Plants and Animals

Pyrgulopsis letsoni Gravel pyrg

Key Characteristics

The gravel pyrg is a small freshwater snail with a smooth brown conic shell averaging 3.5 mm tall and 2.2 mm wide with 4.5 flattened whorls and a blunt apex. The aperture is nearly round with a thickened outer lip. The operculum (lid covering the main shell opening) is paucispiral (having a few broad spirals).

Status and Rank

US Status: No Status/Not Listed
State Status: SC - Special Concern (rare or uncertain; not legally protected)
Global Rank: GU - Unrankable
State Rank: SH - Possibly extirpated


CountyNumber of OccurrencesYear Last Observed
Livingston 1 1943
Monroe 2 1943
Oakland 2 1943
Washtenaw 2 1946

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.


This species has been collected from cobble beaches and large rivers. Very little is known about its habitat requirements.

Natural Community Types

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

Virtually nothing is known about this species, but maintaining high water quality, restoring river hydrology, and maintaining forest cover over streams would probably be beneficial.

Active Period

Active from first week of June to first week of October

Survey Methods

The specific survey period and methodology for this species are unknown, but are likely similar to those of other freshwater riverine snails. 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) anytime from late September through December, but earlier surveys also would likely be effective. 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 June to first week of October

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

Sweep with dip net

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

Visual surveys

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

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


Survey References

  • Berry, E.G. 1943. The Amnicolidae of Michigan: Distribution, Ecology, and Taxonomy. University of Michigan Press, Ann Arbor. 68pp.

Technical References

  • Dillon, R.T. Jr. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge, UK. 509pp.
  • Goodrich, C. 1932. The Mollusca of Michigan. The University of Michigan Press, Ann Arbor. 121pp.