Obovaria subrotunda - (Rafinesque, 1820)
Round Hickorynut
Synonym(s): Obovaria leibii (I. Lea, 1862)
Taxonomic Status: Accepted
Related ITIS Name(s): Lampsilis orbiculata Hildreth (TSN 80005) ;Obovaria retusa lens Lea (TSN 80177) ;Obovaria subrotunda (Rafinesque, 1820) (TSN 80174)
French Common Names: obovarie ronde
Unique Identifier: ELEMENT_GLOBAL.2.117121
Element Code: IMBIV31050
Informal Taxonomy: Animals, Invertebrates - Mollusks - Freshwater Mussels
 
Kingdom Phylum Class Order Family Genus
Animalia Mollusca Bivalvia Unionoida Unionidae Obovaria
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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: Obovaria subrotunda
Taxonomic Comments: This species was originally described in the genus Obliquaria. It has also been placed in the genus Unio.
Conservation Status
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NatureServe Status

Global Status: G4
Global Status Last Reviewed: 03Jan2010
Global Status Last Changed: 30Jan1998
Rounded Global Status: G4 - Apparently Secure
Reasons: This is a wide-ranging species with thousands of individuals that is starting to disappear from many areas where it formerly occurred; declines are particularly evident in last remaining population in Canada (Lake St. Clair) and other Great Lakes localities, as well as western Pennsylvania and New York.
Nation: United States
National Status: N4 (16Jul1998)
Nation: Canada
National Status: N1 (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 Alabama (S2), Arkansas (S1), Georgia (SH), Illinois (SX), Indiana (S1), Kentucky (S2S3), Michigan (S1), Mississippi (S2), New York (SH), Ohio (S4), Pennsylvania (S1), Tennessee (S2S3), West Virginia (S3)
Canada Ontario (S1)

Other Statuses

Canadian Species at Risk Act (SARA) Schedule 1/Annexe 1 Status: E (12Jan2005)
Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Endangered (03May2013)
Comments on COSEWIC: Reason for designation: The Carolinian population of this species has declined by 75-95% over the last 10 years, with an estimated 99% decline over the last 30 years. Populations in the Grand and Thames rivers are extirpated and populations in the Sydenham River and Lake St. Clair have declined to very low levels. Losses and declines are due to the combined effects of pollution from agriculture and residential runoff, and the impacts of invasive species like the Zebra Mussel.


Status history: Designated Endangered in May 2003. Status re-examined and confirmed in May 2013.

IUCN Red List Category: NT - Near threatened
American Fisheries Society Status: Special Concern (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 species is found throughout the Tennessee, Cumberland, and Ohio river systems in the United States from western Pennsylvania and peninsular Michigan west to eastern Illinois, and also occurred in Lake Erie, Lake St. Clair, and some of their tributaries (Parmalee and Bogan, 1998; Clarke, 1981; Watters et al., 2009). In Canada, it was known from the western basin of Lake Erie, Lake St. Clair, and the Welland, Grand, Detroit, Thames and Sydenham rivers but the Lake St. Clair population appears to be the only significant population of O. subrotunda left in Canada. It is now restricted to the East Sydenham River and a portion of the Lake St. Clair delta (COSEWIC, 2003).

Area of Occupancy: 2,501 to >12,500 4-km2 grid cells
Area of Occupancy Comments: In Canada, it is now restricted to the East Sydenham River and a portion of the Lake St. Clair delta. The current extent of occurrence in Canada is approximately 1750 km2 and the area of occupancy is 8 km2

Number of Occurrences: 21 - 300
Number of Occurrences Comments: Just over 100 occurrences have been reported since 1970 with a majority from Indiana. Others are from Tennessee, Michigan and Illinois. However it may now be extirpated from Illinois (collected once in the Vermillion since the 1950s with fresh dead specimens recently- Cummings and Mayer, 1997). Indiana distribution: East Fork White as weathered shells (Harmon, 1992), Muscatatuck (Harmon, 1989), Tippecanoe (Cummings and Berlocher, 1990), St. Joseph (Pryor, 2005). In Ohio, it is widely distributed but sporadic (Watters, 1992; 1995; Lyons et al., 2007; Hoggarth et al., 2007); locally common in Shade River and Middle Fork Salt Creek, uncommon in Big Darby and Fish Creeks, rare in Muskingum River and small southern streams, and found in the Grand, Sandusky, and Maumee River systems, also in the Grand, Sandusky, and Maumee systems (Watters et al., 2009). In Tennessee, it was once found throughout the Tennessee River system in the Powell, Clinch, Holston, Pigeon, Little Tennessee, Sequatchie, Elk, Buffalo, and Duck Rivers and main channel of the Tennessee River, but has disappeared from most of these rivers. In the Cumberland River system, it occurs in the Obey, Stones, Harpeth, and Red Rivers and the mainsteim of the Cumberland River (Parmalee and Bogan, 1998 Gordon and Layzer, 1989). Hubbs (2002) found relict shells in the Elk (RM 105.4) River, Tennessee. In Georgia, it may occur over an extremely limited range in the northern parts in the Tennessee River drainage (P. Hartfield, USFWS, pers. comm., July 2001, in COSEWIC, 2003). In Alabama, it is rare and restricted to the Tennessee River system where it is extant only in the Paint Rock River system (Ahlstedt, 1996; Mirarchi et al., 2004) and Bear Creek in Colbert Co. (Williams et al., 2008). In Mississippi, it occurs in the Mississippi River South, Big Black, and Yazoo drainages (Jones et al., 2005). In Mississippi, it was once common, or apparently so, in the Mississippi River watershed, but is now restricted to short segments of the Big Black; and the only collections since 2000 were in a short reach in Montgomery Co.; while older collections were made downstream in Attala Co. (MS NHP, pers. comm., May 2009). It is not known from Louisiana, but some nearby Mississippi sites include the Big Black River and Bayou Pierre (Vidrine, 1993). In Kentucky, it is occasional to sporadic in the lower Cumberland drainage and eastward (Cicerello and Schuster, 2003; Clark, 1988), including Middle Green (Cochran and Layzer, 1993) and South Fork Kentucky (Evans, 2008). In New York, it is only known from a weathered shell in the Allegheny region in the southwest of the state but is likely now extirpated (Strayer and Jirka, 1997). Although once thought to be extirpated in Pennsylvania (Bogan, 1993), it is historically known from the Upper Ohio, Shenango, Lower Monongahela, Middle Allegheny-Redbank, Beaver, Mahoning, and Lower Alleghany drainages (Ortmann, 1919) and is likely still extant in the Shenango (Bursey, 1987). In West Virginia, it was found in the Upper Ohio/Kanawha (Zeto et al., 1987; Morris and Taylor, 1992)) and Mud Rivers (Guyandotte drainage) (Schmidt and Zeto, 1986). It is historically known from the Clinton River drainage in Michigan (Strayer, 1980); recently Pine and Belle (Badra and Goforth, 2003). In Canada, the current distribution is based on 30 records (live animals and shells collected over the past decade (1991 - 2001). Live specimens were most recently collected from the East Sydenham River and the St. Clair delta in Lake St. Clair in the summer of 2001 (Metcalfe-Smith et al., 2003; COSEWIC, 2003). It is now restricted to the East Sydenham River and a portion of the Lake St. Clair delta. The current extent of occurrence in Canada is approximately 1750 km2 and the area of occupancy is 8 km2.

Population Size: 100,000 - 1,000,000 individuals
Population Size Comments: Due to problems obtaining a unbiased and complete sample, abundance in mussels is always difficult to estimate, and no estimates of population size or abundance have been made for this species.

Number of Occurrences with Good Viability/Integrity: Unknown
Viability/Integrity Comments: In Canada, the Lake St. Clair population appears to be the only significant population of O. subrotunda left in Canada (COSEWIC, 2003). Historically it was plentiful in the Wabash and Tippecanoe River (Blatchley and Daniels, 1903).

Overall Threat Impact: High
Overall Threat Impact Comments: Smith (1971) ranked the causes of extirpation or declines in fish species as follows: siltation, drainage of bottomland lakes, swamps, and prairie marshes, desiccation during drought, species introductions, pollution, impoundments, and increased water temperatures. All of these factors render habitats unsuitable, cause extirpations, and lead to the isolation of populations thereby increasing their vulnerability to extirpation for many aquatic species (including mussels) throughout North America. Zebra mussels DREISSENA POLYMORPHA have destroyed mussel populations in the Great Lakes and significantly reduced mussels in many of the large rivers of the eastern North America and has the potential to severely threaten and other populations especially if it makes its way into smaller streams. Pollution through point (industrial and residential discharge) and non-point (siltation, herbicide and fertilizer run-off) sources is perhaps the greatest on-going threat to this species and most freshwater mussels. Lowered dissolved oxygen content and elevated ammonia levels (frequently associated with agricultural runoff and sewage discharge) have been shown to be lethal to some species of freshwater naiads (Horne and McIntosh, 1979). Residential, mineral and industrial development also pose a significant threat. Destruction of habitat through stream channelization and maintenance and the construction of dams although slowed in recent years is still a threat in some areas. Impoundments reduce currents that are necessary for the most basic physiological activities such as feeding, waste removal and reproduction. In addition, reduced water flow typically results in a reduction in water oxygen levels and a settling out of suspended solids (silt, etc.), both of which are detrimental. Dredging of streams has an immediate effect on existing populations by physically removing and destroying individuals. Dredging also affects the long-term recolonization abilities by destroying much of the potential habitat, making the substrates and flow rates uniform throughout the system. Rotenone, a toxin used to kill fish in bodies of water for increased sport fishery quality, has been shown to be lethal to mussels as well (Heard, 1970). Natural predators include raccoons, otter, mink, muskrats, turtles and some birds, which feed heavily upon freshwater mussels (Simpson, 1899; Boepple and Coker, 1912; Evermann and Clark, 1918; Coker, et al. 1921; Parmalee, 1967; Snyder and Snyder, 1969). Domestic animals such as hogs can root mussel beds to pieces (Meek and Clark, 1912). Fishes, particularly catfish, ICTALURUS SPP. AND AMIEURUS SPP. and freshwater drum, APLODINOTUS GRUNNIENS also consume large numbers of unionids. See the General Freshwater Mussel ESA. Approximately 64% of historical records for O. subrotunda are from waters now infested with zebra mussels; this is probably the greatest threat to this species (COSEWIC, 2003).

Short-term Trend: Decline of 10-30%
Short-term Trend Comments: In Mississippi, it was once common, or apparently so, in the Mississippi River watershed, but is now restricted to short segments of the Big Black; and the only collections since 2000 were in a short reach in Montgomery Co.; while older collections were made downstream in Attala Co. (MS NHP, pers. comm., May 2009). It may be extirpated from Swan Creek (Lower Maumee) in Ohio (Grabarciewicz, 2008). In the Midwest, the round hickorynut is endangered in Illinois and Michigan and is absent from many sites within its former range in Indiana (Cummings and Mayer, 1992; Cummings pers. comm.). This mussel has not been collected alive in Illinois in over 20 years and it is likely extirpated from the state. Declines are evident across its range, particularly in Canada. In Ontario, the species has apparently been lost from the Grand and Thames riversand has declined dramatically in the Sydenham River. The Lake St. Clair population appears to be the only significant population of O. subrotunda left in Canada. In Canada, O. subrotunda is known only from southern Ontario In Ontario, the species has apparently been lost from the Grand and Thames riversand has declined dramatically in the Sydenham River. The Lake St. Clair population appears to be the only significant population of O. subrotunda left in Canada (COSEWIC, 2003). Overall, the round hickorynut has been lost from over 90% of its historical range in Canada. It is now restricted to the East Sydenham River and a portion of the Lake St. Clair delta. The current extent of occurrence is approximately 1750 km2 and the area of occupancy is 8 km2 (COSEWIC, 2003; Metcalfe-Smith and Cudmore-Vokey, 2004).

Long-term Trend: Decline of 30-50%
Long-term Trend Comments: It has been extirpated from the Thames River watershed in Ontario, Canada, since the turn of the century (Cudmore et al., 2004). In New York, it is only known from a weathered shell in the Allegheny region in the southwest of the state but is likely now extirpated (Strayer and Jirka, 1997). Although once thought to be extirpated in Pennsylvania (Bogan, 1993), it is historically known from the Upper Ohio, Shenango, Lower Monongahela, Middle Allegheny-Redbank, Beaver, Mahoning, and Lower Alleghany drainages (Ortmann, 1919) and is likely still extant in the Shenango (Bursey, 1987). In Ohio, historically it was found throughout the Scioto and Hocking Rivers; was rare in Great and Little Miami Rivers, historically from Lake Erie, particularly the western basin (Watters et al., 2009).

Intrinsic Vulnerability: Not intrinsically vulnerable

Environmental Specificity: Broad. Generalist or community with all key requirements common.
Environmental Specificity Comments: Found in medium-sized to large streams in sand and gravel in areas with moderate flow (Cummings and Mayer, 1992). Typically found in medium-sized to large rivers, usually at depths of less than six feet (Parmalee and Bogan 1998), but also occurs in Lake Erie and Lake St. Clair (COSEWIC, 2003).

Other NatureServe Conservation Status Information

Inventory Needs: Populations and geographical limits not well defined in Illinois or Indiana. Additional survey work in those states would better define the range and status of the species. Periodic surveys of known populations should be done to monitor the status of the remaining populations. An inventory of existing museum records should be compiled to provide information on historical sites and potential new ones.

Protection Needs: Obovaria subrotunda is currently listed as endangered in Illinois, Michigan, and Alabama (and proposed for endangered status in Pennsylvania), threatened in Tennessee, and special concern in Indiana, and is therefore afforded some protection in these states (COSEWIC, 2003).

Distribution
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Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) This species is found throughout the Tennessee, Cumberland, and Ohio river systems in the United States from western Pennsylvania and peninsular Michigan west to eastern Illinois, and also occurred in Lake Erie, Lake St. Clair, and some of their tributaries (Parmalee and Bogan, 1998; Clarke, 1981; Watters et al., 2009). In Canada, it was known from the western basin of Lake Erie, Lake St. Clair, and the Welland, Grand, Detroit, Thames and Sydenham rivers but the Lake St. Clair population appears to be the only significant population of O. subrotunda left in Canada. It is now restricted to the East Sydenham River and a portion of the Lake St. Clair delta (COSEWIC, 2003).

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 AL, AR, GA, ILextirpated, IN, KY, MI, MS, NY, OH, PA, TN, WV
Canada ON

Range Map
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U.S. Distribution by County Help
State County Name (FIPS Code)
AL Colbert (01033)*, Franklin (01059)*, Jackson (01071), Lauderdale (01077)*, Madison (01089), Marshall (01095)
IL Vermilion (17183)*
IN Adams (18001), Allen (18003), Bartholomew (18005), Carroll (18015), Cass (18017), Clay (18021), Crawford (18025), Daviess (18027), De Kalb (18033), Dubois (18037), Fountain (18045), Fulton (18049), Gibson (18051), Grant (18053), Greene (18055), Hamilton (18057), Harrison (18061), Huntington (18069), Jackson (18071), Jefferson (18077), Jennings (18079), Johnson (18081), Knox (18083), Kosciusko (18085), Lawrence (18093), Marion (18097), Marshall (18099), Martin (18101), Miami (18103), Monroe (18105), Morgan (18109)*, Owen (18119), Parke (18121), Pike (18125), Posey (18129)*, Pulaski (18131), Putnam (18133), Shelby (18145), Starke (18149), Tippecanoe (18157), Vermillion (18165), Vigo (18167), Wabash (18169), Warren (18171), Washington (18175), Wells (18179), White (18181)
KY Clay (21051), Green (21087)*, Lee (21129), Owsley (21189)
MI Lenawee (26091)*, Macomb (26099), Monroe (26115)*, Sanilac (26151)*, St. Clair (26147), Wayne (26163)
MS Attala (28007), Copiah (28029), Franklin (28037)*, Hinds (28049)*, Holmes (28051), Lincoln (28085)*, Madison (28089), Montgomery (28097), Tishomingo (28141)*, Warren (28149)*, Yazoo (28163)
NY Chautauqua (36013)*
PA Allegheny (42003)*, Armstrong (42005)*, Indiana (42063)*, Lawrence (42073)*, Mercer (42085), Washington (42125)*, Westmoreland (42129)*
TN Bedford (47003), Cheatham (47021), DeKalb (47041)*, Dickson (47043), Franklin (47051)*, Hancock (47067), Jefferson (47089)*, Knox (47093)*, Lincoln (47103), Marshall (47117), Maury (47119), Moore (47127)*, Putnam (47141)*, Robertson (47147)*, Sequatchie (47153)*, Smith (47159)*
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
04 St. Clair (04090001)+, Lake St. Clair (04090002)+, Clinton (04090003)+, Detroit (04090004)+*, Huron (04090005)+*, Ottawa-Stony (04100001)+*, Raisin (04100002)+*, St. Joseph (04100003)+, St. Marys (04100004)+, Upper Maumee (04100005)+*
05 Conewango (05010002)+*, Middle Allegheny-Redbank (05010006)+*, Lower Allegheny (05010009)+*, Lower Monongahela (05020005)+*, Upper Ohio (05030101)+*, Shenango (05030102)+, Mahoning (05030103)+*, Beaver (05030104)+*, South Fork Kentucky (05100203)+, Upper Green (05110001)+*, Upper Wabash (05120101)+, Salamonie (05120102)+, Mississinewa (05120103)+, Eel (05120104)+, Middle Wabash-Deer (05120105)+, Tippecanoe (05120106)+, Wildcat (05120107)+, Middle Wabash-Little Vermilion (05120108)+, Vermilion (05120109)+, Sugar (05120110)+*, Middle Wabash-Busseron (05120111)+, Lower Wabash (05120113)+, Upper White (05120201)+, Lower White (05120202)+, Eel (05120203)+, Driftwood (05120204)+, Flatrock-Haw (05120205)+, Upper East Fork White (05120206)+, Muscatatuck (05120207)+, Lower East Fork White (05120208)+, Caney (05130108)+*, Harpeth (05130204)+, Red (05130206)+*, Blue-Sinking (05140104)+
06 Holston (06010104)+*, Upper Clinch (06010205)+, Sequatchie (06020004)+*, Wheeler Lake (06030002)+, Upper Elk (06030003)+, Pickwick Lake (06030005)+*, Bear (06030006)+*, Upper Duck (06040002)+
08 Upper Big Black (08060201)+, Lower Big Black (08060202)+, Bayou Pierre (08060203)+, Homochitto (08060205)+*
+ 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 relatively small freshwater mussel with an almost perfectly round shell and a smooth, brown, rayless periostracum.
General Description: SHELL: Shell round or circular, moderately thick, and inflated. Anterior and posterior ends rounded in males, somewhat truncated in females. Umbos low and centrally placed, slightly elevated above the hinge line. Beak sculpture consists of a few indistinct, concentric ridges, usually evident only on very young shells. Shell smooth, growth lines dark but often indistinct. Periostracum yellowish brown to dark chestnut brown or blackish, and rayless. Adult size to 3 inches. Pseudocardinal teeth moderately small, serrated, and divergent. Lateral teeth slightly curved, fairly short, and moderately thin. Interdentum narrow or absent. Beak cavity fairly deep. Nacre white, iridescent posteriorly (Cummings and Mayer, 1992).

ANIMAL: "Structure of soft parts essentially the same as in O. retusa. In the sterile female, the ovisacs are slightly narrower than the regular water-tubes. Number of ovisacs up to thirty and more. As in the preceding species the edge of the mantle in front of the branchial is in the female slightly lamellar and crenulated, but has only a brown (not blackish) mark along it." (Ortmann, 1912:323).

From COSEWIC (2003): The round hickorynut, Obovaria subrotunda (Rafinesque, 1820) is easily recognized by its circular shape, centrally located beaks, unsculptured and unrayed periostracum, and relatively small size. It may be occasionally confused with smooth specimens of Quadrula pustulosa, which, however, have a bright golden-yellow periostracum. The type locality is "l'Ohio" (the Ohio River). The following description of the species was adapted from Clarke (1981), Strayer and Jirka (1997) and Parmalee and Bogan (1998). The shell is circular to subcircular and thick. The surface is smooth except for prominent growth rests. The periostracum is generally dark brown or olive-brown and without rays except in some very young specimens. The posterior slope is distinctly lighter than the remainder of the shell. Beaks are centrally placed, curved inward, and elevated well above the hinge line. Beak sculpture is fine and consists of 4-6 short, slightly sinuous bars. Hinge teeth are rather heavy and strong. The left valve has two thick, roughened, triangular pseudocardinal teeth and two short, strong, slightly curved lateral teeth. The right valve has one large, triangular serrated pseudocardinal tooth, usually with two small, compressed teeth on either side, and one short, thick, roughened lateral tooth, often with a secondary inner low, incomplete lateral tooth. The interdentum is narrow or absent. Adductor scars are deeply impressed. The nacre is silvery white, with a tinge of blue or pink in some specimens. Sexual differences in the shell are obscure; in females, the posterior margin of the shell may be truncated. However, there is a distinct difference in size, with females being considerably smaller than males. The species shows considerable ecophenotypic variation in shell inflation among specimens from large rivers, small rivers, and lakes.

Reproduction Comments: This species is bradytictic and egg-bearing females are found in September, and glochidia in late September with glochidia overwintering until the following May and June (Ortmann, 1919). Gravid females were found in the Ohio River drainage of western Pennsylvania in September; females discharging glochidia were found on 27 May. (Ortmann, 1912: 323). The host species for this mussel is unknown (Watters et al., 2009). The round hickorynut, like most freshwater mussels, is considered to be dioecious. Hermaphroditism has not been observed in this species. The lifespan of O. subrotunda is not known, but members of the subfamily Lampsilinae generally grow more rapidly and have shorter life spans than members of the Ambleminae, which can live for over 40 years. For comparison, life spans of three other COSEWIC-listed lampsilines are: 10-20 years for L. fasciola (Metcalfe-Smith et al. 2000c), more than 15 years for Epioblasma torulosa rangiana, and up to 11 years for V. fabalis (from COSEWIC, 2003).
Habitat Type: Freshwater
Non-Migrant: N
Locally Migrant: N
Long Distance Migrant: N
Riverine Habitat(s): BIG RIVER, Low gradient, MEDIUM RIVER, Moderate gradient, Riffle
Lacustrine Habitat(s): Shallow water
Special Habitat Factors: Benthic
Habitat Comments: This species is typically found in medium-sized to large streams and rivers in sand and gravel in areas with moderate flow, usually at depths of up to 2 meters (Cummings and Mayer 1992, Parmalee and Bogan 1998), but also occurs in Lake Erie and Lake St. Clair (COSEWIC, 2003). In Ohio, it is associated with a variety of flow regimes; riffles and runs in Big Darby Creek system, slow-moving water in sandy mud in Shade River and Salt Creek (Watters et al., 2009). In southeastern Michigan, it has mainly been found in turbid, low-gradient, hydrologically unstable rivers with clay/sand or clay/gravel substrates (D. Woolnough pers. comm. 2015).
Length: 6 centimeters
Economic Attributes Not yet assessed
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Management Summary
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Biological Research Needs: In order to effectively manage mussel species it is necessary to work out certain life history characteristics first. Host species for this mussel is unknown. Because of their unusual life-cycle and dependence on fish for completion of that cycle, it is imperative that the host species for the round hickorynut be ascertained. Work needs to be done to identify age and size at sexual maturity, recruitment success, age class structure, and other important life history parameters.

Research is needed to assess the success of watershed protection on mussel populations. Abundance and distribution of selected species needs to be monitored in order to ascertain how species abundanceOs change over time. From that we can assess what land-use changes, conservation practices, and physical/chemical parameters are correlated with, and possibly responsible for, the biological changes.

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: 03Jan2010
NatureServe Conservation Status Factors Author: Cordeiro, J. (2010); Whittaker, J.C.; Cummings, K.S. (1998)
Element Ecology & Life History Edition Date: 03Jan2010
Element Ecology & Life History Author(s): Cordeiro, J.

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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