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Montana Field Guide

Montana Field Guides

Large-mantle Physa - Physa megalochlamys

Species of Concern

Global Rank: G3G4
State Rank: S1
* (see State Rank Reason below)

Agency Status


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State Rank Reason (see State Rank above)
Due to this restricted distribution and only a few known occurrences, this species was placed on the MT Species of Concern list as S1, critically imperiled. It is at high risk of extirpation in the state because of very limited and/or potentially declining population numbers, range and/or habitat.
General Description
See Taylor (1988) for full description.

Diagnostic Characteristics
Physa megalochlamys has two distinctive characters: a large, thin shell and a mantle that is substantially overfolded (Frest and Johannes 1995).

Species Range
Montana Range

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Range Comments
Records are from the National Bison Range, Moiese, MT (Frest and Johannes 1995). Status of current distribution unknown. As of Taylor's 1988 species-description, 16 scattered sites had been identified in North America located in, SK, MT, UT, CO, ID, WY, and OR. Historically, this species probably ranged from southern Canada through the Pacific Northwest and the Rocky Mountain States lying east of the Cascade Mountains as far as western MT, UT, and CO (Taylor 1988).

Summary of Observations Submitted for Montana
Number of Observations: 2

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)


Physa megalochlamys is typically found in marshes or ponds, such as a lily pond in the case of the type species (Frest and Johannes 1995, Taylor 1988). The preferred substrate is characteristically a fine mud type (Frest and Johannes 1995).

Food Habits
Like most snails, these are herbivorous- scrapers of algae, detritus and diatoms from other vegetation of benthic substrates.

Physa megalochlamys forms colonies in the typically muddy conditions found in Typha-Scirpus marshes (Frest and Johannes 1995). Taylor (1988) notes that the ponds and/or marshes chosen by this snail can experience seasonal fluctuations up to and including the point of desification.

Limited distribution. It was considered threatened by Frest and Johannes (1997) in their overview of mollusks of the Interior Columbia basin.

Threats or Limiting Factors
Physa megalochlamys can be considered moderately threatened since it is known from only a few sites with known impact. Threats include draining and dredging of marshes, treatment of wetlands and marshes with insecticides and pesticides, eutropification from agricultural runoff and irrigation diversion and returns, urbanization and other construction-related destruction of marshes and swamps (Frest and Johannes 1995). The major threats to species survival are habitat loss and destruction due to modification of marsh land for human activities. Specific threats include draining and dredging, insecticide spraying, nutrient build up due to agricultural runoff, irrigation systems, and encroaching urbanization (Frest and Johannes 1995).

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Citation for data on this website:
Large-mantle Physa — Physa megalochlamys.  Montana Field Guide.  .  Retrieved on , from