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Montana Field Guide

Montana Field Guides

Triangular Peaclam - Pisidium variabile

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 glossy; beaks prominent; moderately striate (fewer than 30 striae per mm); beaks posteriorly placed." (La Rocque 1967:338). La Rocque (1967:338) cites dimensions from various authors and collection sites in the following range: L. 4.6-1.8, H. 3.9-1.4, D. 2.8-0.9 mm.

Range Comments
La Rocque (1967:339) includes Montana in range, indicates scattered records throughout the northern United States and southern Canada. Burch (1972:8) indicates the species is found throughout the United States and Canada. Sterki (1916b:453) includes Montana in the range listed for the subspecies P. VARIABLE MAGNUM; Henderson (1924:100) cites the same data.

The species is found in creeks, rivers, and lakes, "usually in still waters where soft sediments accumulate." It is frequently found burrowing in gravel, sand, clay and mud in water 1-13 feet deep, "but is more abundant in mud where the water is 4-11 feet deep." The species has been collected in sand and gravel, mud, marly clay, gravel and silt, and sand; in waters with a pH of 5.72-8.37, and fixed carbon dioxide levels of 1.72-30.56 ppm; and depths of 0.3 to 13.5 m. It may perhaps live in almost any kind of lake or river (La Rocque 1967:338). Clarke (1981:418) indicates the species is common and lives in "virtually all natural perennial-water habitats. Found in various substrates (most frequently mud) and usually amid 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
"Litter size varies from at least 12 to 34. No anatomical studies have been done on this species." (Clarke 1981:418).

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Citation for data on this website:
Triangular Peaclam — Pisidium variabile.  Montana Field Guide.  .  Retrieved on , from