Plants and Animals

Planorbella multivolvis Acorn ramshorn

Key Characteristics

The acorn ramshorn is a medium-sized freshwater snail with a discoidal (flat spiral) shell that is 0.6 in (14 mm) in diameter and 0.3 in (8 mm) in height with a spire slightly raised above the main body whorl and a narrow, deep umbilicus (depression or cavity on the central axis or center of the bottom of the shell).

Status and Rank

US Status: No Status/Not Listed
State Status: E - Endangered (legally protected)
Global Rank: GX - Presumed extinct
State Rank: SX - Presumed extirpated


CountyNumber of OccurrencesYear Last Observed
Marquette 1 1907

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.


The acorn ramshorn is believed to be extinct in the wild. It was thought to inhabit deeper water and come to shore to reproduce. Though unlikely, it may still occur in streams and swamps.

Natural Community Types

  • Headwater stream (1st-2nd order), pool
  • Headwater stream (1st-2nd order), run
  • Inland lake, littoral, midwater
  • Submergent marsh

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

Game fish were stocked in Howe Lake beginning in 1905, and acorn ramshorn snails were last seen in 1907. Introduced fish may have led to extinction. If populations still exist, they would benefit from the maintenance of high water quality and avoidance of further fish introductions.

Active Period

Active from first week of August to first week of September

Survey Methods

This species can be surveyed using several techniques. One survey method consists of sweeping aquatic vegetation or scraping the substrate with a fine mesh aquatic sampling net or dip net (e.g., D-frame net). Another survey technique consists of vigorously shaking aquatic vegetation over a pail of water causing the various snails clinging to the plants to drop to the bottom of the pail. Visual surveys also can be conducted for this species by looking for snails attached to vegetation, rocks, woody debris and other cover. Glass bottomed buckets may be used to see snails clinging to vegetation or rocks underwater. Rocks, vegetation, and other cover also should be picked up, examined for snails, and returned to their original positions.

Shaking vegetation survey

Survey Period: From first week of August to first week of September

Sweep with dip net

Survey Period: From first week of August to first week of September

Visual surveys

Survey Period: From first week of August to first week of September


Survey References

  • Berry, E.G. 1943. The Amnicolidae of Michigan: Distribution, Ecology, and Taxonomy. University of Michigan Press, Ann Arbor. 68pp.
  • Clarke, A.H. 1981. The Freshwater Molluscs of Canada. National Museum of Natural Science, National Museums of Canada, Ottawa. 446pp.

Technical References

  • Dillon, R.T. Jr. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge, UK. 509pp.
  • Environmental Protection Agency. 1982. Freshwater Snails (Mollusca: Gastropoda) of North America. EPA Publication 600/3-82-026. Environmental Monitoring and Support Laboratory, Office of Research and Development, U.S. EPA, Cincinnati. 294pp.
  • Evers, D.C. 1994. Endangered and Threatened Wildlife of Michigan. The University of Michigan Press, Ann Arbor. 412pp.
  • NatureServe. 2005. NatureServe Explorer: An online encyclopedia of life [web application]. Version 4.5. NatureServe, Arlington, Virginia. Available