environmental Contanimants, U.S. Fish and Wildlife Service, Annapolis, Maryland. Publication No. CBFO-C97-01. 156 pp.

Strayer, D.L. and J. Ralley. 1993. Microhabitat use by an assemblage of stream-dwelling unionaceans (Bivalvia) including two rare species of Alasmidonta. Journal of the North American Benthological Society 12(3):247-258.

Strayer, D.L., S.J. Sprague, and S. Claypool. 1996. A range-wide assessment of populations of Alasmidonta heterodon, an endangered freshwater mussel (Bivalvia: Unionidae). Journal of the North American Benthological Society, 15(3): 308-317.

Strayer, David L. and D.R. Smith. 2003. A guide to sampling freshwater mussel populations. American Fisheries Society Monograph 8. American Fisheries Society, Bethesda, Maryland. 103 pp.

Strayer, David L. and J. Ralley 1991. The freshwater mussels (Bivalva: Unionidae) of the upper Delaware River drainage. American Malacological Bulletin 9 (1): 21-25.

Strayer, David L. and K.J. Jirka. 1997. The Pearly Mussels (Bivalva: Unionoidea) of New York State. New York State Museum Memoir 26. The New York State Education Department.

Strayer, David L., J.A. Dowling, W.R. Haag, T.L. King, J.B. Layzer, T.J. Newton and S.J. Nichols. 2004. Changing perspectives on Pearly Mussels, North America's most Imperiled Animals. BioScience 54:429-439.

U.S. Fish and Wildlife Service (USFWS). 1993. Dwarf wedge mussel (Alasmidonta heterodon) recovery plan. U.S. Fish and Wildlife Service: Hadley, Massachusetts. 52 pp.


About This Guide

Information for this guide was last updated on: June 22, 2007

Please cite this page as:
New York Natural Heritage Program. 2019. Online Conservation Guide for Alasmidonta heterodon. Available from: Accessed January 18, 2019.

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