Plants and Animals

Obliquaria reflexa Threehorn wartyback

Key Characteristics

The threehorn wartyback has a rounded, small to medium-sized shell (to 3 inches) that varies in color from shades of yellow and green to brown. This species is easily recognized by a central row of 1-4 large knobs on each valve in alternate positions.

Status and Rank

US Status: No Status/Not Listed
State Status: E - Endangered (legally protected)
Global Rank: G5 - Secure
State Rank: S1 - Critically imperiled


CountyNumber of OccurrencesYear Last Observed
Allegan 1 1936
Berrien 1 Historical
Menominee 4 2009
Monroe 5 2011
Ottawa 5 2019
Saginaw 3 2008
St. Clair 1 2011
Van Buren 1 Historical
Wayne 5 2019

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.


Most common in medium to large rivers, the three-horned wartyback occurs in slackwater conditions to swift currents, and substrates of gravel to muddy sand (Oesch 1995, Watters et al. 2009).This species has been found to be relatively tolerant of river impoundments (Grabarkiewicz and Davis 2008).

Natural Community Types

  • Great lake, littoral, benthic
  • Great lake, pelagic, benthic
  • Mainstem stream (3rd-4th order), pool
  • Mainstem stream (3rd-4th order), run
  • Mainstem stream (3rd-4th order), riffle
  • River (5th-6th order), pool
  • 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

Approximately 70% of North American unionids are in decline. Leading threats to this group include water quality degradation, alteration of physical habitat, host fish population declines, and invasive species introduction (Bringolf et al. 2007). The threehorn wartyback, along with other species in this group, exhibit high absorption rates for heavy metals and other contaminants found in herbicides, pesticides (Bringolf et al. 2007), industrial waste, and urban runoff (Levengood et al. 2004). Sedimentation of waterways can be harmful to a wide variety of freshwater organisms. Management of the threehorn wartyback, therefore, must include adhering to high water quality standards and the conservation of healthy habitat structure. Zebra mussels must be prevented from spreading to new waterways through the careful cleaning of fishing gear, boats and trailers, and scuba equipment in order to protect unionid populations from infestation.

Active Period

Gravid from first week of June to fourth week of August

Survey Methods

Visual and tactile search using scuba or glass-bottom buckets. Tactile search (by hand) is especially important where water turbidity and pebbles/rocks make visual detection difficult. After identification, live mussels should be planted back into the substrate anterior end down. Surveys should not take place after heavy rains or during periods of high water as these conditions can make detection much more difficult. Methods of documenting survey effort include: searching a large measured area, e.g. 128m2; taking multiple quadrat samples; and recording search time (person hours). For all methods, at least some excavation of substrate (by hand, 5-10cm down) should be done to detect buried mussels. Searching a large measured area or timed searches are generally better for detecting rare species and generating a species list than quadrat sampling. These two methods allow more types of microhabitats and a larger area to be covered. Quadrat sampling is better suited for documenting changes in density and other statistical analyses at the site level (Strayer and Smith 2003).

Glass-bottom bucket less than waist deep water

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

Time of Day: Daytime
Water Level: Low Water Levels
Water Turbidity: Low Turbidity

SCUBA greater than waist deep water

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

Time of Day: Daytime


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.

Technical References

  • Bringolf, R.B., W.G. Cope, C.B. Eads, P.R. Lazaro, M.C. Barnhart, and D. Shea. 2007. Acute and chronic toxicity of technical-grade pesticides to glochidia and juveniles of freshwater mussels (Unionidae). Environmental Toxicology and Chemistry 26(10):2086-2093.
  • Grabarkiewicz, J. and W. Davis. 2008. An introduction to freshwater mussels as biological indicators. EPA-260-R-08-015. U.S. Environmental Protection Agency, Office of Environmental Information, Washington, DC.
  • Levengood, J.M., D.J. Soucek, J. Esarey, R.J. Hudson, W. Wimer, and R.S. Halbrook. 2004. Contaminants in Mussels from the Mississippi and Illinois Rivers Confluence Area. Final Report to the National GReat Rivers Research and Education Center. 35pp.
  • Oesch, R.D. 1984. Missouri Naiades: a Guide to the Mussels of Missouri. Conservation Commision of the State of Missouri, Jefferson City. 270pp.
  • Sethi, Suresh A., Andrew R. Selle, Martin W. Doyle, Emily H. Stanley, and Helen E. Kitchel. 2004. Response of unionid mussels to dam removal in Koshkonong Creek, Wisconsin (USA). Hydrobiologia 525:157-165.
  • Strayer, D.L. and D.R. Smith. 2003. A Guide to Sampling Freshwater Mussel Populations. American Fisheries Society Monograph 8, Bethesda. 103pp.
  • Watters, G. Thomas, Michael A. Hoggarth, and David H. Stansbery. 2009. The Freshwater Mussels of Ohio. The Ohio State University Press, Columbus. 421 pp.