The dwarf wedgemussel (''Alasmidonta heterodon'') is an

endangered species An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching, inv ...

of freshwater

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

, an aquatic

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

in the family

Unionidae The Unionidae are a Family (biology), family of freshwater mussels, the largest in the order Unionida, the bivalve molluscs sometimes known as river mussels, or simply as unionids. The range of distribution for this family is world-wide. It is a ...

, the river mussels.

Distribution and conservation status

This is a rare species found solely in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

's Atlantic coast streams and rivers of various sizes and moderate current. The dwarf wedge mussel's current range extends from

New Hampshire New Hampshire ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, Vermont to the west, Maine and the Gulf of Maine to the east, and the Canadian province of Quebec t ...

North Carolina North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...

. The dwarf wedge mussel is federally listed as endangered and state-listed as endangered in

Connecticut Connecticut ( ) is a U.S. state, state in the New England region of the Northeastern United States. It borders Rhode Island to the east, Massachusetts to the north, New York (state), New York to the west, and Long Island Sound to the south. ...

Massachusetts Massachusetts ( ; ), officially the Commonwealth of Massachusetts, is a U.S. state, state in the New England region of the Northeastern United States. It borders the Atlantic Ocean and the Gulf of Maine to its east, Connecticut and Rhode ...

Maryland Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It borders the states of Virginia to its south, West Virginia to its west, Pennsylvania to its north, and Delaware to its east ...

New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...

, New York,

Pennsylvania Pennsylvania, officially the Commonwealth of Pennsylvania, is a U.S. state, state spanning the Mid-Atlantic (United States), Mid-Atlantic, Northeastern United States, Northeastern, Appalachian, and Great Lakes region, Great Lakes regions o ...

Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...

, and

Vermont Vermont () is a U.S. state, state in the New England region of the Northeastern United States. It borders Massachusetts to the south, New Hampshire to the east, New York (state), New York to the west, and the Provinces and territories of Ca ...

. It previously lived in

New Brunswick New Brunswick is a Provinces and Territories of Canada, province of Canada, bordering Quebec to the north, Nova Scotia to the east, the Gulf of Saint Lawrence to the northeast, the Bay of Fundy to the southeast, and the U.S. state of Maine to ...

, but it is locally extirpated in Canada since 1968. The Canadian Species at Risk Act listed the dwarf wedgemussel in the

List of Wildlife Species at Risk As of November 2024, the List of Wildlife Species at Risk has more than 600 entries for Canadian wildlife species considered at varying risks of extinction, including 301 classified as endangered species, 147 threatened species, 200 special conc ...

as being

extirpated Local extinction, also extirpation, is the termination of a species (or other taxon) in a chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinctions. Local extinctions mark a chan ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Biotope

This mussel may be found in small creeks to deep rivers in stable habitats with substrates ranging from mixed sand, pebble and gravel, to clay and silty sand. In the southern portion of its range, it is often found buried under logs or root mats in shallow water (USFWS 1993), where in the northern portion of its range, it may be found in firm substrates of mixed sand, gravel or cobble, or embedded in clay banks in water depths of a few inches to greater than 20 feet (Fichtel and Smith 1995; Gabriel 1995; Gabriel 1996; Nedeau and Werle 2003; Nedeau 2004a, 2004b, 2006).

Description

The dwarf wedgemussel is possibly the only freshwater mussel from North America that has two lateral teeth on the right valve and only one on the left side. The dwarf wedgemussel is a small mussel, whose shell size rarely exceeds in length and high. The trapezoidal-shaped shells are colored brown or yellowish-olive, with reddish brown or greenish rays in young or pale specimens. The

nacre Nacre ( , ), also known as mother-of-pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is ...

is bluish or silvery white, and is

iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstruc ...

on the posterior.

Biology and ecology

Its reproductive cycle is typical of other freshwater mussels, requiring a host fish on which its larvae ( glochidia) parasitize and metamorphose into juvenile mussels. The dwarf wedgemussel is not a long-lived species as compared to other freshwater mussels; life expectancy is estimated at 10 to 12 years (Michaelson and Neves 1995). A number of fish species have been positively identified as hosts for the dwarf wedgemussel. Michaelson and Neves (1995) confirmed the

tessellated darter The tessellated darter (''Etheostoma olmstedi'') is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It is native to Ca ...

(''

Etheostoma olmstedi The tessellated darter (''Etheostoma olmstedi'') is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It is native to Ca ...

''),

Johnny darter The johnny darter (''Etheostoma nigrum'') is a species of freshwater ray-finned fish, a darter from the subfamily Etheostomatinae, part of the family Percidae, which also contains the perches, ruffes and pikeperches. It is native to shallow ...

('' Etheostoma nigrum''), and mottled sculpin (''Cottus bairdi'') as host fish for dwarf wedgemussels in the southern part of its range. Wicklow (''in'' New Hampshire Wildlife Action Plan 2005) confirmed the

slimy sculpin The slimy sculpin (''Cottus cognatus'') is a freshwater species of fish belonging to the family Cottidae, which is the largest sculpin family. They usually inhabit cold rocky streams or lakes across North America, ranging from the Great Lakes, s ...

(''Cottus congatus'') and juvenile and parr of the

Atlantic salmon The Atlantic salmon (''Salmo salar'') is a species of ray-finned fish in the family Salmonidae. It is the third largest of the Salmonidae, behind Hucho taimen, Siberian taimen and Pacific Chinook salmon, growing up to a meter in length. Atlan ...

(''Salmo salar'') as host fish for dwarf wedgemussels in New Hampshire. The dwarf wedgemussel is considered to be a long-term brooder. In general, dwarf wedgemussel glochidia may be released between March and June, with peak release times varying from south to north. Michaelson and Neves (1995) documented the reproductive cycle of the dwarf wedgemussel from North Carolina and observed that this species spawns in late summer, becomes gravid in September, and releases glochidia in April. Wicklow (''in'' New Hampshire Wildlife Action Plan 2005) observed glochidia release beginning in March and continuing through June in the

Ashuelot River The Ashuelot River is a tributary of the Connecticut River, approximately long, in southwestern New Hampshire in the United States. It drains a mountainous area of , including much of the area known as the Monadnock Region. It is the longest tr ...

in New Hampshire. In a study of dwarf wedgemussel reproduction in the Mill River, Massachusetts, McLain and Ross (2005) observed that most glochidia were released in April and May. Reproductive output appears to be correlated with local population abundance. McLain and Ross (2005) documented that sites with the highest abundance of adult dwarf wedgemussels also demonstrated the highest proportion of gravid females, glochidial density, host infection, and density of juvenile mussels.

Populations

At one time, this species was recorded from 70 localities in 15 major drainages ranging from North Carolina to

, Canada. Since the species was first listed as endangered by the U.S. Fish and Wildlife Service in 1990 (55 FR 9447 9451), a number of new locations have been discovered and a number of known locations are possibly no longer extant. Based on preliminary information, the dwarf wedgemussel is currently found in 15 major drainages, comprising approximately 70 “sites” (one site may have multiple occurrences). At least 45 of these sites are based on less than five individuals or solely on spent shells. The only known occurrence in New Brunswick, Canada (Petticodiac River) appears to be historic; no live mussels or spent shells were found during a 1997 survey (Hanson 1998). The mainstem of the

Connecticut River The Connecticut River is the longest river in the New England region of the United States, flowing roughly southward for through four states. It rises 300 yards (270 m) south of the U.S. border with Quebec, Canada, and discharges into Long Isl ...

in New Hampshire and

is considered to have the largest remaining dwarf wedgemussel population, consisting of three distinct stretches of sporadically occupied habitat segmented by hydroelectric dams. It is estimated that there are hundreds of thousands of dwarf wedgemussels scattered within an approximate 75-mile stretch of the Connecticut River. The Ashuelot River in New Hampshire, the Farmington River in Connecticut, and the Neversink River in New York harbor large populations, but these number in the thousands only. The remaining populations from

south to North Carolina are estimated at a few individuals to a few hundred individuals. Strayer et al. (1996) speculated that many dwarf wedgemussel populations, particularly in the southern portion of the range, may be threatened by low densities, small ranges. and linear structure (i.e., an entire population in one stream with no possibility of refuge from catastrophes or stochastic events). Low-density populations may lead to a loss of productivity due to reproductive impediments (e.g., the distance between mussels being too great) or loss of genetic variability. The Mill River in Hatfield and Whately, Massachusetts is an example of a river with a dwarf wedgemussel population patchily distributed over an approximate 16-mile stretch. The most reproductively robust patch is limited to a small stretch (< 1 mile) making it extremely vulnerable to a catastrophic event. The remainder dwarf wedgemussels are sparsely scattered and may demonstrate a reduced capability to reproduce as indicated by McLain and Ross (2005). Agricultural run-off has been identified as a significant threat to dwarf wedgemussel populations in Massachusetts,

, and North Carolina. In 2001, more than 25 dwarf wedgemussels and hundreds of other mussels (including state-listed species) were killed in the Mill River, Massachusetts, by waste run-off from a small farm. Recently, severe flooding in the Baltimore and Neversink Rivers in

and New York, respectively, resulted in the destruction of occupied habitat and loss of dwarf wedgemussels. Surveys conducted at two sites on the Neversink River below a dam in Cuddebackville, New York, derived abundance estimates ranging from 60 to 500 dwarf wedgemussels per site (Cole et al. 2004) prior to 2005. Severe flooding in the spring of 2005 scoured the river channel and deposited cobble in at least one of the sites previously surveyed. Resurveys in 2005 of the two sites conducted after the flood event detected one fresh dead dwarf wedgemussel and no live mussels (Cole and White 2006). Surveys in 2006 indicated that the dwarf wedgemussel population in the Neversink River was adversely affected by flood events, although some live mussels were detected. Little riverine habitat adjacent to extant populations is protected other than by state shoreline protection regulations or local land use regulations. Development of adjacent uplands continues to be a significant and pervasive threat to southern populations. In summary, it appears that the populations in North Carolina, Virginia, and Maryland are declining as evidenced by low densities, lack of reproduction, or inability to relocate any dwarf wedgemussels in follow-up surveys. Populations in New Hampshire, Massachusetts, and Connecticut appear to be stable, while the status of populations in the

Delaware River The Delaware River is a major river in the Mid-Atlantic region of the United States and is the longest free-flowing (undammed) river in the Eastern United States. From the meeting of its branches in Hancock, New York, the river flows for a ...

watershed affected by the recent floods of 2005 is uncertain at this time.

References

Footnotes

Bibliography

Dwarf Wedgemussel Recovery Plan: US Fish and Wildlife Service
(PDF document)
NatureServe Explorer report

* Andrew Moser, G. and the Fish and Wildlife Service. 14 March 1990. Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Dwarf Wedge Mussel. ''Federal Register'', 55(50):9447-9451. * Cole, J. C., B. S. White and C. Apse. 2004. A quantitative assessment of freshwater mussels (Bivalvia:Unionidae) in the Neversink River prior to the Cuddebackville Dam removal. Report to The Nature Conservancy, Cuddebackville, New York. 16 pp. * Cole, J. and B. S. White. 2006. An assessment of freshwater mussels in the Neversink River following removal of the dam at Cuddebackville, NY and a severe spring flood event. Report to The Nature Conservancy, Cuddebackville, NY. 12 pp. * Fichtel, C. and D. G. Smith. 1995. The Freshwater Mussels of Vermont. Nongame and Natural Heritage Program, Vermont Fish and Wildlife Department. Technical Report 18. 53 pp. * Gabriel, M. 1995. Freshwater mussel distribution in the rivers and streams of Cheshire, Hillsborough, Merrimack and Rockingham, Counties, New Hampshire. Report submitted to U.S. Fish and Wildlife Service, New England Field Office and New Hampshire Fish and Game Department. 60 pp. * Gabriel, M. 1996. 1996 Monitoring of the dwarf wedgemussel (Alasmidonta heterodon) in the Ashuelot and Connecticut Rivers, New Hampshire. Report submitted to The Nature Conservancy, Eastern Regional Office, Boston, Massachusetts. 27 pp. * Hanson, M. J. 1998. March 24. Re: status of dwarf wedgemussel ersonal email Accessed March 24, 1998. * McLain, D. 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. Jour. N. Am. Benthol. Soc., 24(1):139-147. * Michaelson, D. L. and R. J. Neves. 1995. Life History and habitat of the endangered dwarf wedgemussel Alasmidonta heterodon (Bivalvia:Unionidae). Jour. N. Am. Benthol. Soc. 14:324-340. * Nedeau, E. J. 2004a. A Fourth Investigation of the Survival of Dwarf Wedgemussels (Alasmidonta heterodon) for the Relocation Project on the Connecticut River, Route 2 Stabilization Project, Lunenburg, Vermont. Unpublished report submitted to the U.S. Fish and Wildlife Service, Concord, New Hampshire. 7 pp. * Nedeau, E. J. 2004b. Quantitative survey of dwarf wedgemussel (Alasmidonta heterodon) populations downstream of the Surry Mountain Flood Control Dam on the Ashuelot River. . Unpublished report submitted to the U.S. Fish and Wildlife Service, Concord, New Hampshire. 12 pp. * Nedeau, E. 2006. Characterizing the Range and Habitat of Dwarf Wedgemussels in the “Middle Macrosite” of the Upper Connecticut River. Unpublished report submitted to the U.S. Fish and Wildlife Service, Concord, New Hampshire. 6 pp. * Nedeau, E. J. and S. Werle. 2003. Freshwater Mussels of the Ashuelot River: Keene to Hinsdale. Unpublished report submitted to the U.S. Fish and Wildlife Service, Concord, New Hampshire. 50 pp. * Strayer, D. L., S. J. Sprague and S. Claypool. 1996. A range-wide assessment of populations of the dwarf wedgemussel Alasmidonta heterodon. J. N. Am. Benthol. Soc. 15(3):308-317. * U.S. Fish and Wildlife Service. 1993. Dwarf Wedge Mussel Alasmidonta heterodon Recovery Plan. Hadley, Massachusetts. 52 pp. * Wicklow, B. 2005. in New Hampshire Wildlife Action Plan. New Hampshire Fish and Game Department, 11 Hazen Dr., Concord, NH, 03301. pp. A26-A35. {{Authority control Alasmidonta IUCN Red List vulnerable species Bivalves described in 1830 Taxa named by Isaac Lea