Alasmidonta varicosa - (Lamarck, 1819)
Brook Floater
Taxonomic Status: Accepted
Related ITIS Name(s): Alasmidonta varicosa (Lamarck, 1819) (TSN 79920)
French Common Names: alasmidonte renflée
Unique Identifier: ELEMENT_GLOBAL.2.111437
Element Code: IMBIV02100
Informal Taxonomy: Animals, Invertebrates - Mollusks - Freshwater Mussels
 
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 varicosa
Taxonomic Comments: Alasmidonta varicosa was originally described under the genus Monodonta and is closely related to Alasmidonta marginata (Simpson, 1914). Current nomenclature follows Turgeon et al. (1998). It can sometimes be confused with Alasmidonta undulata and Alasmidonta heterodon (Nedeau, 2008). Systematics of the genus has not been reviewed genetically.
Conservation Status
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NatureServe Status

Global Status: G3
Global Status Last Reviewed: 23Dec2011
Global Status Last Changed: 24Oct1998
Rounded Global Status: G3 - Vulnerable
Reasons: Significant declines have been noted in Massachusetts, New York, Pennsylvania, New Jersey, Rhode Island, Virginia, North Carolina, and South Carolina. Approximately 70-90 site extirpations (of 150 or more known historically) have occurred globally with only a portion of the remaining sites holding healthy, viable populations. Although precise area of occupancy is not known and precise extent of decline is not known with accuracy, the loss of historical sites is indicative of a significant decline in area of occupancy over the last century (exact numbers not known but likely greater than 50% area of occupancy and range). Some good populations are known in the north (Vermont, and particularly Maine and a very large population just discovered in New Hampshire plus nine new populations in New Brunswick and Nova Scotia) where the species is more stable but declines continue even in the more stable portions of its range.
Nation: United States
National Status: N3 (24Oct1998)
Nation: Canada
National Status: N2 (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 (SX), District of Columbia (SNR), Georgia (S2), Maine (S3), Maryland (S1), Massachusetts (S1), New Hampshire (S1), New Jersey (S1), New York (S1), North Carolina (S3), Pennsylvania (S1S2), Rhode Island (SH), South Carolina (SNR), Vermont (S1), Virginia (S1), West Virginia (S2)
Canada New Brunswick (S2), Nova Scotia (S1S2)

Other Statuses

Canadian Species at Risk Act (SARA) Schedule 1/Annexe 1 Status: SC (07Mar2013)
Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Special Concern (26Apr2009)
Comments on COSEWIC: Reason for designation: A medium-sized freshwater mussel that is confined to 15 widely scattered watersheds in Nova Scotia and New Brunswick. Thiswas never abundant, usually representing only 1-5% of the total freshwater mussel fauna present. The habitat is subject to impacts (shoreline development, poor agricultural practices, and other water quality issues) with potential cumulative degradation on larger stretches of rivers. Populations appear to have been lost from two historic locations, although new populations have been formed recently. Because this mussel has disappeared from approximately half of its USA locations, the Canadian population now represents an important global stronghold for the species.

Status history: Designated Special Concern in April 2009.

IUCN Red List Category: VU - Vulnerable
American Fisheries Society Status: Threatened (01Jan1993)

NatureServe Global Conservation Status Factors

Range Extent: 200,000-2,500,000 square km (about 80,000-1,000,000 square miles)
Range Extent Comments: This northeastern North American species was historically found 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). Present distribution is spotty, 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 (8 drainages) and is rare in Nova Scotia (6 drainages) (COSEWIC, 2009).

Number of Occurrences: 21 - 80
Number of Occurrences Comments: Approximately 150 historic collections are known with more populations believed destroyed than will yet be discovered. Maine Natural Areas Inventory (1997) reports 37 populations over 73 survey sites, after a four year assessment; surveys continue in this state (see Nedeau et al., 2000). Nedeau et al. (2000) cites Aroostook, Cumberland, Hancock, Kennebec, Knox, Lincoln, Penobscot, Piscataquis, Somerset, Waldo, and Washington Cos. in Maine (absent from north, west, far south). In Massachusetts, most of the few remaining populations (4) are from the Connecticut River, with Merrimack and Charles River now extirpated (Smith, 2000). In Vermont, it is known from the lower reaches of the West River (Fichtel and Smith, 1995; Nedeau, 2008) and historically in the Connecticut River (Johnson, 1915). It remains in the North Branch Sugar River, New Hampshire (Nedeau, 2008) and a large population of a few thousand individuals was recently found in the Suncook River (Conaboy, 2006) with 14-20 occurrences overall (2-3 decent viability). In Connecticut, it is in a handful of streams in the Connecticut and Thames River watersheds (Nedeau and Victoria, 2003; J. Cordeiro, NatureServe, pers. obs., 2006; Nedeau, 2008). In Pennsylvania, it is known from the Potomac, Susquehanna, and Delaware basins (Bogan, 1993) in about 12 counties. In the Delaware River basin, it has been recorded in the Middle Delaware- Mongaup- Broadhead drainage in New York to bordering Pennsylvania (Strayer and Ralley, 1991). In New Jersey, it was reported from Stony Brook, Musconetcong, Raritan, Lamington and upper Delaware Rivers. In Maryland, it was from the North Branch Potomac River, Upper Potomac River, Middle Potomac River, and Washington Metro drainages (Bogan and Proch, 1995). In Virginia it is in the northern part of the state in the Middle James-Willis, the North Fork Shenandoah and parts of the Potomac, but is extirpated from the Middle Potomac-Anacostia-Occoquan, South Fork Shenandoah, and Shenandoah drainages (VA NHP, pers. comm., 2007). It occurs in Patterson Creek (North Branch Potomac drainage) (Clayton et al., 2001) and South and North Branch Potomac Plus Cacapon-Town drainages in West Virginia (Taylor, 1985; WV NHP, pers. comm., 2007). Some 12 populations are considered extant in North Carolina (NC Wildlife Resources Commission, 1998). Bogan (2002) cites the Roanoke, Neuse, Cape Fear, Pee Dee, and Catawba River basins for North Carolina while LeGrand et al. (2006) list occurrences in that state in Anson, Burke, Caldwell, Chatham, Forsyth, Granville, Moore, Orange, Randolph, Surry, and Yadkin Cos. During the past decade, 4 populations are considered extant in South Carolina (Alderman, 1998) including the Steven's Creek basin (Beaverdam, Stevens, Turkey, Mountain Creeks), Chattooga River and Flat Creek and nearby Lynches River (Catena Group, 2006), and Savannah basin (Bogan and Alderman, 2004) into neighboring Georgia (GA NHP, pers. comm., March 2007), and historically in the Cooper-Santee and Pee Dee River basins (Bogan and Alderman, 2004). In Canada, it has a limited distribution in the Bay of Fundy drainage in New Brunswick (Athearn, 1961; 1963) including Petitcodiac (Hanson and Locke, 2001), rare in Nova Scotia (7 locations in central, northern, and eastern counties) (Davis, 1999; Metcalfe-Smith and Cadmore-Vokey, 2004). Overall 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).

Population Size: 10,000 - 1,000,000 individuals
Population Size Comments: Most historic localities have yet to be searched. The Maine Natural Areas Inventory reports that although it is rare and localized at most sites in the state, several populations are extensive with 100s of individuals found. In North Carolina and South Carolina, populations are small, isolated, and have limited extents. A relatively large population of a few thousand individuals were recently found in Suncook, New Hampshire, 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 (over a thousand were subsequently relocated to another section of the river upstream). There were likely a few thousand (perhaps 10,000) live individuals in this population and the area warrants further study (Conaboy, 2006). Populations in the Connecticut River watershed are isolated and fragmented (with the rivers themselves isolated by the Connecticut River main stem) and several populations are no longer considered viable (e.g. Stony Brook, Eightmile River, West Branch Farmington River) (Nedeau, 2008). New populations have been discovered in New Brunswick in the last 15 years (St. Croix, Magaguadavic, Southwest Miramichi, Kouchibouguacis, Bouctouche, Shediac, and Scoudouc Rivers) and Nova Scotia (Annapolis, Stweiacke, Gays, Wallace, Mattattal Lake, LaHave, St. Marys, Salmon Rivers, Bordens Lake) (COSEWIC, 2009).

Number of Occurrences with Good Viability/Integrity: Few to some (4-40)
Viability/Integrity Comments: A relatively large population of a few hundred individuals were recently found in Suncook, New Hampshire, 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 (they were subsequently relocated to another section of the river) (Conaboy, 2006). Other than that, the best populations in the northeast are in Maine which has more populations than the remainder of the northeast combined (Nedeau et al., 2000). Some populations in the West River in Vermont have excellent viability (VT NHP, pers. comm., 2005). The most viable southern population may be in the Chattooga River near the South Carolina/Georgia state line. New surveys in New Brunswick and Nova Scotia have revealed about nine new populations with good viability that may serve as important global strongholds for the future, however, New Brunswick supports most of Canada's populations (COSEWIC, 2009; Martel et al., 2010).

Overall Threat Impact: High
Overall Threat Impact Comments: Pollution of small rivers is believed to have impacted this species. More specifically impoundments; waste water plant releases; releases from poultry processing plants; point sources of pollutants; rip-rapping and siltation. Also, collection for biological supply (population in Penobscot River in Maine is thought to have been harvested in 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).

Short-term Trend: Decline of 50-70%
Short-term Trend Comments: This species has disappeared from 60-80 sites range-wide. There has been a sharp decline in numbers where present. The Potomac River system was surveyed in 1994 reproductive success was limited. It also 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) and is nearly extirpated in Connecticut (Nedeau and Victoria, 2003). 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 1 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 1990's (Strayer and Jirka 1997). Nedeau et al. (2000) report it extirpated in some of Maine's watersheds including the Dennys River and Presumpscot River. Similar declines are reported for Massachusetts (Smith, 2000) and the Connecticut River in Massachusetts, Connecticut, New Hampshire, and Vermont (Nedeau, 2008). 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).

Long-term Trend: Decline of 50-70%
Long-term Trend Comments: In Virginia it occurs in the northern part of the state in the Middle James-Willis, the North Fork Shenandoah and parts of the Potomac, but is extirpated from the Middle Potomac-Anacostia-Occoquan, South Fork Shenandoah, and Shenandoah drainages (VA NHP, pers. comm., 2007). A historical record in Vermont exists for the Connecticut River main stem in Norwich (Kart et al., 2005; Johnson, 1915), but currently it only occurs in the West River and the North Branch Sugar River in neighboring New Hampshire (Nedeau, 2008).

Intrinsic Vulnerability: Moderately vulnerable
Intrinsic Vulnerability Comments: Many populations may be lost due to small population sizes and significant isolation of populations.

Environmental Specificity: Narrow. Specialist or community with key requirements common.
Environmental Specificity Comments: Like most riffle-inhabiting freshwater unios, this species is believed to be sensitive to low oxygen, pollution, and silt; and prefers stable bank environments.

Other NatureServe Conservation Status Information

Inventory Needs: Survey all historic sites for presence/absence and connectedness, and conduct de novo inventory in promising areas. More specifically northern New Hampshire, Maine (as well as mid-coast), Connecticut, Massachusetts, Ontario; in West Virginia need to survey panhandle sites; Pennsylvania and Delaware have historic sites that need survey as well as coastal Virginia sites. Surveys in and around a recent large population in New Hampshire (probably the largest in existence) merit further investigation.

Distribution
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Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) This northeastern North American species was historically found 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). Present distribution is spotty, 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 (8 drainages) and is rare in Nova Scotia (6 drainages) (COSEWIC, 2009).

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, DEextirpated, GA, MA, MD, ME, NC, NH, NJ, NY, PA, RI, SC, VA, VT, WV
Canada NB, NS

Range Map
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U.S. Distribution by County Help
State County Name (FIPS Code)
CT Hartford (09003), Litchfield (09005), New London (09011), Tolland (09013), Windham (09015)
DE New Castle (10003)*
GA Rabun (13241)
MA Berkshire (25003), Bristol (25005)*, Essex (25009)*, Franklin (25011)*, Hampden (25013), Hampshire (25015), Middlesex (25017), Worcester (25027)*
MD Allegany (24001), Baltimore (city) (24510)*, Baltimore County (24005)*, Carroll (24013), Frederick (24021), Montgomery (24031)*, Washington (24043)
NC Anson (37007), Burke (37023), Caldwell (37027), Chatham (37037), Forsyth (37067), Granville (37077), McDowell (37111), Montgomery (37123), Moore (37125), Orange (37135), Randolph (37151), Surry (37171), Wilkes (37193), Yadkin (37197)
NH Grafton (33009)*, Hillsborough (33011), Merrimack (33013), Rockingham (33015), Strafford (33017), Sullivan (33019)
NJ Burlington (34005)*, Hunterdon (34019), Mercer (34021), Morris (34027), Somerset (34035), Sussex (34037), Warren (34041)
NY Broome (36007), Chemung (36015), Chenango (36017), Cortland (36023), Delaware (36025), Dutchess (36027), Madison (36053), Orange (36071), Otsego (36077), Rockland (36087), Steuben (36101), Sullivan (36105), Tioga (36107), Ulster (36111)
PA Adams (42001), Bedford (42009), Berks (42011)*, Blair (42013)*, Bradford (42015)*, Bucks (42017)*, Cameron (42023)*, Clinton (42035), Cumberland (42041), Dauphin (42043), Franklin (42055)*, Fulton (42057)*, Huntingdon (42061), Indiana (42063)*, Lancaster (42071)*, Lebanon (42075), Lycoming (42081)*, Monroe (42089), Montgomery (42091)*, Northampton (42095), Perry (42099)*, Philadelphia (42101)*, Pike (42103), Snyder (42109), Tioga (42117), Union (42119), Wayne (42127), York (42133)
RI Providence (44007)*
SC Chesterfield (45025), Edgefield (45037), Lancaster (45057), McCormick (45065), Oconee (45073)
VA Augusta (51015)*, Clarke (51043)*, Fairfax (51059)*, Manassas (City) (51683), Prince William (51153), Rockingham (51165)*, Shenandoah (51171)*, Warren (51187)*, Waynesboro (City) (51820)*
VT Windham (50025), Windsor (50027)*
WV Grant (54023), Hampshire (54027), Mineral (54057), Morgan (54065)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
01 Aroostook (01010004), West Branch Penobscot (01020001), East Branch Penobscot (01020002), Mattawamkeag (01020003), Piscataquis (01020004), Lower Penobscot (01020005), Upper Kennebec (01030001), Dead (01030002), Lower Kennebec (01030003), St. Croix (01050001), Maine Coastal (01050002), St. George-Sheepscot (01050003), Presumpscot (01060001), Piscataqua-Salmon Falls (01060003)+, Pemigewasset (01070001)+, Merrimack (01070002)+, Contoocook (01070003)+, Nashua (01070004)+, Concord (01070005)+*, Merrimack (01070006)+, Upper Connecticut (01080101)*, Upper Connecticut-Mascoma (01080104)+, Black-Ottauquechee (01080106)+, West (01080107)+, Middle Connecticut (01080201)+, Chicopee (01080204)+, Lower Connecticut (01080205)+, Westfield (01080206)*, Farmington (01080207)+, Charles (01090001)+*, Blackstone (01090003)+*, Narragansett (01090004)*, Quinebaug (01100001)+, Shetucket (01100002)+, Thames (01100003), Housatonic (01100005)+
02 Rondout (02020007)+, Hackensack-Passaic (02030103)+, Raritan (02030105)+, Upper Delaware (02040101)+, East Branch Delaware (02040102)+, Middle Delaware-Mongaup-Brodhead (02040104)+, Middle Delaware-Musconetcong (02040105)+, Crosswicks-Neshaminy (02040201)+*, Lower Delaware (02040202)+, Schuylkill (02040203)+*, Brandywine-Christina (02040205)+*, Upper Susquehanna (02050101)+, Chenango (02050102)+, Owego-Wappasening (02050103)+, Tioga (02050104)+, Chemung (02050105)+, Upper Susquehanna-Tunkhannock (02050106)+*, Upper West Branch Susquehanna (02050201)+*, Sinnemahoning (02050202)+*, Middle West Branch Susquehanna (02050203)+, Pine (02050205)+, Lower West Branch Susquehanna (02050206)+*, Lower Susquehanna-Penns (02050301)+, Upper Juniata (02050302)+*, Raystown (02050303)+, Lower Juniata (02050304)+, Lower Susquehanna-Swatara (02050305)+, Lower Susquehanna (02050306)+, Gunpowder-Patapsco (02060003)+*, South Branch Potomac (02070001), North Branch Potomac (02070002)+, Cacapon-Town (02070003)+, Conococheague-Opequon (02070004)+, South Fork Shenandoah (02070005)+*, North Fork Shenandoah (02070006)+, Shenandoah (02070007)+*, Middle Potomac-Catoctin (02070008)+, Monocacy (02070009)+, Middle Potomac-Anacostia-Occoquan (02070010)+, Middle James-Willis (02080205)*
03 Middle Roanoke (03010102)+, Lower Dan (03010104)+, Upper Neuse (03020201), Middle Neuse (03020202), Haw (03030002)+, Deep (03030003)+, Upper Cape Fear (03030004), Lower Cape Fear (03030005), Upper Yadkin (03040101)+, Lower Yadkin (03040103)+, Upper Pee Dee (03040104)+, Lynches (03040202)+, Upper Catawba (03050101)+, Santee (03050112)*, Cooper (03050201)*, Tugaloo (03060102)+, Middle Savannah (03060106), Stevens (03060107)+, Lower Savannah (03060109)
06 Upper Little Tennessee (06010202)
CP CP-32 (CP-32)+*
ME ME-19 (ME-19)+*
NS NS-08 (NS-08)+*
WE WE-39 (WE-39)+*
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
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Basic Description: Freshwater mussel
General Description: The shell is kidney-shaped. The anterior margin is abruptly curved. The ventral margin is straight to slightly concave, descending somewhat. The posterior margin is biangulate below and broadly curved to straight above. The dorsal margin is slightly convex. The beaks are located well forward, approximately one fourth of length from anterior margin. There are numerous short, low, corrugations or ridges on the posterior slope. The ridges tend to be oriented radially. The shell is moderately elongate and is firm but not thick. The periostracum tends to be smooth centrally and rough elsewhere. In adults, the periostracum is brownish with dark fine rays sometimes present; in juveniles it is yellowish with green rays (Clarke, 1981). The ligament is located posterior of beaks, and is well developed. The nacre is rose or yellowish olive centrally and bluish white or blue at margins. Dentition is rudimentary. There is typically one small pseudocardinal in each valve that is poorly developed and of compressed pyramidal shape. If present, the lateral teeth are vestigial. See Clarke (1981) or Johnson (1970) for a complete description. Identification keys are provided in Bogan (2002) for North Carolina, Bogan and Alderman (2004) for South Carolina, Clarke (1981) for Canada, Fichtel and Smith (1995) for Vermont, Nedeau et al. (2000) for Maine, Nedeau and Victoria (2000) for Connecticut, Smith (2000) for New England, and Strayer and Jirka (1997) for New York.
Diagnostic Characteristics: Diagnostic characters include a flat or slightly indented ventral margin, swollen to moderately inflated valves, yellowish-green (young specimens) to brownish-black periostracum with prominent green color rays (eroded away in older specimens), series of corrugations along the dorso-posterior slope perpendicular to growth lines, pseudocardinal teeth poorly developed with only one small knob-like tooth in each valve and no lateral teeth (Strayer and Jirka, 1997; Nedeau et al., 2000; Smith, 2000; Nedeau, 2008).

This species can sometimes be confused with Alasmidonta undulata, Strophitus undulatus, and Alasmidonta heterodon (Nedeau, 2008). Alasmidonta marginata is similar but is more inflated, grows larger, and has a distinctly truncate posterior slope. No other shell within its range has distinctive radial ridging on posterior slope except Alasmidonta robusta, a localized and believed extinct species of the Santee River system in North and South Carolina. Alasmidonta robusta has distinctive wide and wavy green rays on its shell (Strayer and Jirka, 1997).

Reproduction Comments: Fertilization occurs in summer with glochidia released the following spring (Nedeau, 2008). The period of time required by glochidia to complete metamorphosis to juveniles varies according to species. Time period for related mussels is generally between 30-60 days, but is not known for this species. It is a long-term brooder with gravid females found from August to May (Clarke, 1981, Ortmann, 1919). In Maine, release of glochidia occurs from April to June (and possibly later) (Nedeau et al., 2000). Wicklow (in Strayer and Jirka, 1997) and Schulz and Marbain (1998) found the longnose dace (Rhinichthys cataractae), golden shiners (Notemigonus crysoleucas), pumpkinseed (Lepomis gibbosus), marginated madtom (Noturus insignis), yellow perch (Perca flavescens), blacknosed dace (Rhinichthys atratulus), and slimy sculpin (Cottus cognatus) serve as glochidial hosts. No studies, however, have confirmed glochidial host transformation under natural conditions.
Ecology Comments: Mussel literature consists primarily of taxonomy, systematics, distribution, and life history of the taxa. No ecological studies of this species exist. Alasmidonta varicosa appears to be a species negatively affected by eutrophication and siltation, and to be sensitive to impacts. Given the large number of potential widely distributed host fish, it is unlikely that the rarity of this species can be attributed to some aspects of host fish biology or ecology (Nedeau, 2008).
Habitat Type: Freshwater
Non-Migrant: Y
Locally Migrant: N
Long Distance Migrant: N
Mobility and Migration Comments: Adults are essentially sessile. Passive movement downstream may occur. Dispersal is essential via glochidia encysted on fish (Williams et al., 2008). Some freshwater mussel species have developed elaborate methods to increase chances of glochidial contact with potential host fish. Some species package glochidia into bundles resembling food items called conglutinates (Ortmann, 1912; Lefevre and Curtis, 1912; Utterback, 1931; Morrison, 1973; Haag and Straton, 2003; Haag and Warren, 2003). Conglutinates are believed to aid in delivering glochidia to potential host fishes. Long-term brooders (not yet observed in Alasmidonta varicosa), have evolved modifications to the posterioventral mantle margin that are utilized to lure potential host fish and often resemble food items (e.g. fishes, crayfish, insect larvae, worms) for the hosts (Kraemer, 1970; Haag et al., 1999).

Riverine Habitat(s): CREEK, High gradient, MEDIUM RIVER, Moderate gradient, Riffle
Special Habitat Factors: Benthic
Habitat Comments: 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).
Adult Food Habits: Detritivore
Immature Food Habits: Parasitic
Food Comments: Little is known about food and feeding of freshwater mussels with early information summarized by Fuller (1974). Stomach content analysis indicates freshwater mussels generally feed on mud, desmids, diatoms, rotifers, flagellates, and other unicellular organisms (Lefevre and Curtis, 1910; 1912; Wilson and Clark, 1912; Allen, 1914; 1921; Evermann and Clark, 1918, 1920; Coker et al., 1921; Howard, 1922; Churchill and Lewis, 1924). Glochidia (larval form) of freshwater mussels are typically parasitic on fish. Host specificity varies among species; but the host remains unidentified for Alasmidonta varicosa although several potential species have been identified in lab conditions. Adult mussels are filter feeders. General literature has assumed that plankton constitutes the majority of food for mussels. Recent studies indicate that detritus is the primary energy source (James, 1987).
Adult Phenology: Circadian
Phenology Comments: Freshwater mussel life cycles are very complex. Males release sperm into the water column to be taken in by females via the incurrent aperture and fertilization is internal (Yokley, 1972). Fertilized eggs are brooded in marupial spaces between the gills to complete development into glochidia (Lefevre and Curtis, 1910; Heard, 1975). Glochidia are discharged from the excurrent siphon and most are obligate parasites on fish or amphibians. While encysted on the host, glochidia transform into juvenile mussels. Mussel species are either short-term brooders, which produce gametes over an extended period, from autumn to the following winter; or long-term brooders, which produce gametes over an extended period, usually from late winter or early spring to early summer (Haggerty et al., 1995; Garner et al., 1999). Alasmidonta varicosa is a long-term brooder (Ortmann, 1919). Most species of freshwater mussels are long-lived with many species living 30 to 70 years (Bauer, 1992). Growth and life history information have not been published for Alasmidonta varicosa.
Length: 7.5 centimeters
Economic Attributes Not yet assessed
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Management Summary
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Stewardship Overview: Population status and trends of local occurrences need to be determined. Overall systematics of the genus needs to be reviewed. Element occurrences that represent best known occurrences in a given state should be monitored biannually as resources allow. Fish host(s) need to be confirmed. Documentation of differences between extirpated and extant occurrences should be performed as possible to determine causes of extirpation.
Restoration Potential: Greater knowledge of tolerance to impacts and of competitive interactions with introduced bivalves is needed before active restoration is undertaken.
Preserve Selection & Design Considerations: Elimination of impacts to water quality is the main consideration. This includes the provision for sufficient buffers to prevent siltation, eutrophication, and toxic runoff into waters in which there is an element occurrence. For a relatively large watershed, this is possibly unfeasible; however, local protection can be significant. Protection from take could be a significant benefit of a preserve. A significant consideration is the ability to protect the site from colonization by zebra mussel, Dreissena polymorpha.
Management Requirements: Maintain water quality by reducting siltation, pollution, and eutrophication. The host fish species must be maintained within the element occurrence for successful reproduction. Transplantation can be done successfully for adult mussels; however, the long term fate of such transplanted populations is uncertain. See General Freshwater Mussel ESA, Fuller (1974), or Havlik and Marking (1987) for additional information on pollutants requiring control.
Monitoring Requirements: Monitoring requires determination of the density or relative density of individuals and the presence of young cohorts. For accurately determining the presence of mussels, waterscoping is required and snorkeling is recommended. Populations should be monitored either with density measures such as quadrat sampling or by recording the number of individuals found along a transect or within a specific time interval. Sampling effort to find rare mussels is frequently difficult to quantify; however, Kovalak et. al. (1986) provides a useful review of the topic and guidance on the issue. In general, it will be best to concentrate effort on the known habitats of this species. Due to its small size and preferred habitat, Alasmidonta varicosa can be a difficult species to find. Monitoring is best accomplished during periods of low flow, typically in summer. Unusual drought or water level drops should be taken advantage of to reach areas typically not accessible. Also, a mussel population may have a significant portion of the population in a temporarily endobenthic (buried) state not visible from the surface (Amyot and Downing, 1991). To avoid excessive disturbance from annual monitoring, monitoring programs may require only biannual visits. It is extremely important that live mussels handled during monitoring are appropriately replaced in the substrate. Failing to replace mussels into the substrate or placing them in an improper orientation can be fatal. Proper orientation requires that the anterior end be in the substrate. It is typically best to avoid handling mussels when they are at the end of the gravid period and ready to discharge glochidia. When fish host(s) are reported, monitoring should include these fish(es) in some manner.
Management Research Needs: Fish host for glochidia is unknown. Research into the impacts of siltation, pollution, and eutrophication are required. Habitat requirements and interaction with host habitat requirements are needed. Effects of exotic bivalves on native mussels needs research as does control measures for the exotics.

Glochidial host should be identified; however, the broad range of A. VARICOSA indicates a common host species or a number of hosts are used.

Ongoing research into the ecology of the DREISSENA POLYMORPHA should be examined as this species has been predicted to exhibit thermal tolerance such that it can potentially invade the entirety of the range of ALASMIDONTA VARICOSA (Strayer, 1990). DREISSENA POLYMORPHA appears to be excluded from streams under 10 meters in width (Strayer, 1990).

Biological Research Needs: Determine possible genetic differences between northern populations and those found in North and South Carolina. Determine population viability. What is the least invasive way to monitor populations to determine between year comparisons, recruitment, to be able to identify a drop in population numbers? Also, what is the best way to assess fresh dead animals and how do shells fit into population estimates? Determine host fish. Determine sensitivities to pollutants, low dissolved oxygen. Note, current studies easily miss the pulses of e.g., chloride surge, or manure run-off that happen within a few days.
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: 07Jun2010
NatureServe Conservation Status Factors Author: Cordeiro, J. (2011); Master, L.; Alderman, J. [1998 edition]
Management Information Edition Date: 23Mar1992
Management Information Edition Author: Stevenson, Phil
Element Ecology & Life History Edition Date: 23Dec2011
Element Ecology & Life History Author(s): Cordeiro, J. (2011); STEVENSON, P. (1994)

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

References
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  • Williams, J.D., A.E. Bogan, and J.T. Garner. 2008. Freshwater Mussels of Alabama & the Mobile Basin in Georgia, Mississippi & Tennessee. University of Alabama Press: Tuscaloosa, Alabama. 908 pp.

  • Williams, J.D., M.L. Warren, Jr., K.S. Cummings, J.L. Harris, and R.J. Neves. 1993b. Conservation status of freshwater mussels of the United States and Canada. Fisheries 18(9): 6-22.

  • Wilson, C.B. and H.W. Clark. 1912. The mussel fauna of the Kankakee basin. Report and Special Papers of the U.S. Fish Commission [Issued separately as U.S. Bureau of Fisheries Document 758] 1911:1-52 + 1 map.

  • Yokley, P. 1972b. Life history of Pleurobema cordatum (Rafinesque, 1820) (Bivalvia:Unionacea). Malacologia 11:351-364.

References for Watershed Distribution Map
  • Athearn, H.D. 1961. Additions to the New Brunswick checklist. Sterkiana 4:33-34.

  • Athearn, H.D. 1963. Some new records of naiades from eastern North America. Sterkiana 9:39.

  • Athearn, H.D. and A.H. Clarke, Jr. 1962. The freshwater mussels of Nova Scotia. National Museum of Canada Bulletin 183:11-41.

  • Bogan, A.E. 2002. Workbook and key to the freshwater bivalves of North Carolina. North Carolina Museum of Natural Sciences: Raleigh, North Carolina. 101 pp.

  • Bogan, A.E. and J.M. Alderman. 2004. Workbook and key to the freshwater bivalves of South Carolina. North Carolina Museum of Natural Sciences: Raleigh, North Carolina. 64 pp.

  • Bogan, A.E. and T. Proch. 1995. Manual of the freshwater bivalves of Maryland. Prepared for a workshop held at Versar, Inc., Columbia, Maryland, 9 March 1995. 68 pp.

  • Conaboy, C. 2006. Mussels lose their river home. Concord Monitor [Concord, New Hampshire], 23 May 2006. 2 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. 54 pp.

  • Hanson, J.M. and A. Locke. 2000. The status of the dwarf wedgemussel, Alasmidonta heterodon, in Canada. The Canadian Field-Naturalist 114:271-278.

  • Johnson, C.W. 1915. Fauna of New England. 13. List of the Mollusca. Occasional Papers of the Boston Society of Natural History 7(13):1-231.

  • LeGrand, H.E., Jr., S.P. Hall, S.E. McRae, and J.T. Finnegan. 2006. Natural Heritage Program List of the Rare Animal Species of North Carolina. North Carolina Natural Heritage Program, Raleigh, North Carolina. 104 pp.

  • Metcalfe-Smith, J.L. and B. Cudmore-Vokey. 2004. National general status assessment of freshwater mussels (Unionacea). National Water Research Institute / NWRI Contribution No. 04-027. Environment Canada, March 2004. Paginated separately.

  • Nedeau, E.J. and J. Victoria. 2003. A Field Guide to the Freshwater Mussels of Connecticut. Connecticut Department of Environmental Protection, Hartford, Connecticut. 31 pp.

  • Nedeau, E.J., M.A. McCollough, and B.I. Swartz. 2000. The Freshwater Mussels of Maine. Maine Department of Inland Fisheries and Wildlife, Augusta, Maine. 118 pp.

  • Raithel, C.J. and R.H. Hartenstein. 2006. The status of freshwater mussels in Rhode Island. Northeastern Naturalist 13(1):103-116.

  • Strayer, D.L. and J. Ralley. 1993. Microhabitat use by an assemblage of stream-dwelling unionaceans (Bivalvia) including two rare species of Alasmidonta. Journal of the North American Benthological Society 12(3):247-258.

  • Taylor, R.W. 1985. Comments on the distribution of freshwater mussels (Unionacea) of the Potomac River headwaters in West Virginia. The Nautilus 99(2-3):84-87.

  • The Catena Group. 2006. Freshwater mussel surveys of the Pee Dee River basin in South Carolina. Unpublished report prepared for the Nature Conservancy- South Carolina Chapter, January 3, 2006. 47 pp.

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