Dwarf wedgemussel
Encyclopedia
The dwarf wedgemussel, scientific name Alasmidonta heterodon, is an endangered species
of freshwater mussel
, an aquatic
bivalve mollusk in the family Unionidae
, the river mussels.
's Atlantic coast
streams and rivers of various sizes and moderate current.
The dwarf wedge mussel's current range extends from New Hampshire
to North Carolina
. The dwarf wedge mussel is federally-listed as endangered, and state-listed as endangered, in Connecticut
, Massachusetts
, Maryland
, North Carolina
, New Hampshire
, New Jersey
, New York
, Pennsylvania
, Virginia
, and Vermont
.
It previously lived in New Brunswick
, but it is locally extinct in Canada since 1968. The Canadian Species at Risk Act
listed the Dwarf Wedge Mussel in the List of Wildlife Species at Risk as being extirpated
in Canada
.
is bluish or silvery white, and is iridescent on the posterior.
A number of fish species have been positively identified as hosts for the dwarf wedgemussel. Michaelson and Neves (1995) confirmed the tessellated darter (Etheostoma olmstedi), Johnny darter
(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
(. congatus) and juvenile and parr of the Atlantic salmon
(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
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.
, 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
in New Hampshire and Vermont
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 New Jersey
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, Maryland
, 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 Delaware and Neversink Rivers in Pennsylvania
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
watershed affected by the recent floods of 2005 is uncertain at this time.
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...
of freshwater mussel
Mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...
, an aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
bivalve mollusk in the family Unionidae
Unionidae
Unionidae is a family of freshwater mussels, the largest in the order Unionoida, the bivalve mollusks sometimes known as river mussels, naiads, or simply as unionids.The range of distribution for this family is world-wide...
, the river mussels.
Distribution and conservation status
This is a rare species found solely in North AmericaNorth America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
's Atlantic coast
Atlantic Coast
The Atlantic Coast is any coast fronting the Atlantic Ocean. The term differentiates the coasts of countries or continents with coastlines on more than one body of water, such as North America, South America, Africa and Europe.-See also:*Indian Ocean...
streams and rivers of various sizes and moderate current.
The dwarf wedge mussel's current range extends from New Hampshire
New Hampshire
New Hampshire is a state in the New England region of the northeastern United States of America. The state was named after the southern English county of Hampshire. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Atlantic Ocean to the east, and the Canadian...
to North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
. The dwarf wedge mussel is federally-listed as endangered, and state-listed as endangered, in Connecticut
Connecticut
Connecticut is a state in the New England region of the northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, and the state of New York to the west and the south .Connecticut is named for the Connecticut River, the major U.S. river that approximately...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
, Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
, North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
, New Hampshire
New Hampshire
New Hampshire is a state in the New England region of the northeastern United States of America. The state was named after the southern English county of Hampshire. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Atlantic Ocean to the east, and the Canadian...
, New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
, New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
, Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
, Virginia
Virginia
The Commonwealth of Virginia , is a U.S. state on the Atlantic Coast of the Southern United States. Virginia is nicknamed the "Old Dominion" and sometimes the "Mother of Presidents" after the eight U.S. presidents born there...
, and Vermont
Vermont
Vermont is a state in the New England region of the northeastern United States of America. The state ranks 43rd in land area, , and 45th in total area. Its population according to the 2010 census, 630,337, is the second smallest in the country, larger only than Wyoming. It is the only New England...
.
It previously lived in New Brunswick
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...
, but it is locally extinct in Canada since 1968. The Canadian Species at Risk Act
Species at Risk Act
The Species at Risk Act is a piece of Canadian federal legislation which became law in Canada on December 12, 2002. It is designed to meet one of Canada's key commitments under the International Convention on Biological Diversity. The goal of the Act is to protect endangered or threatened...
listed the Dwarf Wedge Mussel in the List of Wildlife Species at Risk as being extirpated
Local extinction
Local extinction, also known as extirpation, is the condition of a species which ceases to exist in the chosen geographic area of study, though it still exists elsewhere...
in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
.
Biotope
This mussel may be found in small creeks to deep rivers in stable habitat 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 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 45 mm in length and 25 mm high. The trapezoidal-shaped shells are colored brown or yellowish-olive, with reddish brown or greenish rays in young or pale specimens. The nacreNacre
Nacre , also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks as an inner shell layer; it is also what makes up pearls. It is very strong, resilient, and iridescent....
is bluish or silvery white, and is iridescent 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 (Etheostoma olmstedi), Johnny darter
Johnny darter
The Johnny Darter, Etheostoma nigrum, is a species of ray-finned fish that are found in shallow water throughout eastern North America. It's name comes from the Greek and Latin root words of etheo meaning to filter, stoma meaning mouth, nigrum meaning black in Latin...
(Etheostoma nigrum), and mottled sculpin
Mottled sculpin
The mottled sculpin, Cottus bairdii, is a freshwater sculpin found widely although unevenly throughout North America.As the name suggests, its coloration is a combination of bars, spots, and speckles randomly distributed. The large pectoral fins are banded...
(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
Slimy Sculpin
The slimy sculpin, Cottus cognatus, is a freshwater species of fish that inhabits cool, rocky streams, rivers and lakes throughout northern North America and eastern Siberia. The slimy sculpin is found in freshwater and sometimes brackish water in areas with rocky or gravel type bottoms...
(. congatus) and juvenile and parr of the Atlantic salmon
Atlantic salmon
The Atlantic salmon is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and the north Pacific....
(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
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...
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 New BrunswickNew Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...
, 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
Connecticut River
The Connecticut River is the largest and longest river in New England, and also an American Heritage River. It flows roughly south, starting from the Fourth Connecticut Lake in New Hampshire. After flowing through the remaining Connecticut Lakes and Lake Francis, it defines the border between the...
in New Hampshire and Vermont
Vermont
Vermont is a state in the New England region of the northeastern United States of America. The state ranks 43rd in land area, , and 45th in total area. Its population according to the 2010 census, 630,337, is the second smallest in the country, larger only than Wyoming. It is the only New England...
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
Farmington River
The Farmington River is a river located in northwest Connecticut, with major tributaries extending into southwest Massachusetts. Via its longest branch , the Farmington's length increases to , making it the Connecticut River's longest tributary by a mere over the major river directly to its...
in Connecticut, and the Neversink River
Neversink River
The Neversink River is a tributary of the Delaware River in southeastern New York in the United States...
in New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
harbor large populations, but these number in the thousands only. The remaining populations from New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...
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, Maryland
Maryland
Maryland is a U.S. state located in the Mid Atlantic region of the United States, bordering Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware to its east...
, 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 Delaware and Neversink Rivers in Pennsylvania
Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...
and New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
, 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
Cuddebackville Dam
The Cuddebackville Dam was a concrete dam on the Neversink River in Orange County, New York near the town of Cuddebackville, removed in October, 2004 to benefit aquatic life. It was the first dam removed in New York State for environmental reasons.-History:...
, 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
Delaware River
The Delaware River is a major river on the Atlantic coast of the United States.A Dutch expedition led by Henry Hudson in 1609 first mapped the river. The river was christened the South River in the New Netherland colony that followed, in contrast to the North River, as the Hudson River was then...
watershed affected by the recent floods of 2005 is uncertain at this time.