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Montana Field Guide

Montana Field Guides

Rhomboid Fingernailclam - Sphaerium rhomboideum

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
Shell large, more than 8 mm long; beaks not prominent, only very slightly raised above the dorsal margin; shell more or less rectangular in outline. L. 13.1-3.6 mm., H. 10.5-3.1 mm., 8.5-1.6 mm. (La Rocque 1967).

Range Comments
Northern United States from Maine and Pennsylvania to Idaho; also in southern Canada from New Brunswick to British Columbia (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 ponds, eddies in creeks and rivers, and sheltered areas in small lakes. Seems to have a preference for muddy bottoms among weeds and algae. It has been collected in mud, gravel and coarse sand bottoms; at depths of 0.3 - 2 m (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.

Reproductive Characteristics
Adults ordinarily contain approximately 2 to 11 juveniles (Clarke 1981).

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Citation for data on this website:
Rhomboid Fingernailclam — Sphaerium rhomboideum.  Montana Field Guide.  .  Retrieved on , from