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Montana Field Guides

Ridged-beak Peaclam - Pisidium compressum

Native Species

Global Rank: G5
State Rank: SNR

Agency Status


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General Description
Fingernail clams are small "mostly about the size of a finger or thumbnail" bottom-dwelling, filter-feeders found in ponds, lakes and streams throughout Montana. They are native and can be quite abundant, providing food for a variety of animals and producing large accumulations of empty shells. These shells can be quite fragile compared to introduced Asian clams of the family Corbiculidae, which have not been reported in Montana, yet.

Diagnostic Characteristics
Anterior cusp of left valve not twisted, but parallel to the dorsal margin; shell tapering ventrally in end view; hinge long (more than 3/4 shell length); anterior end rounded; shell without heavy ridges or with ridges only on the beaks; cardinals central or subcentral; anterior and posterior ends sloped at different angles, i.e., not parallel; shell dull; shell with ridges on the beaks; ridges straight or slightly curved, but not U-shaped. Dimensions from various authors and collection sites in the following ranges: L. 3.8-1.8, H. 3.8-1.6, D. 2.7-1.0 mm (La Rocque 1967).

Range Comments
La Rocque (1967) includes Montana in range, indicates scattered records for the species throughout the United States. Burch (1972) states the species is present throughout most of the United States, Canada, and into Mexico. Berry (1916) indicates the species was collected near Oxford, Montana. Henderson (1924) cites the same data. Elrod (1902) records a collection at Wibaux, Montana.

Observations in Montana Natural Heritage Program Database
Number of Observations: 4

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Relative Density



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

This is the second most common Pisidium in North America (after P. casertnum). One source indicates the species is found in lakes, rivers, and creeks; never in ponds, swamps, lagoons, or bog ponds (Herrington and Taylor 1958), while another source states that the species prefers streams but is occasionally found in ponds. The species presence in lakes and ponds may be explained by being "washed in from streams emptying into the lake." It has been recorded in sandy bottoms with vegetation, and in mud and clay bottoms; in shallow water and at depths up to 20 m; with pH levels of 7.0-8.37; and fixed carbon dioxide levels of 9.3-30.56 ppm (La Rocque 1967). Clarke (1981) indicates the species occurs in permanent ponds, lakes, rivers, and streams; it is found on a variety of substrates. It is usually found in shallow water among vegetation.

Food Habits
Fingernail clams are mostly filter-feeders, siphoning in floating particulate organic materials ( small plant or animal) from the water column and straining out the particles and expel the strained water. Pedal feeding from the bottom with the foot muscle has also been observed.

Reproductive Characteristics
One litter containing various numbers of young (up to at least 42) is produced each year (Clarke 1981).

  • Literature Cited AboveLegend:   View Online Publication
    • Berry, S.S. 1916. Notes of Mollusca of central Montana. Nautilus 29:124-128.
    • Burch, J.B. 1972. Freshwater Sphaeriacean clams (Mollusca:Pelecypoda) of North America. EPA Biota of Freshwater Ecosystems Identification Manual No. 3. 31 pp.
    • Clarke, A.H. 1981. The freshwater molluscs of Canada. National Museum of Natural Sciences, National Museums of Canada, Ottawa. 446 pp.
    • Elrod, M.J. 1902. A biological reconnoissance in the vicinity of Flathead Lake. Bulletin of the University of Montana Number, Biological Series 10(3):89-182.
    • Henderson, J. 1924. Mollusca of Colorado, Utah, Montana, Idaho, and Wyoming. University of Colorado Studies 13(2):65-223.
    • Herrington, H.B. and D.W. Taylor. 1958. Pliocene and Pleistocene Sphaeriidae (Pelecypoda) from the central United States. Occasional Papers of the Museum of Zoology University of Michigan. 596.
    • La Rocque, A. 1967. Pleistocene Mollusca of Ohio. Department of Natural Resources, Division of Geological Survey Bulletin 62, Part 2. 113-365 + 8 plates.
  • Additional ReferencesLegend:   View Online Publication
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    • Squyer, H. 1894. List of shells from the vicinity of Mingusville, Montana. The Nautilus 8:63-65.
  • Web Search Engines for Articles on "Ridged-beak Peaclam"
  • Additional Sources of Information Related to "Mussels / Clams"
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Citation for data on this website:
Ridged-beak Peaclam — Pisidium compressum.  Montana Field Guide.  .  Retrieved on , from