Global Range: (20,000-200,000 square km (about 8000-80,000 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).
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Regularity: Regularly occurring
Type of Residency: Year-round
Length: 7.5 cm
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).
Habitat Type: Freshwater
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).
Habitat and Ecology
Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
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).
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).
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 21 - 80
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).
10,000 - 1,000,000 individuals
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).
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).
Life History and Behavior
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.
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.
NatureServe Conservation Status
Rounded Global Status Rank: 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.
Intrinsic Vulnerability: Moderately vulnerable
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.
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.
IUCN Red List Assessment
Red List Category
Red List Criteria
- 1996Data Deficient
- 1994Indeterminate(Groombridge 1994)
National NatureServe Conservation Status
Rounded National Status Rank: N2 - Imperiled
Rounded National Status Rank: N3 - Vulnerable
Global Short Term Trend: Decline of 50-70%
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).
Global Long Term Trend: Decline of 50-70%
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).
Degree of Threat: High
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).
Restoration Potential: Greater knowledge of tolerance to impacts and of competitive interactions with introduced bivalves is needed before active restoration is undertaken.
Preserve Selection and 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.
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.
Global Protection: Few to several (1-12) occurrences appropriately protected and managed
Comments: No occurrences are currently known to be adequately protected. Some North and South Carolina occurrences are on Forest Service land although they are not managed. One historic site (Sideling Hill Creek) is part of designated TNC Bioreserve and live individuals have been found downstream of Sideling Hill Creek. It is listed as endangered in Massachusetts, Connecticut, and New Hampshire and threatened in Vermont. The species was recently assessed as special concern in Canada (Martel et al., 2010).
Relevance to Humans and Ecosystems
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.
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This mussel lives in high relief streams, under boulders and in sand.
The Brook Floater is sensitive to sedimentation, flow alteration, and low oxygen conditions.
- Bogan, A.E 2000. Alasmidonta varicosa. 2006 IUCN Red List of Threatened Species. Downloaded on 6 August 2007.
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Names and Taxonomy
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.