Alasmidonta varicosa 

Scope: Global
Language: English

Translate page into:

Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Mollusca Bivalvia Unionoida Unionidae

Scientific Name: Alasmidonta varicosa (Lamarck, 1819)
Common Name(s):
English Brook Floater
Alismodonta varicosa (Lamarck, 1819)

Assessment Information [top]

Red List Category & Criteria: Vulnerable A2bc ver 3.1
Year Published: 2017
Date Assessed: 2015-06-10
Assessor(s): Woolnough, D. & Bogan, A.E.
Reviewer(s): Ormes, M., Seddon, M.B. & Lopes-Lima , M.
Contributor(s): Cordeiro, J. & Seddon, M.B.
This North American freshwater mussel is listed as Vulnerable based on loss of subpopulations throughout its range. The species has disappeared from 60‑80 sites range‑wide, out of about 150 known historic collections, suggesting a possible loss of area of occupancy (AOO) of between 35-50%. In addition there has been a significant decline in reproductive success; for example, in New Hampshire only 2-3 of the 14-20 subpopulations are now considered to be viable. The loss of historical sites is indicative of a significant decline in AOO over the past three generations (around 30 years). Monitoring is required as it is possible, given the continued decline in AOO and number of mature individuals, that the species may qualify for Endangered in the near future.
Previously published Red List assessments:

Geographic Range [top]

Range Description:This northeastern North American species was originally known from Nova Scotia to South Carolina in the Atlantic drainages, with an isolated record in Greenbrier River of West Virginia, part of Ohio drainage (Clarke 1981). The current distribution is scattered, including the Potomac drainage in Virginia, small populations in North and South Carolina, several populations farther north in New York (Neversink River) and elsewhere, and numerous large populations in Maine (Nedeau et al. 2000) and small populations in Massachusetts (Smith 2000), Connecticut (Nedeau and Victoria, 2003), and New Hampshire (Fichtel and Smith 1995).

Although it was reported in Rhode Island over 100 years ago, there has never been a documented occurrence in the state since (Raithel and Hartenstein 2006).

A discontinuity also exists in northern New Hampshire and southern Maine (COSEWIC 2009). A relatively large population of a few thousand individuals were recently found in Suncook, New Hampshire (Conaboy 2006).

In Canada, it has a limited distribution in the Bay of Fundy drainage in New Brunswick (eight drainages) and is rare in Nova Scotia (six drainages) (COSEWIC 2009). The Area of Occupancy in Canada is 3.0215 km².
Countries occurrence:
Canada (New Brunswick, Nova Scotia); United States (Connecticut, Delaware, District of Columbia, Georgia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, South Carolina, Vermont, Virginia, West Virginia)
Additional data:
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:This species has disappeared from 60‑80 sites range‑wide (out of about 150 known historic collections) and there has been a sharp decline in numbers where present. Only a portion of the remaining sites have healthy, viable populations, particularly those in the north (Vermont, Maine, New Hampshire in the United States and New Brunswick and Nova Scotia in Canada).

In Canada, it is confined to 14 widely scattered watersheds in New Brunswick (St. Croix, Magaguadavic, Petitcodiac, Southwest Miramichi, Shediac, Scoudouc, Bouctouche, and Kouchibouguacis watersheds), and Nova Scotia (Annapolis, LeHave, Gays, Wallace, East St. Marys and Salmon Rivers) (COSEWIC, 2009; Martel et al. 2010) where it is rare in all (COSEWIC 2009).

Maine Natural Areas Inventory (1997) reported 37 populations over 73 survey sites, after a four year assessment. Of the 14-20 occurrences in New Hampshire, only 2-3 are considered viable and the largest population (several hundred individuals, discovered when the Suncook River overflowed its banks and left the main watercourse leaving about a mile of the old riverbed nearly dry and the mussels exposed) was relocated to another section of the river (Conaboy 2006).

It appears to have disappeared from most of the Susquehanna River Basin in New York since the 1990s (Strayer and Jirka 1997, Strayer and Fetterman 1999). Elsewhere in New York, populations in the Housatonic and Passaic basins have apparently disappeared and surveys of nearly a dozen historical populations throughout the Susquehanna River watershed in 1991 turned up only one living animal. Populations in the Shawangunk Kill and Delaware River basins (Lellis 2001) are sparse and limited in extent. Only the Neversink River population currently appears healthy although it also apparently declined by an estimated 38,000 individuals during the mid 1990s (Strayer and Jirka 1997).

The Potomac River system was surveyed in 1994 reproductive success was limited. In North Carolina and South Carolina, populations are small, isolated, and have limited extents.

Although it was reported in Rhode Island over 100 years ago, there has never been a documented occurrence in the state since (Raithel and Hartenstein 2006).

Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes
Extreme fluctuations:No
Continuing decline in subpopulations:Yes

Habitat and Ecology [top]

Habitat and Ecology:Considered to be a species of creeks and small rivers where it is found among rocks in gravel substrates and in sandy shoals, the Brook Floater inhabits flowing-water habitats only (Nedeau et al. 2000, Nedeau, 2008). It occurs in running water and, although typically found in riffles and moderate rapids with sandy shoals or riffles with gravel bottoms (Clark and Berg 1959, Athearn and Clark 1962), it can also be found in a range of flow conditions (usually not in very slow flow conditions). Strayer and Ralley (1993) found no consistent substrate preference but it is thought to prefer stable habitats such as coarse sand and gravel. It is more common in small to mid-sized streams or creeks than in large rivers (Clarke 1981) and is more common in upper portions of large watersheds with intact upland forest but is absent from headwater streams (Nedeau 2008).
Generation Length (years):10

Use and Trade [top]

Use and Trade: Unknown but this group of organisms (Unionidae) has been used for pearl and buttons.

Threats [top]

Major Threat(s): Pollution of small rivers from waste water plant releases, releases from poultry processing plants and other point sources of pollutants is believed to have impacted this species. Other threats come from impoundments, rip-rapping and siltation. In some parts of the range, (population in Penobscot River in Maine), collection for biological supply was harvesting until 1993. It has also been proposed that the Asiatic clam, Corbicula fluminea, is a competitor of Unionidae (Clarke, 1984) and the introduced zebra mussel, Dreissena polymorpha, may have negative impacts on the species. Threats in Canada include habitat degradation (silt, nutrient and sewage loads, poor agricultural practices), increased residential development, and loss of riparian corridors (COSEWIC 2009).

Conservation Actions [top]

Conservation Actions: This species requires active monitoring as it is potentially close to meeting the Endangered threshold. As with all unionid bivalves, maintaining water quality, flow, and ensuring that the host fish species are present is essential. Several common fish species are known as hosts therefore water quality and maintaining adequate habitat would be of primary importance.

Canada: COSEWIC listed this species as "Special Concern". Canadian Provinces: New Brunswick (S1S2), Nova Scotia (S1S2)

USA: USFWS This species was originally considered as G3 over the range, but this status is currently under review.  It is considered threatened in many of the states throughout the range. USA States: Connecticut (S1), Delaware (SX), District of Columbia (SNR), Georgia (S2), Maine (S3), Maryland (S1), Massachusetts (S1), New Hampshire (S1), New Jersey (S1), New York (S1), North Carolina (S1), Pennsylvania (S2), Rhode Island (SH), South Carolina (SNR), Vermont (S1), Virginia (S1), West Virginia (S1)

Citation: Woolnough, D. & Bogan, A.E. 2017. Alasmidonta varicosa. The IUCN Red List of Threatened Species 2017: e.T781A69490583. . Downloaded on 26 April 2018.
Disclaimer: To make use of this information, please check the <Terms of Use>.
Feedback: If you see any errors or have any questions or suggestions on what is shown on this page, please provide us with feedback so that we can correct or extend the information provided