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Montana Field Guide

Montana Field Guides

Fat Peaclam - Pisidium rotundatum

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
Shell small, up to about 3.3 mm long, relatively high (H/L 0.80-0.92), greatly inflated (W/L 0.70-0.76), ovate, and thin. Beaks prominent, large, inflated, capped in some specimens, and located about 1/3 of the distance from posterior to anterior or (usually) more centrally. All margins rounded, the anterior and posterior margins more sharply rounded than the dorsal and ventral. Hinge plate short, narrow, curved, and posterior of center. Surface covered with moderate-to-fine striae that are evenly spaced. Periostracum yellowish brown and glossy. The hinge teeth, according to (Herrington 1962), are as follows: Laterals short; cusps short and high with near-vertical ends; cusps of A2 proximal, of P2 and A1 central or on distal side of center; cardinals close to anterior cusps; C3 curved, but not much enlarged at posterior end...;C2 and C4 short; C2 almost parallel with hinge-plate, straight, sometimes slightly curved, or just a peg; C4 straight or slightly curved, sometimes parallel with hinge-plate, but more often directed slightly downward, then not parallel with C2; proximal end of posterior sulcus of right valve closed by a pseudocallus on inner side of proximal end of P3 and, therefore, does not run out on top of hinge-plate. Distinguished from its close relative P. ventricosum by the more central position of the beaks and the narrow hinge plate (between the cardinals and A2.) In P. ventricosum the beaks are nearly posterior, and the hinge plate is broad between the cardinals and A2 (Clarke 1981).

Range Comments
The species is found from northern Canada to the northern United States from Maine to Washington, south in the Rocky Mountains to Mexico (Burch 1972)

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 in lakes, rivers, permanent ponds, and streams. It is usually found in mud among vegetation, but occurs in various other substrates as well (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:
Fat Peaclam — Pisidium rotundatum.  Montana Field Guide.  .  Retrieved on , from