The round pigtoe was historically found in the upper Mississippi drainages and the St. Lawrence drainage. Generally it is found from western New York and Ontario west through Wisconsin and Minnesota to the southeastern portion of South Dakota, south to Oklahoma, Arkansas, Mississippi and Alabama. Its eastern range includes western Virginia, West Virginia, and western Pennsylvania.
In Michigan P. sintoxia is found in mid-Michigan drainages, mainly from the Muskegon and Saginaw River systems on through the southern river systems. Specimens have also been collected in the Sturgeon River in the upper peninsula.
Biogeographic Regions: nearctic (Native )
- Burch, J. 1975. Freshwater unionacean clams (Mollusca: Pelecypoda) of North America. Hamburg, Michigan: Malacological Publications.
- Cummings, K., C. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Champaign, Illinois: Illinois Natural History Survey Manual 5. Accessed August 25, 2005 at http://www.inhs.uiuc.edu/cbd/collections/mollusk/fieldguide.html.
- COSEWIC, 2004. COSEWIC assessment and status report on the round pigtoe, Pleurobema sintoxia, in Canada. Committee on the Status of Endangered Wildlife in Canada, 2004: 1-33. Accessed September 12, 2006 at http://www.sararegistry.gc.ca/virtual_sara/files/cosewic/sr_round_pigtoe_e.pdf.
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
Global Range: (200,000-2,500,000 square km (about 80,000-1,000,000 square miles)) This species was historically distributed from New York and Ontario west to South Dakota, Kansas and Oklahoma, and south to Louisiana and Alabama. Large river populations largely eliminated in the upper Midwest, but many populations still survive in Mississippi River tributaries. The current distribution of the Round Pigtoe is similar to the historical range. Although large river populations have for the most part disappeared from the upper Midwest, many populations still survive in tributaries of the Mississippi and Ohio rivers. In Canada Pleurobema sintoxia is only known from southern Ontario (COSEWIC, 2004) including the Thames River (Cudmore et al., 2004). Recently this species has been confirmed to be nearly extirpated (last live specimens probably do not represent a viable population) in the main channel of the Detroit River between Lake St. Clair and Lake Erie, Michigan/Ontario; due to zebra mussel invasion (Schloesser et al., 2006). Long-standing population exist in the Poteau River and tributaries, Arkansas and Oklahoma (Vaughn and Spooner, 2004).
The round pigtoe is up to 10.2 cm (4 inches) long , and is round and compressed in headwaters, triangular and inflated in rivers. The shell is usually fairly thick. The anterior end is broadly rounded, the posterior end rounded to bluntly pointed. The dorsal margin is straight to slightly curved and the ventral margin is gently curved.
Umbos are low, being raised only slightly above the hinge line, and can be centered on the shell or more anterior. The beak sculpture with two elevated ridges on the umbo or posterior ridge.
The periostracum (outer shell layer) is green-brown, red-brown or light brown with green rays. Older specimens tend to be more brown and lack rays.
On the inner shell, the left valve has two pseudocardinal teeth, which are rough, large, and grooved. The anterior tooth is about one third the size of the posterior tooth. The two lateral teeth are straight, short, and somewhat striated. The right valve has one large, erect, notched and grooved pseudocardinal tooth. The one lateral tooth has high, broad and striated.
The beak cavity is shallow to moderately deep. Although the nacre is white, occasionally it is has a pink or salmon tint and is iridescent at the posterior end.
In Michigan, this species is most difficult to distinguish from the headwater forms of the Wabash pigtoe, and is often only distinguished by looking at the teeth on the inner part of the valves of the shells. The Wabash pigtoe has a deeper beak cavity. When present in females, glochidial packets from the Wabash pigtoe also tend to be orangish while the round pigtoe has white glochidial packets. Other similar species include the hickorynut and the round hickorynut, which are generally more inflated. The hickorynut is more slightly more oval in shape. The round hickorynut is more rounded and generally lighter at the posterior end.
Range length: 10.2 (high) cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike
- Oesch, R. 1984. Missouri naiades, a guide to the mussels of Missouri. Jefferson City, Missouri: Missouri Department of Conservation.
Length: 1.2 cm
In Canada the Round Pigtoe may be confused with the Wabash Pigtoe (Fusconaia flava). The main features distinguishing F. flava from P. sintoxia are a lower and more centrally located beak, deeper lateral sulcus, and deeper beak cavity in F. flava. The lake form of P. sintoxia is somewhat similar to Obovaria olivaria, but has flatter valves, a duller periostracum, and is less likely to have coloured rays (COSEWIC, 2004).
The round pigtoe is generally widespread from small to large rivers. Substrates it inhabits include mud, sand and gravel with moderate flows.
In Lake St. Clair it is found in nearshore areas less than 1 m with firm, sandy substrates.
Habitat Regions: freshwater
Aquatic Biomes: rivers and streams
- Watters, G. 1995. A guide to the freshwater mussels of Ohio. Columbus, Ohio: Ohio Department of Natural Resources.
- Zanatta, D., G. Mackie, J. Metcalfe-Smith, D. Woolnough. 2002. A refuge for native freshwater mussels (Bivalvia:Unionidae) from impacts of the exotic zebra mussel (Dreissena polymorpha) in Lake St. Clair. Journal of Great Lakes Research, 28: 479-489.
Habitat Type: Freshwater
Comments: This species is found in medium to large rivers in mixed mud, sand, and gravel (Cummings and Mayer, 1992). In Canada, the Round Pigtoe is typically found in medium-sized to large rivers but also occurs in Lake Erie and Lake St. Clair (COSEWIC, 2004). Parmalee and Bogan (1998) reported Tennessee occurrences most abundantly, and almost exclusively, in medium-sized and big rivers and in current on a firm substrate of coarse gravel and sand at depths of less than three feet to more than 20 feet.
Non-Migrant: No. All populations of this species make significant seasonal migrations.
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.
In general, unionids are filter feeders. The mussels use cilia to pump water into the incurrent siphon where food is caught in a mucus lining in the demibranchs. Particles are sorted by the labial palps and then directed to the mouth. Mussels have been cultured on algae, but they may also ingest bacteria, protozoans and other organic particles.
The parasitic glochidial stage absorbs blood and nutrients from hosts after attachment. Mantle cells within the glochidia feed off of the host’s tissue through phagocytocis.
Plant Foods: algae; phytoplankton
Other Foods: detritus ; microbes
Foraging Behavior: filter-feeding
Primary Diet: planktivore ; detritivore
- Meglitsch, P., F. Schram. 1991. Invertebrate Zoology, Third Edition. New York, NY: Oxford University Press, Inc.
Comments: Round Pigtoes, like all species of freshwater mussels, are filter feeders as adults. Their primary food sources are bacteria, algae, particles of organic detritus, and some protozoans. Food availability may be a limiting factor for the Lake St. Clair population due to the presence of high densities of Zebra Mussels, which are also filter-feeders. During the parasitic larval stage, glochidia feed on the body fluids of the host (COSEWIC, 2004).
Fish hosts are determined by looking at both lab metamorphosis and natural infestations. Looking at both is necessary, as lab transformations from glochidia to juvenile may occur, but the mussel may not actually infect a particular species in a natural situation. Natural infestations may also be found, but glochidia will attach to almost any fish, including those that are not suitable hosts. Lab transformations involve isolating one particular fish species and introducing glochidia either into the fish tank or directly inoculating the fish gills with glochidia. Tanks are monitored and if juveniles are later found the fish species is considered a suitable host.
In lab trials, Pleurobema sintoxia glochidia metamorphosed on the bluntnose minnow, northern redbelly dace, southern redbelly dace and spotfin shiner.
Ecosystem Impact: parasite
Species Used as Host:
- Hove, M., R. Engelking, M. Peteler, E. Peterson, Kapuscinski, A.R., Sovell, L.A. and E.R. Evers. 1995. Suitable fish hosts for glochidia of four freshwater mussels. Conservation and Management of Freshwater Mussels II. Proceedings of a UMRCC Symposium, 16-18 October 1995, St. Louis, Missouri: 21-25.
- Hove, M. 1995. Host research on round pigtoe glochidia. Triannual Unionid Report, 8: 8. Accessed October 04, 2005 at http://ellipse.inhs.uiuc.edu/FMCS/TUR/TUR8.html#p6.
Unionids in general are preyed upon by muskrats, raccoons, minks, otters, and some birds. Juveniles are probably also fed upon by freshwater drum, sheepshead, lake sturgeon, spotted suckers, redhorses, and pumpkinseeds.
Unionid mortality and reproduction is affected by unionicolid mites and monogenic trematodes feeding on gill and mantle tissue. Parasitic chironomid larvae may destroy up to half the mussel gill.
- muskrat, Ondatra zibethicus
- mink, Neovison vison
- raccoon Procyon lotor
- otter, Lontra canadensis
- turtles, Testudines
- hellbenders, Cryptobranchus
- freshwater drum, Aplodinotus grunniens
- sheepshead, Archosargus probatocephalus
- lake sturgeon, Acipenser fulvescens
- shortnosed sturgeon, Acipenser brevirostrum
- spotted suckers, Minytrema melanops
- common red-horse, Moxostoma
- catfish, Siluriformes
- pumpkinseed, Lepomis gibbosus
100,000 to >1,000,000 individuals
Comments: Many healthy populations are still extant in the upper Mississippi River drainage. Due to problems obtaining a unbiased and complete sample, abundance in mussels is difficult to estimate, and no estimates of population size or abundance have been made. Smith and Crabtree (2010) found this species at 7 of 32 sites (1 with recruitment) along the entire length of Pennsylvania's French Creek.
Number of Occurrences
Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: 81 to >300
Comments: In Pennsylvania, it is in 12 sites in the Allegheny (Little Mahoning Creek- Chapman and Smith, 2008), 14 in the French Creek, and 2 in the Lake Erie drainage. In New York, it is widespread in the Allegheny basin, but likely gone from the Niagara basin. There are several records throughout West Virginia (nowhere in abundance) in the Monongahela and Little Kanawha River drainages, Elk, Kanawha, Ohio Rivers, and Middle Island Creek. It is not common in Indiana (Muscatatuck- Harmon, 1989; Tippecanoe- Cummings and Berlocher, 1990; St. Joseph- Pryor, 2005) and Illinois (Mackinaw, Kankakee, Sangamon, Embarras River drainages (Cummings and Mayer, 1997; Schanzle and Cummings, 1991; Sietman et al., 2001), but is widespread in the Mississippi and Ohio drainages; recently Fox basin in Illinois and Wisconsin (Schanzle et al., 2004). In Ohio, it was widespread (Watters, 1992; 1995) but has declined with only a few recent records: St. Joseph and tribs. in Williams Co., Olentangy, Big Darby, Little Darby, Caesar Creeks, Walhonding River, lower Muskingum River, and western basin of Lake Erie (Watters et al., 2009). In Tennessee, it is restricted to the Cumberland, Big South Fork Cumberland, and Stones rivers (Parmalee and Bogan, 1998) including Muscle Shoals region (Tennessee/Alabama) again in 2001. In Alabama, it is rare (Wilson and Guntersville Dam tailwaters) but was historic through the Tennessee River (Mirarchi, 2004; Williams et al., 2008). It is in Kentucky in the S Fork Kentucky (Evans, 2008), Middle Green and Barren Rivers (Cochran and Layzer, 1993), but in the Green River eastward (Cicerello and Schuster, 2003; Gordon, 1991); also Ohio River (not widespread nor abundant), upper Green and major tribs., Rolling Fork (Salt River trib.), upper Cumberland River's major tribs., and Big Sandy River. Oklahoma distribution (as P. cordatum): Chikaskia (as P. coccineum), Little, Kiamichi, Verdigris (Boeckman and Bidwell, 2008), Neosho, Poteau, Mountain Fork, and Spring Rivers (Branson, 1966; 1983) into Missouri, but N occurrences likely P. sintoxia; a small population in the Red River. In Kansas, range includes the Neosho, Verdigris, Elk, Fall, Spring, and Marais des Cygnes Rivers but scattered (Couch, 1997); historical in Wakarusa (Tiemann, 2006); ongoing work on the Verdigris (Miller and Lynott, 2006). In the Big Blue system of SE Nebraska and NE Kansas, it was historically rare in Kansas (1 museum plus 1 from Delaware River); but not over a century (Hoke, 2005). Recently only dead shells in the Marais des Cygnes, Elk, and Fall Rivers in Kansas (Combes and Edds, 2005). It was recently collected in Poteau (Vaughn and Spooner, 2004), St. Francis (Ahlstedt and Jenkinson, 1991), Cache and White Rivers, Arkansas (Christian, 1995; Christian et al., 2005). It is relatively common in Missouri, in 13 major drainages throughout the S half (Oesch, 1995); but little recruitment (MO DOC 359 records from 1977-1999, 79% live/ fresh dead). It has not been seen in recent surveys of Iowa, but reported historically. It has not been found alive recently in Presque Isle Bay or Thompson Bay (outer harbour of Presque Isle Bay). In Michigan, it is fairly widespread in the Great Lakes and Ohio River drainages (incl. Clinton River drainage- Trdan and Hoeh, 1993), but seldom common, but still persists in Lake St. Clair (COSEWIC, 2004; Strayer, 1980) incl. Pine (Badra and Goforth, 2003). In Canada in the Thames River, in 8 of 16 sites in a 150 km reach in 1997-1998 and historically broadly distributed, but now limited to the upper reaches of the Middle Thames near Thamesford and upstream to near the S Thames confluence (Cudmore et al., 2004). It also occurs in Ontario's Sydenham River (Metcalfe-Smith et al., 2003). Recently confirmed nearly extirpated (last live specimens probably not viable) in the main channel Detroit River between Lake St. Clair and Lake Erie, Michigan/Ontario; due to zebra mussel invasion (Schloesser et al., 2006).
Life History and Behavior
Communication and Perception
The middle lobe of the mantle edge has most of a bivalve's sensory organs. Paired statocysts, which are fluid filled chambers with a solid granule or pellet (a statolity) are in the mussel's foot. The statocysts help the mussel with georeception, or orientation.
Mussels are heterothermic, and therefore are sensitive and responsive to temperature.
Unionids in general may have some form of chemical reception to recognize fish hosts. How the round pigote attracts its fish host is unknown. However, white glochidial packets that are released resemble invertebrates that fish would eat.
Glochidia respond to both touch, light and some chemical cues. In general, when touched or a fluid is introduced, they will respond by clamping shut.
Communication Channels: visual ; chemical
Perception Channels: visual ; tactile ; vibrations ; chemical
- Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
Fertilized eggs are brooded in the marsupia (water tubes) up to three months, where they develop into larvae, called glochidia. The glochidia are then released into the water where they must attach to the gill filaments and/or general body surface of the host fish. After attachment, epithelial tissue from the host fish grows over and encapsulates a glochidium, usually within a few hours. The glochidia then metamorphoses into a juvenile mussel within a few days or weeks. After metamorphosis, the juvenile is sloughed off as a free-living organism. Juveniles are found in the substrate where they develop into adults.
Development - Life Cycle: metamorphosis
- Arey, L. 1921. An experimental study on glochidia and the factors underlying encystment. J. Exp. Zool., 33: 463-499.
- Lefevre, G., W. Curtis. 1910. Reproduction and parasitism in the Unionidae. J. Expt. Biol., 9: 79-115.
The age of mussels can be determined by looking at annual rings on the shell. However, no demographic data on this species has been recorded.
Age to sexual maturity for this species is unknown. Unionids are gonochoristic (sexes are separate) and viviparous. The glochidia, which are the larval stage of the mussels, are released live from the female after they are fully developed.
In general, gametogenesis in unionids is initiated by increasing water temperatures. The general life cycle of a unionid, includes open fertilization. Males release sperm into the water, which is taken in by the females through their respiratory current. The eggs are internally fertilized in the suprabranchial chambers, then pass into water tubes of the gills, where they develop into glochidia.
Pleurobema sintoxia is a short-term brooder. In southeastern Michigan, it was gravid from mid May to late July, and probably spawns in early May.
Breeding interval: The round pigtoe breeds once in the warmer months of the year.
Breeding season: In Michigan, the breeding season is probably early to mid-May.
Range gestation period: 2.5 (high) months.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (External ); viviparous
Females brood fertilized eggs in their marsupial pouch. The fertilized eggs develop into glochidia. There is no parental investment after the female releases the glochidia.
Parental Investment: pre-fertilization (Provisioning); pre-hatching/birth (Provisioning: Female)
- van der Schalie, H. 1938. The naiad fauna of the Huron River, in southeastern Michigan. Miscellaneous Publications of the Museum of Zoology, University of Michigan, 40: 1-83.
- Watters, G. 1995. A guide to the freshwater mussels of Ohio. Columbus, Ohio: Ohio Department of Natural Resources.
- Lefevre, G., W. Curtis. 1912. Experiments in the artificial propagation of fresh-water mussels. Proc. Internat. Fishery Congress, Washington. Bull. Bur. Fisheries, 28: 617-626.
Pleurobema sintoxia is believed to be sexually dioecious, but is not sexually dimorphic. The lifespan of P. sintoxia has not yet been determined, but other members of the Subfamily Ambleminae are known to live for more than 30 years. Age to maturity for this species is not known, but the juvenile stage for most unionids lasts 2-5 years. The Round Pigtoe is a short-term brooder (tachytictic) with the breeding season lasting from early May to late July in Wisconsin (Parmalee and Bogan 1998). The glochidia are subovate, without hooks, measuring 150 µm in both height and width according to Clarke (1981) and 160 µm according to Hoggarth (1992). The lack of hooks suggests that they are gill parasites. Early reported hosts include the bluegill (Lepomis macrochirus) (Surber, 1913; Coker et al., 1921). Recently identified host fishes for this mussel are the spotfin shiner (Cyprinella spiloptera), bluntnose minnow (Pimephales notatus), northern redbelly dace (Phoxinus eos), and southern redbelly dace (Phoxinus erythrogaster) (Hove, 1995; Hove et al., 1997). New host fish confirmations wer identified from Watters et al. (2005): central stoneroller (Campostoma anomalum), bluegill (Lepomis macrochirus), creek chub, southern redbelly dace (Phoxinus erythrogaster).
Molecular Biology and Genetics
Statistics of barcoding coverage: Pleurobema sintoxia
Public Records: 6
Specimens with Barcodes: 6
Species With Barcodes: 1
Barcode data: Pleurobema sintoxia
There are 5 barcode sequences available from BOLD and GenBank. Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species. See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
The round pigtoe is listed as state endangered in Iowa, threatened in Minnesota, and special concern in Michigan.
US Federal List: no special status
CITES: no special status
- Hove, M. 2004. "Links to each state's listed freshwater mussels, invertebrates, or fauna" (On-line). Accessed September 21, 2005 at http://www.fw.umn.edu/Personnel/staff/Hove/State.TE.mussels.
National NatureServe Conservation Status
Rounded National Status Rank: N1 - Critically Imperiled
Rounded National Status Rank: N4 - Apparently Secure
NatureServe Conservation Status
Rounded Global Status Rank: G4 - Apparently Secure
Reasons: The large river populations have become increasingly rare. Distribution is greatly fragmented but remains relatively wide, much as it was historically. Long-term viability of many populations is questionable, especially those in large rivers where zebra mussel populations are now established; but outside these areas the species appears to maintain stable populations.
Intrinsic Vulnerability: Not intrinsically vulnerable
Environmental Specificity: Moderate. Generalist or community with some key requirements scarce.
Comments: This species is somewhat sensitive to pollution, siltation, habitat perturbation, inundation, and loss of glochidial hosts. This species is found in medium to large rivers in mixed mud, sand, and gravel (Cummings and Mayer, 1992). In Canada, the Round Pigtoe is typically found in medium-sized to large rivers but also occurs in Lake Erie and Lake St. Clair (COSEWIC, 2004). Parmalee and Bogan (1998) reported Tennessee occurrences most abundantly, and almost exclusively, in medium-sized and big rivers and in current on a firm substrate of coarse gravel and sand at depths of less than three feet to more than 20 feet.
Other Considerations: It was considered to be Currently Stable by the freshwater mussel subcommittee of the endangered species committee of the American Fisheries Society (Williams et al., 1993). In the Midwest, it is endangered in Iowa and a species of special interest in Ohio (Cummings and Mayer, 1992).
Global Short Term Trend: Decline of 10-30%
Comments: Large river populations are exceedingly rare in the upper Midwest. Recent information suggests that the current distribution and abundance of the Round Pigtoe in the U.S. is generally the same as it was historically, although declines seem evident in a number of areas. Overall, the Round Pigtoe has been lost from about 54% of its former range (in terms of extent of occurrence) in Canada; the historical extent of occurrence was 26,592 km2 and the current EO is 12,360 km2, with an estimated area of occupancy of about 15 km2 (COSEWIC, 2004). In Minnesota, Sietman (2003) reports this species has recently expanded above St. Anthony Falls on the Mississippi River. Some declines have been noted in Illinois (Schanzle et al., 2004). In Canada, only remnant populations still occur in Lakes Erie and St. Clair and it is represented by a few relic specimens in the Grand and Thames Rivers; it is also declining in the Sydenham River and St. Clair delta, Ontario (Metcalfe-Smith and Cudmore-Vokey, 2004).
Global Long Term Trend: Relatively stable to decline of 50%
Comments: It was extirpated from the Black River, Ohio, over 100 years ago (Lyons et al., 2007). It is likely extirpated from the Wakarusa basin in Kansas (Tiemann, 2006). In Alabama, it is known historically from the Tennessee River across the northern part of the state and the Paint Rock River but is currently confined to the tailwaters of Guntersville and Wilson Dams on the Tennessee River (Williams et al., 2008).
Degree of Threat: High
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 eastern North America. Zebra mussels have the potential to severely threaten other populations especially if they make their 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. 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). Destruction of habitat through stream channelization and maintenance and the construction of dams is still a threat in some areas. Impoundments reduce currents that are necessary for 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. Natural predators include raccoons, otter, mink, muskrats, turtles and some birds (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. The introduction and spread of the exotic Zebra Mussel (Dreissena polymorpha) throughout the Great Lakes drainage has led to dramatic declines of native freshwater mussels in colonized areas. Zebra Mussels have infested 63% of sites where P. sintoxia was known to occur prior to 1990. The Zebra Mussel invasion of Lake St. Clair, Lake Erie and the Detroit and Niagara rivers has led to the reduction or elimination of P. sintoxia and other native mussel species from these waters. Zebra Mussels may threaten the population of Round Pigtoes that still survives in the delta area of Lake St. Clair, as it is not known if the unionid community is stable or if the process of extirpation by Zebra Mussels is just slower in this area. P. sintoxia in Ontario, i.e., the population in the Sydenham River, because the river is not navigable by boats and has few impoundments that could support a permanent colony. Nevertheless, the reservoirs at Coldstream and Strathroy in the headwaters of the East Sydenham River are of some concern (from COSEWIC, 2004).
Biological Research Needs: In order to effectively manage mussel species it is necessary to work out certain life history characteristics first. 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 pigtoe be ascertained. Life history studies need 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 abundances 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.
As was stated in the taxonomic section above, this is an extremely difficult genus to identify. Arguments arise even among taxonomists regarding the "species" represented in the genus Pleurobema. Although a few "morphs" have been variously identified and named, no rigorous genetic, anatomic, or conchologic study has ever been published on this group to help elucidate species boundaries or relationships.
Global Protection: Few to several (1-12) occurrences appropriately protected and managed
Comments: In Wisconsin, some of the reported EO's have been within the St. Croix National Scenic Riverway and Nicolet National Forest. Although some populations occur in "protected" sanctuaries or natural areas, disturbances in the watershed impact populations regardless of site protection. No site is adequately protected. The population of P. sintoxia in the Canadian waters of the St. Clair delta, is located entirely within the territory of the Walpole Island First Nation. The area is largely undisturbed and is likely to remain so in the future. The Walpole Island Heritage Centre is aware of the presence of P. sintoxia within their territory, and of the national significance of the population (COSEWIC, 2004). This species occurs in Muddy Creek (French Creek drainage) in the Erie NWR in Crawford Co., Pennsylvania (Mohler et al., 2006). Specimens from the Black River (St. Clair drainage), Michigan, were relocated to the Detroit River in 1992 (Trdan and Hoeh, 1993).
Relevance to Humans and Ecosystems
Economic Importance for Humans: Negative
There are no significant negative impacts of mussels on humans.
Economic Importance for Humans: Positive
Mussels are ecological indicators. Their presence in a water body usually indicates good water quality.
Distribution and conservation status
- COSEWIC. 2005. Canadian Species at Risk. Committee on the Status of Endangered Wildlife in Canada. 64 pp., page 13.
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Names and Taxonomy
Comments: Recently, Oesch (1984) introduced the combination Pleurobema sintoxia (Rafinesque, 1820) as the senior synonym of Pleurobema coccineum (Conrad, 1836). Oesch did not provide any discussion or justification in resurrecting the Rafinesque name.
The members of the genus Pleurobema are among the most difficult to identify in North America. Arguments arise even among taxonomists regarding the "species" represented in the genus Pleurobema. Stansbery (1983) summarized many of the problems and identified a few of the shell characters used to separate P. sintoxia from the morphologically similar and often co-occurring P. plenum, P. cordatum, and P. rubrum. A few "morphs" have been variously identified and named but no rigorous genetic, anatomic, or conchologic study has ever been published on this group to help elucidate species boundaries or relationships.ema sintoxia from the morphologically similar and often co-occurring Pleurobema plenum, Pleurobema cordatum, and Pleurobema rubrum. A few "morphs" have been variously identified and named but no rigorous genetic, anatomic, or conchologic study has ever been published on this group to help elucidate species boundaries or relationships.