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Montana Field Guide

Montana Field Guides

A Peaclam - Pisidium obtusale

Global Rank: G3G5
State Rank: SNR

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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; shell tapering ventrally in end view; hinge short (less that 3/4 shell length). Cusp of P II distal or on distal side of center: anterior end curves gently into the dorsal margin; beaks never ridged; dorsal margin well rounded. In specimens with cusp of P II central or on proximal side of center: surface glossy, finely striate (more than 30 striae per mm); anterior (proximal) end of posterior sulcus in right valve closed. (La Rocque 1967). (La Rocque 1967) cites dimensions in the range: L. 3.0-3.75, H. 4.0-2.5 mm.

Range Comments
(La Rocque 1967) includes Montana in range, indicates scattered records throughout the United States and Canada.

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

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Relative Density



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

Most collections in North America are from lakes, while data from Europe indicates the species is lacking in lakes and rivers and is found in stagnant waters choked with aquatic plants. North American specimens have been collected from 1.6 to 12 m, in mud, gravel, sand and clay, and sand bottoms (La Rocque 1967).

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:
A Peaclam — Pisidium obtusale.  Montana Field Guide.  .  Retrieved on , from