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Montana Field Guide

Montana Field Guides

River Peaclam - Pisidium fallax

Native Species

Global Rank: G5
State Rank: SU

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 small, up to about 3.5 mm long, moderately high (H/L 0.80-0.90), rather compressed (W/L 0.50-0.62), subovate, and of medium thickness. Beak a little behind centre and, in some specimens, flattened apically and with a low ridge. Dorsal margin curves; anterior margin flatly curved above and rather sharply convex centrally; and ventral margin curved and passing smoothly into posterior margin that is flatly rounded or roundly truncated. Hinge broad and long. Anterior lateral cusp of the left valve abrupt and slightly twisted counterclockwise toward the interior of the shell. Surface of shell with rather coarse concentric striae (20-30 per mm). Periostracum yellowish brown and dull. The hinge teeth, according to Herrington (1962), are as follows: 'Laterals not very long; cusps mostly low and not very sharp; cusps of A1 central or on proximal side of center, usually leaning inward (the hinge-plate is much widened here); cusps of P1 central, of P2 central or on proximal side of center, of A2 central or on proximal side of center, not at center of hinge-plate but well inside, which gives it the appearance of a twist so that it is not parallel with the shell margin or directed across it, but directed somewhat inward. This accounts for the great width at A1. A1 and A3 not parallel, but somewhat V-shaped. The cardinals are subcentral to near the anterior cusps; C2 an inverted D; C4 straight or a little curved and directed slightly inside cusps of P2; C3 mostly short, much curved and directed across the hinge-plate, but it varies considerably.' Best recognized by its subovate shape, dull periostracum, and characteristic left anterior lateral hinge tooth (Clarke 1981).

Range Comments
No known records for Montana; Burch (1972) indicates the species is "distributed sporadically from Great Slave Lake, Alberta and western James Bay south through southern Canada and the northern United States from Washington to New Jersey."

Clarke (1981) indicates the species is uncommon, and occurs in streams, rivers, and exposed habitats in lakes. Usually found on sand or gravel.

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
Young are born in the spring and, like all other species of Cyclocalyx, individuals live for only about 1 year (Clarke 1981).

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