Alasmidonta heterodon 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Bivalvia Unionoida Unionidae

Scientific Name: Alasmidonta heterodon
Species Authority: Lea, 1830
Common Name(s):
English Dwarf Wedgemussel, Dwarf Wedge Mussel
Taxonomic Notes: It is possible that genetic analyses will split this species into more than one taxon (Bogan 2000). Based on the analysis of the gene sequences, Bogan et al. (2008) have placed Alasmidonta heterodon in a clade separate from the type species of Alasmidonta, A. undulata. They argue for the elevation of the subgenus Prolasmidonta (Ortmann, 1914) with the type species Alasmidonta heterdon, to create Prolasmidonta heterodon.

A list of synonyms for this species can be found on The MUSSEL project web site (Graf and Cummings 2011).

Assessment Information [top]

Red List Category & Criteria: Vulnerable B2ab(i,ii,iii,iv) ver 3.1
Year Published: 2011
Date Assessed: 2011-07-22
Assessor(s): Cummings, K. & Cordeiro, J.
Reviewer(s): Böhm, M. & Collen, B.
Contributor(s): Dyer, E., Soulsby, A.-M., Whitton, F., McGuinness, S., De Silva, R., Milligan, H.T., Kasthala, G., Herdson, R., Thorley, J., McMillan, K. & Collins, A.
Alasmidonta heterdon has been assessed as Vulnerable. This species continues to be threatened throughout its range, although the threat level is generally more severe in southern areas. Declining populations and loss of viable habitat in the southern portions of its range are not compensated for by the extensive, but geographically-limited populations found in New Hampshire. Decline has continued, especially over the last 10 years, and it is inferred that A. heterdon currently occupies only 20–25% of the sites it once occupied, with populations severely fragmented. As a result, its area of occupancy is estimated as between 500 and 2,000 km², thus qualifying the species for a Vulnerable listing. Without significant recovery activities targeted at southern populations, there is a real possibility of further range contraction.
Previously published Red List assessments:

Geographic Range [top]

Range Description:In the United States, NatureServe (2009) have classified this species as Critically Imperilled in Connecticut, Maryland, Massachusetts, North Carolina, New Hampshire, New Jersey, New York, Pennsylvania, Virginia, Vermont; and Possibly Extirpated in District of Columbia and Delaware. The U.S Fish and Wildlife Service found this species to be declining in Virginia and North Carolina. They found small populations remaining in Maryland, while large populations were found in New Hampshire, and additional populations were discovered in the Delaware River watershed in Pennsylvania, New Jersey and New York (USFWS 2007). Since the publication of the U.S. Fish and Wildlife Service report, new populations have been reported in North Carolina (A. Bogan pers. comm. 2010).

In Canada, this species is extirpated in New Brunswick. 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).

Because of its severely fragmented distribution, the area of occupancy for this species is estimated as between 500 and 2,000 km².
Countries occurrence:
United States (Connecticut, Delaware - Possibly Extinct, District of Columbia - Possibly Extinct, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Vermont, Virginia)
Possibly extinct:
Canada (New Brunswick)
Additional data:
Estimated area of occupancy (AOO) - km2:500-2000Continuing decline in area of occupancy (AOO):Yes
Estimated extent of occurrence (EOO) - km2:>20,000
Continuing decline in extent of occurrence (EOO):Yes
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Historically, this species occurred from the Neuse River basin, North Carolina, in the south, north to the Petitcodiac River Basin, New Brunswick, Canada, with sporadic distribution in the river basins in between (Bogan et al. 2008). No historical population estimates exist, but findings by Strayer et al. (1996) are similar to observations by Clark (1981) that the species forms sparse populations and was never numerous.

However, the species has experienced significant declines including regional extirpations (e.g., the last remaining population in Canada) and there are only a small number of extant occurrences remaining (NatureServe 2009).

Viability is questionable at most (nearly all) sites. One hundred years ago the species was known from about 70 Atlantic seaboard river systems, but now from only fifteen major drainages (USFWS 2007). Of the 70–80 known sites in 2007 (one site may have multiple occurrences) only 16 were believed to support reproducing populations, while at least 45 were based on observations of five or fewer individuals, or solely on spent shells (USFWS 2007, NatureServe 2009). However, 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 thirteen streams throughout the species' range and concluded that all populations had low densities, although five to six of the populations were large (1,000 to 100,000 animals). Three linked patch sites on the Connecticut River on the Vermont/New Hampshire border were found to have decent viability and these are likely to present the largest population of this species (perhaps a few hundred thousand in a 75 km stretch in three patches) (USFWS 2007).
Current Population Trend:Decreasing
Additional data:
Population severely fragmented:Yes
Continuing decline in subpopulations:Yes

Habitat and Ecology [top]

Habitat and Ecology:This species occurs in medium or slow-flowing creeks or rivers of varying size. It can be found on gravel, sand or muddy sand bottoms and sometimes among submersed aquatic plants (Clarke 1981). Where it occurs, this species forms distinct, widely separate patches (USFWS 1993, NatureServe 2009). To survive, this species needs a silt-free, stable stream bed and well oxygenated water free of pollutants (USFWS 2007).

The fish hosts of this species are the Tessellated Darter (Etheostoma olmsted); the Slimy Sculpin (Cottus congatus); juvenile and parr of the Atlantic Salmon (Salmo salar); and probably the Johnny Darter (Etheostoma nigrum), and Mottled Sculpin (Cottus bairdi) (Michaelson and Neves 1995, McLain and Ross 2005, USFWS 2007). However McLain and Ross (2005) postulate that storm-assisted dispersal could be a more important dispersal mechanism for this species than its host fish.
Continuing decline in area, extent and/or quality of habitat:Yes

Threats [top]

Major Threat(s): Water pollution and impoundments are the primary threats to this species. 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 (USFWS 1993). It has been speculated that the reason this species disappeared from the Petticodiac River in Canada was the building of a large causeway near the river's mouth, blocking the fish host of this species (USFWS 2007). In addition, a wide array of industrial, agricultural and domestic pollutants have been responsible for this species' disappearance from much of its historical range and this continues to be a problem in most of its habitat (USFWS 1993, Strayer et al. 1996).

The low densities (<0.5 per square metre) in which this species occurs and the small ranges they occupy could be problematic since successful reproduction is density dependent (Strayer et al. 1996, USFWS 2007). This species is sensitive to disappearance or decline of its host fishes which are known to be largely sedentary with very poor dispersal capability limited to only their immediate vicinity with a narrow window of host infection time during each year (McLain and Ross 2005).

Conservation Actions [top]

Conservation Actions: This species has been given a NatureServe Global Heritage Status Rank of G1G2 - critically imperiled to imperiled (NatureServe 2009); an American Fisheries Society Status of Endangered (1 Jan 1993); and a previous IUCN Red List Category of EN (2000 ver 2.3). The American Fisheries Society (2010) lists the species as threatened (Williams et al. in press, from K. Cummings pers. comm. 2011). Under the U.S. Endangered Species Act it is listed as LE: Listed endangered (14 Mar 1990) and it was assigned a U.S. Fish and Wildlife Service Lead Region of R5 for the Northeast. In Canada, it has been given a Canadian Species at Risk Act (SARA) Schedule 1/Annexe 1 Status of XT: Extirpated (5 Jun 2003) and under the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) it is listed as Extirpated (1 May 2000).

This species is listed as a U.S. Federally Endangered species, although no sites are adequately protected. The Nature Conservancy has a management agreement at sites in Vermont, New Hampshire and Maryland. 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 (NatureServe 2009). However, the outcome of this management strategy on the mussel populations has not been evaluated.

A recovery plan has been drafted for this species (USFWS 1993). The five year review (USFWS 2007) has determined that certain criteria from the recovery plan (USFWS 1993) have been partially met as viable populations have been found in the mainstem Connecticut River and Ashuelot River.

Further research is recommended to monitor existing populations of this species, and determine their long term viability. Research is also needed to resolve outstanding taxonomic questions. Conservation planning and protection is necessary to avoid the extirpation of this species.

Citation: Cummings, K. & Cordeiro, J. 2011. Alasmidonta heterodon. The IUCN Red List of Threatened Species 2011: e.T775A13077570. . Downloaded on 21 January 2017.
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