Alasmidonta heterodon - (I. Lea, 1830)
Dwarf Wedgemussel
Other English Common Names: dwarf wedgemussel
Taxonomic Status: Accepted
Related ITIS Name(s): Alasmidonta heterodon (I. Lea, 1830) (TSN 79917)
Unique Identifier: ELEMENT_GLOBAL.2.108301
Element Code: IMBIV02030
Informal Taxonomy: Animals, Invertebrates - Mollusks - Freshwater Mussels
Image 12031

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Kingdom Phylum Class Order Family Genus
Animalia Mollusca Bivalvia Unionoida Unionidae Alasmidonta
Genus Size: C - Small genus (6-20 species)
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Concept Reference
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Concept Reference: Turgeon, D.D., J.F. Quinn, Jr., A.E. Bogan, E.V. Coan, F.G. Hochberg, W.G. Lyons, P.M. Mikkelsen, R.J. Neves, C.F.E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F.G. Thompson, M. Vecchione, and J.D. Williams. 1998. Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks. 2nd Edition. American Fisheries Society Special Publication 26, Bethesda, Maryland: 526 pp.
Concept Reference Code: B98TUR01EHUS
Name Used in Concept Reference: Alasmidonta heterodon
Conservation Status
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NatureServe Status

Global Status: G1G2
Global Status Last Reviewed: 31Dec2011
Global Status Last Changed: 01Jun1998
Rounded Global Status: G1 - Critically Imperiled
Reasons: Historically, this species was widespread, though never common, along the Atlantic Slope from New Brunswick to the Carolinas. The species has experienced significant decline including regional extirpations such as the last remaining population in Canada and there are only a small number of extant occurrences remaining whose long-term viability is questionable, given continuing declines and difficult-to-manage threats. Decline has continued, especially over the last 10 years to where it currently occupies only 20-25% of the sites it once occupied, with populations severely fragmented. Declines are even more pronounced from New Jersey south to North Carolina with individual populations numbering only in the tens to hundreds of individuals. The species continues to face significant threats from habitat loss primarily due to human encroachment throughout its range and, without intervention, may decline to the point of critical imperilment soon.
Nation: United States
National Status: N1N2 (14Jul2006)
Nation: Canada
National Status: NX (01Aug2017)

U.S. & Canada State/Province Status
Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
United States Connecticut (S1), Delaware (SH), District of Columbia (SH), Maryland (S1), Massachusetts (S1), New Hampshire (S1), New Jersey (S1), New York (S1), North Carolina (S1), Pennsylvania (S1), Vermont (S1), Virginia (S1)
Canada New Brunswick (SX)

Other Statuses

U.S. Endangered Species Act (USESA): LE: Listed endangered (14Mar1990)
U.S. Fish & Wildlife Service Lead Region: R5 - Northeast
Canadian Species at Risk Act (SARA) Schedule 1/Annexe 1 Status: XT (05Jun2003)
Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Extirpated (27Nov2009)
Comments on COSEWIC: Extirpated by 1968. Designated Extirpated in April 1999. Status re-examined and confirmed in May 2000 and November 2009.
IUCN Red List Category: VU - Vulnerable
American Fisheries Society Status: Endangered (01Jan1993)

NatureServe Global Conservation Status Factors

Range Extent: 20,000-200,000 square km (about 8000-80,000 square miles)
Range Extent Comments: This species is discontinuously distributed in the Atlantic coast drainages from Maine to North Carolina. The population in the Petitcodiac River in New Brunswick has been searched for since 1994 but not relocated (Hanson and Locke, 2000) and is likely extirpated (Metcalfe-Smith and Cadmore-Vokey, 2004), thereby restricting its current range to the United States only. Historically known from approximately 70 sites in streams, the species is now known from 25-30 streams and rivers (Strayer et al., 1996). It is extant in ten states and likely extirpated from Canada and possibly Pennsylvania. Throughout its range, especially the north Atlantic Slope, populations have declined to only a few viable occurrences in each of the remaining states where it is extant.

Area of Occupancy: 501-12,500 4-km2 grid cells
Area of Occupancy Comments:  

Number of Occurrences: 6 - 20
Number of Occurrences Comments: jerIn the late 1990s, this species was known from 25-30 streams and rivers (Strayer et al., 1996); but likely occurs in only 20 remaining sites. Viability is questionable at most (nearly all) sites. Historically, more than 70 occurrences were known, and all were searched from 1983-1985. About 100 years ago the species was known from about 70 Atlantic seaboard river systems, but now from only 25-30 (USFWS, 1993). However, since the Recovery Plan was published (USFWS, 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. Of the 70- 80 known locales in 2004 only 16 were believed to support reproducing populations, while 31 were based on observations of of five or fewer individuals, or solely on spent shells (Strayer et al., 1996). Also, some 45 of the 70 historical sites from USFWS (1993) are based on less than 5 individuals or spent shells (USFWS, 2007). Where it occurs, it forms distinct, widely separate patches. The species is extirpated from its only location in New Brunswick (Athearn, 1961), Canada (Hanson and Locke, 2000). In Massachusetts, it is almost extirpated, confined to only a few streams in the Connecticut River drainage system (Smith, 2000) and is also extremely rare in Vermont with a few shells known recently from the Connecticut River (and also on the other side of the river in New Hampshire- NH NHP, pers. comm., 2007) in Claremont, Northumberland, Windsor, Hartland, Springfield, Brattleboro, Dummerston, Rockingham, Weathersfield (Fichtel and Smith, 1995; VT NHP) and historically at Essex, Bloomfield and Thetford; also a relocation site at South Lunenberg on the Connecticut River in Essex Co. (VT NHP, pers. comm., 2007). Three New Hampshire occurrences mentioned in USFWS (1993) now function as one large (10+ mile) occurrence (USFWS, 2007). In Connecticut it is known from only the Connecticut River and a few tributaries such as the Farmington (Nedeau and Victoria, 2003). In the Delaware River basin, it has been recoreded in the Neversink River (Middle Delaware- Mongaup- Brodhead drainage) in New York (Strayer and Ralley, 1991). In the Delmarva peninsula, this species was found in Norwich Creek (Choptank River system) in Queen Anne's Co., Maryland (Counts et al., 1991). In Maryland, it has been reported from the Washington Metro, Lower Potomac River, and Chester River drainages (Bogan and Proch, 1995). In North Carolina, it is known from the Neuse and Pamlico River basins (Bogan, 2002; Athearn, 1992) in Franklin, Granville, Halifax, Johnston, Nash, Orange, Person, Pitt, Vance, Wake, Warren, and Wilson Cos. (LeGrand et al., 2006). Historical Maryland records exist for St. Mary's Co. (Athearn, 1992). It is only known from a single site in the Pequest River (anonymous, 1995; 2000; Strano et al., 1996) and Delaware River (Delaware-Mongaup-Brodhead drainage), Sussex Co. (anonymous, 2000; NJ NHP, pers. comm., 2007), in New Jersey. Although formerly thought to be extirpated in Pennsylvania (Bogan, 1993), this species occurs in the Upper Delaware drainage (at six or more locations in the upper Delaware River, Wayne County and from north of Equinunk, Pennsylvania to south of Callicoon, New York- PA NHP, pers. comm., 2006) with historical occurrences in the Crosswicks-Neshaminy, Lehigh, Schuylkill, Middle Delaware-Musconetcong, and Lower Susquehanna drainages (USFWS, 1993; PA NHP, pers. comm., 2006). In Virginia, extant occurrences are known from the Nottaway, Mattaponi, Lower Potomac, and Lower Rappahannock basins (VA NHP, pers. comm., 2007) with historical occurrences in the Pamunkey, Rapidon-Upper Rappahannock, and Maury basins (Ortmann, 1919; Clarke, 1981).

Population Size: 100,000 - 1,000,000 individuals
Population Size Comments: It is important to note that because a portion of the population is always found below the substrate, population estimates must take into account undetected mussels (USFWS, 2007). Strayer et al. (1996) studied 13 streams throughout the species' range and concluded that all populations had low densities (CPUE = 0-4 mussels/hr.), although five to six of the populations were large (1,000 to 100,000 animals). All populations studied had evidence of recent reproduction. During a 1997 survey of the Connecticut River in Massachusetts, 585 individuals were found in a 500 by 30 ft. area with 50% or more of the individuals submerged in the substrate. No historical population estimates exist, but findings by Strayer et al. (1996) are similar to observations by Ortmann (1919) and Clark (1981) that it forms sparse populations and was never numerous. In at least one site it was historically much more abundant (e.g., at one site in New Hampshire, 100 specimens were collected in 15 minutes in 1954; in 1989, only 1 specimen could be expected to be found in 15 minutes of searching). The Neversink River population in New York, at 20,000 individuals, is one of the largest known, but it dropped by an estimated 60,000 individuals between 1990 to 1994 (80,000 individuals to 50,000 in 1991, then to 20,000 in 1994) and continues to be vulnerable to a myraid of stressors that affect aquatic systems (Strayer et al., 1996). McLain and Ross (2005) found only one small stretch of the Mill River (of 5 study sites all within 10 km = single occurrence) had juvenile density high enough to be considered adequately viable (CPUE 25.6/h; 46.3% gravid, juvenile density 1.7/square meter). During mussel surveys of the Upper Delaware Scenic and Recreation River from July to August 2000, the dwarf wedgemussel was discovered at six locations in the upper Delaware River, Wayne County while thirteen live specimens and four shells were located from north of Equinunk, Pennsylvania to south of Callicoon, New York; and subsequent surveys in 2001 and 2002 have documented additional occurrences in the Upper Delaware River basin (PA NHP, pers. comm., 2006). Three linked patch sites on the Connecticut River on the Vermont/New Hampshire border also have decent viability (VT NHP) and this is likely the largest population (perhaps a few hundred thousand in a 75 km stretch in 3 patches) (USFWS, 2007). The Ashuelot River (NH), Farmington River (CT), and Neversink River (NY) harbor large populations but only in the thousands, while the ramaining populations from New Jersey south to North Carolina have a few individuals to a few hundred individuals (USFWS, 2007). Most recently, resurveys of several sites revealed the following: Connecticut River (NH/VT) population present and additional populations found with higher densities, Ashuelot River (NH) population present with densities from 0.31 to 1.257, Neversink River (NY) population present but affected by 2005 floods; McIntosh Run (MD) population present with no change, Aquia Creek (VA) no live animals since 2003 and believed to be in decline, Po River (VA) population present in very low numbers), Tar River/Shelton Creek (NC) population present and no change, Crooked Creek (NC) only 1 live in 2004, Little River (NC) population absent, Swift Creek (NC) population present but only 3 animals found 2002, and Moccasin Creek (NC) population presence questionable and only spent shells found (USFWS, 2007).

Number of Occurrences with Good Viability/Integrity: Very few to few (1-12)
Viability/Integrity Comments: There are very few populations with good viability but reproduction has been documented for the largest populations in New Hampshire (Ashuelot River), Massachusetts (Connecticut River), Connecticut (Farmington and lower Connecticut River) and New York (Neversink River) (USFWS, 2007). The Neversink River population in New York, though it occurs in only a short stretch of the river, is probably the best population globally (at 20,000 individuals, down from 80,000) though it is threatened with habitat alteration (USFWS, 1993; Strayer et al., 1996). Populations in New Hampshire, as well as Connecticut (Cordeiro, pers. obs.) are very small with poor viability. A population in Massachusetts on the Connecticut River is viable but limited to a very short stretch of the Mill River with only a single portion of that stretch exhibiting decent abundance and reproductive output (CPUE 25.6/h; 46.3% gravid, juvenile density 1.7/square meter) (McLain and Ross, 2005). A relocated population in South Lunenberg, Vermont, as well as a couple other sites on the Connecticut River on the Vermont/New Hampshire border also have decent viability (VT NHP) and this is likely the largest population (perhaps a few hundred thousand in a 75 km stretch in 3 patches) (USFWS, 2007). Viability of new populations in the Upper Delaware River in Pennsylvania have not been assessed. The Swift Creek (part of Neuse River watershed) is likely further impacted due to increased development pressure as a result of expanded water treatment facility; and recent surveys of the nearby Little River indicate there are few if any individuals remaining (USFWS, 2007).

Overall Threat Impact: Very high - high
Overall Threat Impact Comments: 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 many Atlantic slope watersheds. Chemical and organic pollution, siltation, removal of stream bank vegetation, and impounding and regulating water flow of major rivers apparently continue to impact the species, as well as poor land use practices and urbanization in proximity to extant populations. By far the largest threat is damming and channelization of rivers throughout the species' range. Siltation, generated by road construction, agriculture, forestry activities, and removal of streambank vegetation is also an important factor in decline. Historically, and in some cases now, agricultural, domestic, and industrial pollution have also contributed to this species' decline. Collecting may also be a potential threat (USFWS, 1993; Strayer et al., 1996). The Swift Creek (part of Neuse River watershed) is likely further impacted due to increased development pressure as a result of expanded water treatment facility; and recent surveys of the nearby Little River indicate there are few if any individuals remaining (USFWS, 2007). Added threats listed in the recovery plan 5-year review (USFWS, 2007) include agricultural run-off and flooding resulting in loss of habitat.

Short-term Trend: Decline of 50-70%
Short-term Trend Comments: About 100 years ago the species was known from about 70 Atlantic seaboard river systems, but now from only 25-30 (USFWS, 1993). However, since the Recovery Plan was published (USFWS, 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. Of the 70- 80 known locales in 2004 only 16 were believed to support reproducing populations, while 31 were based on observations of of five or fewer individuals, or solely on spent shells (Strayer et al., 1996). Historically known from approximately 70 sites in streams, the species is now known from 25-30 streams and rivers (Strayer et al. 1996) and is considered viable in only very few of these. It is extant in ten states and likely extirpated from Canada (Metcalfe-Smith and Cadmore-Vokey, 2004; Hanson and Locke, 2000) where it formerly occurred in New Brunswick (Davis, 1999) in a branch of the Petitcodiac River (Athearn, 1963) and possibly Pennsylvania and is nearly extirpated from Massachusetts and Connecticut. The Petitcodiac River, New Brunswick, population was considered "common" based on 1960s surveys but subsequent surveys in 1984 and 1997/1998 failed to locate specimens and it is now considered extirpated (Hanson and Locke, 2000; 2001). This population was geographically isolated from U.S. populations (possibly for as long as 50,000 years) and may have developed unique genetic characteristics (Canada Department of Fisheries and Oceans, 2007). The number of occurrences and abundance have declined dramatically as indicated above. Significant declines continued throughout the 1980's. The species appears to be declining most in the southern extent of its range. Most recently, resurveys of several sites revealed the following: Connecticut River (NH/VT) population present and additional populations found with higher densities, Ashuelot River (NH) population present with densities from 0.31 to 1.257, Neversink River (NY) population present but affected by 2005 floods; McIntosh Run (MD) population present with no change, Aquia Creek (VA) no live animals since 2003 and believed to be in decline, Po River (VA) population present in very low numbers), Tar River/Shelton Creek (NC) population present and no change, Crooked Creek (NC) only 1 live in 2004, Little River (NC) population absent, Swift Creek (NC) population present but only 3 animals found 2002, and Moccasin Creek (NC) population presence questionable and only spent shells found (USFWS, 2007).

Long-term Trend: Decline of 50-70%
Long-term Trend Comments: Historically known from approximately 70 sites in streams, the species is now known from 25-30 streams and rivers, many of which are not viable (Strayer et al. 1996). It is extant in ten states and likely extirpated from Canada (Hanson and Locke, 2000; Metcalfe-Smith and Cadmore-Vokey, 2004) and possibly Pennsylvania and is nearly extirpated from Massachusetts and Connecticut. In Vermont,it was historically found from Bloomfield to Brattleboro in the Connecticut system (Hart et al., 2005).

Intrinsic Vulnerability: Highly vulnerable
Intrinsic Vulnerability Comments: Many populations, especially in the southern portion of the range, may be threatened by low densities, small ranges, and linear structure (Strayer et al., 1996; USFWS, 2007). Apparently, this species is sensitive to disappearance or decline of host fish(es); as host fish are largely sedentary with very poor dispersal capability limited to only their immediate vicinity (meters) with a narrow window of host infection time during each year (McLain and Ross, 2005). Reliance on the tessellated darter as a host may be advantageous because of high darter abundance, but limited darter dispersal may cause patchy distributions of mussels, leading to metapopulations.

Environmental Specificity: Narrow. Specialist or community with key requirements common.
Environmental Specificity Comments: Apparently, this species is relatively sensitive to pollution, siltation, low dissolved oxygen and is limited to flowing water conditions only (McLain and Ross, 2005).

Other NatureServe Conservation Status Information

Inventory Needs: Determine extent of existing populations, and continue searching for new occurrences.

Protection Needs: All populations should receive protection through acquisition, easements, registry, and/or working with local, state, and federal government agencies on issues relating to zoning and streamside development, water quality, regulation of water flows, land use practices, etc.

Distribution
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Global Range: (20,000-200,000 square km (about 8000-80,000 square miles)) This species is discontinuously distributed in the Atlantic coast drainages from Maine to North Carolina. The population in the Petitcodiac River in New Brunswick has been searched for since 1994 but not relocated (Hanson and Locke, 2000) and is likely extirpated (Metcalfe-Smith and Cadmore-Vokey, 2004), thereby restricting its current range to the United States only. Historically known from approximately 70 sites in streams, the species is now known from 25-30 streams and rivers (Strayer et al., 1996). It is extant in ten states and likely extirpated from Canada and possibly Pennsylvania. Throughout its range, especially the north Atlantic Slope, populations have declined to only a few viable occurrences in each of the remaining states where it is extant.

U.S. States and Canadian Provinces

Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
Color legend for Distribution Map
Endemism: occurs (regularly, as a native taxon) in multiple nations

U.S. & Canada State/Province Distribution
United States CT, DC, DE, MA, MD, NC, NH, NJ, NY, PA, VA, VT
Canada NBextirpated

Range Map
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U.S. Distribution by County Help
State County Name (FIPS Code)
CT Hartford (09003)
DE Kent (10001)*
MA Bristol (25005)*, Essex (25009)*, Franklin (25011), Hampden (25013)*, Hampshire (25015), Plymouth (25023)*
MD Caroline (24011), Charles (24017), Montgomery (24031)*, Queen Annes (24035), St. Marys (24037), Talbot (24041)
NC Franklin (37069), Granville (37077), Halifax (37083), Johnston (37101), Nash (37127), Orange (37135)*, Vance (37181), Wake (37183), Warren (37185), Wilson (37195)
NH Cheshire (33005), Coos (33007), Grafton (33009), Sullivan (33019)
NJ Bergen (34003)*, Essex (34013)*, Mercer (34021)*, Morris (34027)*, Sussex (34037), Warren (34041)
NY Delaware (36025), Dutchess (36027), Orange (36071), Sullivan (36105)
PA Bucks (42017)*, Carbon (42025)*, Chester (42029)*, Lancaster (42071)*, Montgomery (42091)*, Philadelphia (42101)*, Pike (42103), Wayne (42127)
VA Culpeper (51047)*, Fauquier (51061)*, Hanover (51085)*, Lexington (City) (51678)*, Louisa (51109), Lunenburg (51111), Nottoway (51135), Orange (51137)*, Rockbridge (51163)*, Spotsylvania (51177), Stafford (51179), Sussex (51183)
VT Caledonia (50005), Essex (50009), Orange (50017), Windham (50025), Windsor (50027)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
01 Merrimack (01070002)*, Upper Connecticut (01080101)+, Waits (01080103)+, Upper Connecticut-Mascoma (01080104)+, White (01080105)+, Black-Ottauquechee (01080106)+, West (01080107)+, Middle Connecticut (01080201)+, Lower Connecticut (01080205)+, Westfield (01080206)*, Farmington (01080207)+, Cape Cod (01090002)*, Narragansett (01090004)+*, Housatonic (01100005)+
02 Lower Hudson (02030101)+*, Hackensack-Passaic (02030103)+*, Sandy Hook-Staten Island (02030104)+*, Upper Delaware (02040101)+, Middle Delaware-Mongaup-Brodhead (02040104)+, Middle Delaware-Musconetcong (02040105)+, Lehigh (02040106)+*, Crosswicks-Neshaminy (02040201)+*, Lower Delaware (02040202)*, Schuylkill (02040203)+, Lower Susquehanna (02050306)+*, Chester-Sassafras (02060002)+, Choptank (02060005)+, Patuxent (02060006)*, Middle Potomac-Catoctin (02070008)+*, Middle Potomac-Anacostia-Occoquan (02070010)*, Lower Potomac (02070011)+, Rapidan-Upper Rappahannock (02080103)+, Lower Rappahannock (02080104)+, Mattaponi (02080105)+, Pamunkey (02080106)+, Maury (02080202)+
03 Nottoway (03010201)+, Upper Tar (03020101)+, Fishing (03020102)+, Lower Tar (03020103), Pamlico (03020104), Upper Neuse (03020201)+, Middle Neuse (03020202), Contentnea (03020203)+, Lower Neuse (03020204)
BB BB-41 (BB-41)+*
CV CV-19 (CV-19)+*, CV-26 (CV-26)+*
NC NC-08 (NC-08)+*
WE WE-39 (WE-39)+*
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
U.S. Distribution by Watershed (based on multiple information sources) Help
Ecology & Life History
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Basic Description: A small freshwater mussel with a trapezoidal-shaped shell.
General Description: Small freshwater mussel, usually less than 45 mm length and 25 mm high. Shell subtrapezoidal, thick anteriorly and thinning posteriorly; ventral margin mostly straight; posterior margin pointed near base; dorsal margin slightly curved; beaks low and rounded, projecting only slightly above the hinge line; posterior ridge rounded, somewhat inflated and prominent; periostracum brownish or yellowish brown, with variable width reddish brown or greenish rays in young or pale colored specimens. Nacre bluish or silvery white, and iridescent posteriorly. Hinge teeth small but distinct; pseudocardinal teeth compressed, 1 or 2 in the right valve and 2 in the left; lateral teeth gently curved and reversed, that is, in most specimens, 2 in the right valve and 1 in the left. Ventral mantle margin plain; papillae flesh-colored; exhalent aperture without papillae. (Clarke, 1981; Smith, 1986; Bogan, 1993)
Diagnostic Characteristics: The small size, roundly pointed posterio-basal margin, and reversed lateral hinge teeth readily distinguish this species (Clarke, 1981).
Reproduction Comments: This species is a long term brooder that spawns in late summer and becomes gravid in the fall. In laboratory experiments, Michaelson and Neves (1995) indentified three fish species as possible glochicial hosts: tesselated darter, Etheostoma olmstedti, Johnny darter, Etheostoma nigrum, and mottled sculpin, Cottus bairdi. Schulz and Marbain (1998) recently also identified the slimy sculpin, Cottus cognatus, as a host fish. Wicklow (1999) identified the Atlantic salmon (Salmo salar) as a host, as well. Recent published work indicates (1) although it has multiple hosts, it prefers the tessellated darter, (2) darter hosts tend to remain close to the area where they were infested with mussel glochidia (order of meters) indicating low dispersal capability, (3) sites with high mussel patch density were associated with high juvenile recruitment, multiple size classes, high glochidial density on darter hosts (McLain and Ross, 2005; Strayer et al., 2006).
Habitat Type: Freshwater
Non-Migrant: Y
Locally Migrant: N
Long Distance Migrant: N
Mobility and Migration Comments: This species probably is rather sessile with only limited movement in the substrate. Passive downstream movement may occur when mussels are displaced from the substrate during floods. Major dispersal occurs while glochidia are encysted on their hosts.
Riverine Habitat(s): BIG RIVER, CREEK, MEDIUM RIVER, Moderate gradient
Special Habitat Factors: Benthic
Habitat Comments: Typically, this species is found in shallow to deep quick running water on cobble, fine gravel, or on firm silt or sandy bottoms. Other habitats included are amongst submerged aquatic plants, and near stream banks underneath overhanging tree limbs. USFWS (1993) states the species commonly lives on muddy sand, sand, and gravel bottoms in creeks and rivers of various sizes. It requires areas of slow to moderate current, good water quality, and little silt deposits.
Adult Food Habits: Detritivore
Immature Food Habits: Parasitic
Food Comments: Larvae (glochidia) of freshwater mussels generally are parasitic on fish and display varying degrees of host specificity. No specific trophic studies have been conducted on this species. General literature claims that mussels are filter-feeders that remove phytoplankton from the water column. These assumptions appear to be based on a casual observations of mussels in situ and a few examinations of rectal contents. Baker (1928) speculated that detritus was the primary energy source. This has been substantiated by James (1987) and correlates well with observed microhabitat utilization. This suggests that mussels may occupy a variety of trophic guilds such as postulated for the Sphaeriidae (see Lopez and Holopaien, 1987; Gordon and Layzer, 1989).
Phenology Comments: Little is known concerning the phenology of mussels other than when eggs/glochida are held in the branchial marsupia. Being poikilothermic, activity levels would expectedly be reduced greatly during cold-temperature months.
Length: 4.5 centimeters
Economic Attributes Not yet assessed
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Management Summary
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Stewardship Overview: This species was listed as federally endangered in the U.S. in 1990. A recovery plan has been drafted for this species (USFWS, 1993) with the following objectives: (1) reclassify from endangered to threatened status when the likelihood of extinction has been eliminated (populations in mainstem Connecticut River, Ashuelot River, Neversink River, upper Tar River, Little River, Swift Creek (Neuse system), and Turkey Creek, as well as six other rivers shown to be viable), (2) remove from federal list when criteria are met (at least ten of rivers or creeks must support a viable population widely enough dispersed within its habitat such that a single adverse event in a given river would be unlikely to result in total loss of that river's population; all populations must be protected from present and foreseeable anthropogenic and natural threats that could interfere with survival). Recovery plan criteria include: (1) the following populations of the DWM must be shown to be viable: mainstem Connecticut River (NH/VT), Ashuelot River (NH), Neversink River (NY), Upper Tar River (NC), Little River (NC), Swift Creek (NC), Turkey Creek (NC), and six other rivers/creeks representative of the species range; (2), at least 10 of the rivers/creeks must support a widely dispersed viable population so that a single catastrophic event in a given river will be unlikely to result in the total loss of that river's population; (3) the rivers should be distributed throughotu teh species' current range with at least two in New England, one in New York, and four south of Pennsylvania; (4) all populations referred must be protected from present and foreseeable anthropogenic and natural threats that could interfere with their survival.

In Canada, this species was designated as extirpated by COSEWIC in 2000. Recovery in Canada is considered not feasible (Canada Department of Fisheries and Oceans, 2007). In order for recovery to be feasible, (1) the causeway would have to be re-engineered to permit fish passage, (2) the fish host species would have to be re-established in the Petitcodiac system, (3) dwarf wedgemussels from another population or from captivity would have to be reintroduced in numbers adequate to permit establishment of a viable population. Microsatellite marklers have been developed for this species (Shaw et al., 2006).

The 5-year review (USFWS, 2007) has determined that criterion 1 (from the recovery plan, USFWS, 1993) has been partially met as viable populations have been found in the mainstem Connecticut River and Ashuelot River; while criteria 2-4 have not been met.

Biological Research Needs: Investigate the effects of low population densities on reproduction rates; determine sensitivities to land use and pollution threats.
Population/Occurrence Delineation
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Group Name: Freshwater Mussels

Use Class: Not applicable
Minimum Criteria for an Occurrence: Occurrences are based on some evidence of historical or current presence of single or multiple specimens, including live specimens or recently dead shells (i.e., soft tissue still attached and/or nacre still glossy and iridescent without signs of external weathering or staining), at a given location with potentially recurring existence. Weathered shells constitute a historic occurrence. Evidence is derived from reliable published observation or collection data; unpublished, though documented (i.e. government or agency reports, web sites, etc.) observation or collection data; or museum specimen information.
Mapping Guidance: Based on the separation distances outlined herein, for freshwater mussels in STANDING WATER (or backwater areas of flowing water such as oxbows and sloughs), all standing water bodies with either (1) greater than 2 km linear distance of unsuitable habitat between (i.e. lotic connections), or (2) more than 10 km of apparently unoccupied though suitable habitat (including lentic shoreline, linear distance across water bodies, and lentic water bodies with proper lotic connections), are considered separate element occurrences. Only the largest standing water bodies (with 20 km linear shoreline or greater) may have greater than one element occurrence within each. Multiple collection or observation locations in one lake, for example, would only constitute multiple occurrences in the largest lakes, and only then if there was some likelihood that unsurveyed areas between collections did not contain the element.

For freshwater mussels in FLOWING WATER conditions, occurrences are separated by a distance of more than 2 stream km of unsuitable habitat, or a distance of more than 10 stream km of apparently unoccupied though suitable habitat. Standing water between occurrences is considered suitable habitat when calculating separation distance for flowing water mussel species unless dispersal barriers (see Separation Barriers) are in place.

Several mussel species in North America occur in both standing and flowing water (see Specs Notes). Calculation of separation distance and determination of separation barriers for these taxa should take into account the environment in which the element was collected. Juvenile mussels do not follow this pattern and juveniles are typically missed by most standard sampling methods (Hastie and Cosgrove, 2002; Neves and Widlak, 1987), therefore juvenile movement is not considered when calculating separation distance.

Separation Barriers: Separation barriers within standing water bodies are based solely on separation distance (see Separation Distance-suitable, below). Separation barriers between standing water bodies and within flowing water systems include lack of lotic connections, natural barriers such as upland habitat, absence of appropriate species specific fish hosts, water depth greater than 10 meters (Cvancara, 1972; Moyle and Bacon, 1969) or anthropogenic barriers to water flow such as dams or other impoundments and high waterfalls.
Separation Distance for Unsuitable Habitat: 2 km
Separation Distance for Suitable Habitat: 10 km
Alternate Separation Procedure: None
Separation Justification: Adult freshwater mussels are largely sedentary spending their entire lives very near to the place where they first successfully settled (Coker et al., 1921; Watters, 1992). Strayer (1999) demonstrated in field trials that mussels in streams occur chiefly in flow refuges, or relatively stable areas that displayed little movement of particles during flood events. Flow refuges conceivably allow relatively immobile mussels to remain in the same general location throughout their entire lives. Movement occurs with the impetus of some stimulus (nearby water disturbance, physical removal from the water such as during collection, exposure conditions during low water, seasonal temperature change or associated diurnal cycles) and during spawning. Movement is confined to either vertical movement burrowing deeper into sediments though rarely completely beneath the surface, or horizontal movement in a distinct path often away from the area of stimulus. Vertical movement is generally seasonal with rapid descent into the sediment in autumn and gradual reappearance at the surface during spring (Amyot and Downing, 1991; 1997). Horizontal movement is generally on the order of a few meters at most and is associated with day length and during times of spawning (Amyot and Downing, 1997). Such locomotion plays little, if any, part in the distribution of freshwater mussels as these limited movements are not dispersal mechanisms. Dispersal patterns are largely speculative but have been attributed to stream size and surface geology (Strayer, 1983; Strayer and Ralley, 1993; van der Schalie, 1938), utilization of flow refuges during flood stages (Strayer, 1999), and patterns of host fish distribution during spawning periods (Haag and Warren, 1998; Watters, 1992). Lee and DeAngelis (1997) modeled the dispersal of freshwater into unoccupied habitats as a traveling wave front with a velocity ranging from 0.87 to 2.47 km/year (depending on mussel life span) with increase in glochidial attachment rate to fish having no effect on wave velocity.

Nearly all mussels require a host or hosts during the parasitic larval portion of their life cycle. Hosts are usually fish, but a few exceptional species utilize amphibians as hosts (Van Snik Gray et al., 2002; Howard, 1915) or may metamorphose without a host (Allen, 1924; Barfield et al., 1998; Lefevre and Curtis, 1911; 1912). Haag and Warren (1998) found that densities of host generalist mussels (using a variety of hosts from many different families) and displaying host specialists (using a small number of hosts usually in the same family but mussel females have behavioral modifications to attract hosts to the gravid female) were independent of the densities of their hosts. Densities of non-displaying host specialist mussels (using a small number of hosts usually in the same family but without host-attracting behavior) were correlated positively with densities of their hosts. Upstream dispersal of host fish for non-displaying host specialist mussels could, theoretically, transport mussel larvae (glochidia) over long distances through unsuitable habitat, but it is unlikely that this occurs very often. D. Strayer (personal communication) suggested a distance of at least 10 km, but a greater distance between occurrences may be necessary to constitute genetic separation of populations. As such, separation distance is based on a set, though arbitrary, distance between two known points of occurrence.

Date: 18Oct2004
Author: Cordeiro, J.
Notes: Contact Jay Cordeiro (jay_cordeiro@natureserve.org) for a complete list of freshwater mussel taxa sorted by flow regime.
Population/Occurrence Viability
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U.S. Invasive Species Impact Rank (I-Rank) Not yet assessed
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Authors/Contributors
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NatureServe Conservation Status Factors Edition Date: 23Dec2011
NatureServe Conservation Status Factors Author: Cordeiro, J. (2011); Master, L.; Morrison, M. (1998)
Management Information Edition Date: 30Sep2008
Management Information Edition Author: Cordeiro, J.
Element Ecology & Life History Edition Date: 23Dec2011
Element Ecology & Life History Author(s): Cordeiro, J. (2011); MASTER, L. (1994)

Zoological data developed by NatureServe and its network of natural heritage programs (see Local Programs) and other contributors and cooperators (see Sources).

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References for Watershed Distribution Map
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