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Montana Field Guide

Montana Field Guides

Walker Peaclam - Pisidium walkeri

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 short (less than 3/4 shell length); cusp of P II central or on proximal side of center; surface dull; moderately striate (fewer that 30 striae per mm) (La Rocque 1967). (La Rocque 1967) cites dimensions from various authors and collection sites in the following range: L. 5.9-1.7, H. 4.9-1.4, D. 3.6-0.8 mm.

Range Comments
(Burch 1972) records the species from northern Canada south to Arizona and Virginia.

The species is found in creeks, rivers, and small lakes and ponds with soft bottoms. It is generally not abundant, although it can be in small lakes that are filling up with marl. It seems to prefer small lakes or ponds. The species has been collected in mud, sand and boulder, and marly clay bottoms; at depths of 0.6 to 6.1 m (La Rocque 1967). (Clarke 1981) states the species is rather uncommon and occurs in permanent ponds, lakes, rivers, and streams. It is generally found among vegetation, and on a variety of substrates.

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
A single litter of young are born during the 1-year lifetime of each individual. The anatomy of this species has not been studied (Clarke 1981).

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