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Montana Field Guide

Montana Field Guides

Giant Northern Peaclam - Pisidium idahoense

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
Shell high in outline, height 90 percent of length; surface glossy; striae fine (15 or more per mm); cusps of P I distal; cusp of P II central or on proximal side of center (La Rocque 1967).

Range Comments
La Rocque (1967) includes Montana in range, indicates the species occurs in the Pacific Northwest and Great Lakes regions. Burch (1972) records the species from the Great Lakes region of the United States west to the Aleutian Islands, Washington, and California; also in southern Canada and British Columbia. Clarke (1981) indicates the species occurs from Alaska, throughout Canada (except in the Interior Basin) and south in the Rocky Mountains to California.

Observations in Montana Natural Heritage Program Database
Number of Observations: 1

(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)

The species is found most often in cold arctic and mountain lakes, but in the southern part of its range, also occurs in small relatively warm lakes. Usually found on sand among vegetation (Clarke 1981).

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.

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Citation for data on this website:
Giant Northern Peaclam — Pisidium idahoense.  Montana Field Guide.  .  Retrieved on , from