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Montana Field Guide

Montana Field Guides

Quadrangular Pillclam - Pisidium milium

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 the introduced Asian clams , which have not be reported in Montana yet.

Diagnostic Characteristics
Anterior cusp of left valve not twisted, but parallel to the dorsal margin. Ventral aspect of shell very truncate in end view (La Rocque 1967). La Rocque (1967) cites dimensions in the range: L. 2.9-1.7, H. 2.3-1.4, D. 2.0-1.0 mm.

Range Comments
La Rocque (1967) includes Montana in range, indicates scattered records in the Pacific Northwest, Great Lakes region and Canada. Burch (1972) records the species for the "Aleutian Islands and Great Slave Lake, south to British Columbia, James Bay, Prince Edward Island and Maine, and south in the Rocky Mountains to Utah and Colorado."

The species is uncommon and occurs in ponds, lakes, and slow-moving streams. It is found on muddy bottoms 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.

Reproductive Characteristics
Swedish specimens were observed to produce young from June to September (Clarke 1981).

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Citation for data on this website:
Quadrangular Pillclam — Pisidium milium.  Montana Field Guide.  .  Retrieved on , from