Plants and Animals

Pleurobema sintoxia Round pigtoe

Key Characteristics

The round pigtoe is a relatively large (to 4 inches) mussel with a moderately thick, round shell and a rounded anterior end and a rounded or bluntly pointed posterior end. The beak is low and only slightly elevated above the hinge line. This species has a shallow beak cavity and beak sculpture consisting of 2 to 3 elevated ridges. The lateral teeth are straight and cardinal teeth are well developed with 2 in the left valve and 1 in the right valve. The shell is smooth and brown with faint green rays visible near the beak. The nacre is variable in color, from white to pink to rose colored.

Status and Rank

US Status: No Status/Not Listed
State Status: SC - Special Concern (rare or uncertain; not legally protected)
Global Rank: G4G5 - Rank is uncertain, ranging from apparently secure to secure
State Rank: S3 - Vulnerable

Occurrences

CountyNumber of OccurrencesYear Last Observed
Allegan 1 2000
Barry 1 2010
Berrien 8 2004
Branch 1 2000
Calhoun 8 2018
Cass 2 2009
Clare 1 1934
Clinton 4 2010
Dickinson 3 2009
Eaton 4 2016
Genesee 1 2001
Gratiot 3 2010
Hillsdale 14 2018
Huron 1 Historical
Ingham 5 2001
Ionia 12 2016
Jackson 10 2018
Kalamazoo 6 2018
Kent 13 2017
Lapeer 1 1926
Lenawee 10 2016
Livingston 1 2001
Macomb 10 2007
Manistee 1 1918
Mecosta 2 1934
Menominee 4 2009
Midland 1 2011
Missaukee 1 2002
Monroe 11 2000
Newaygo 3 1934
Oakland 11 2016
Osceola 2 2002
Ottawa 2 1960
Saginaw 3 2011
Sanilac 2 2010
Shiawassee 4 2001
St. Clair 8 2016
St. Joseph 9 2016
Tuscola 5 2004
Van Buren 3 2009
Washtenaw 6 2010
Wayne 7 2012

Information is summarized from MNFI's database of rare species and community occurrences. Data may not reflect true distribution since much of the state has not been thoroughly surveyed.

Habitat

The round pigtoe occurs in mud, sand, or gravel substrates of medium to large rivers.

Specific Habitat Needs

Sandy or gravel substrates needed in: Mainstem stream (3rd-4th order), run; Mainstem stream (3rd-4th order), riffle; River (5th-6th order), run; River (5th-6th order), riffle.

Natural Community Types

  • Mainstem stream (3rd-4th order), run
  • Mainstem stream (3rd-4th order), riffle
  • River (5th-6th order), run
  • River (5th-6th order), riffle

For each species, lists of natural communities were derived from review of the nearly 6,500 element occurrences in the MNFI database, in addition to herbarium label data for some taxa. In most cases, at least one specimen record exists for each listed natural community. For certain taxa, especially poorly collected or extirpated species of prairie and savanna habitats, natural community lists were derived from inferences from collection sites and habitat preferences in immediately adjacent states (particularly Indiana and Illinois). Natural communities are not listed for those species documented only from altered or ruderal habitats in Michigan, especially for taxa that occur in a variety of habitats outside of the state.

Natural communities are not listed in order of frequency of occurrence, but are rather derived from the full set of natural communities, organized by Ecological Group. In many cases, the general habitat descriptions should provide greater clarity and direction to the surveyor. In future versions of the Rare Species Explorer, we hope to incorporate natural community fidelity ranks for each taxon.

Management Recommendations

Like other mussels, threats to the round pigtoe include: natural flow alterations, siltation, channel disturbance, point and non-point source pollution, and exotic species. Maintenance or establishment of vegetated riparian buffers can help protect mussel habitats from many of their threats. Control of zebra mussels is critical to preserving native mussels. And as with all mussels, protection of their hosts habitat is also crucial.

Survey Methods

Glass-bottom bucket less than waist deep water

Survey Period: From first week of June to first week of October

SCUBA searches

Survey Period: From first week of June to first week of October

Snorkeling searches

Survey Period: From first week of June to first week of October

References

Survey References

  • Cummings, K.S. and C.A. Mayer. 1992. Field Guide to Freshwater Mussels of the Midwest. Illinois Natural History Survey Manual 5, Champaign. 194pp.
  • Strayer, D.L. and D.R. Smith. 2003. A Guide to Sampling Freshwater Mussel Populations. American Fisheries Society Monograph 8, Bethesda. 103pp.

Technical References

  • Clarke, A.H. 1981. The Freshwater Molluscs of Canada. National Museum of Natural Science, National Museums of Canada, Ottawa. 446pp.
  • Cummings, K.S. and C.A. Mayer. 1992. Field Guide to Freshwater Mussels of the Midwest. Illinois Natural History Survey Manual 5, Champaign. 194pp.
  • Smith, P.W. 1961. The amphibians and reptiles of Illinois. Illinois Natural History Survey, Carbondale. Bulletin No. 28. 298 pp.