Montana Field Guide

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.

Anterior cusp of left valve not twisted, but parallel to the dorsal margin; shell tapering ventrally in end view; hinge long (more than 3/4 shell length); anterior end rounded; shell without heavy ridges or with ridges only on the beaks; cardinals central or subcentral; anterior and posterior ends of shell sloped at different angles, i.e., not parallel; shell glossy; beaks not prominent; finely striate (more than 30 striae per mm) and frequently with several coarse striae at outer edge of embryonic shell. Dimensions in the range: L. 3.0-1.6, H. 2.7-1.4, D. 1.4-0.9 mm (La Rocque 1967).

The species is fairly common and prefers shallow water, although has been found at depths of 20-25 m. It is found only in perennial water bodies such as lakes, large ponds, bog ponds, creeks, and rivers. Seems to prefer fairly clear waters with aquatic plants; is rare or absent in marshes. Has been collected in soft sand, clay, and mud bottoms (La Rocque 1967).

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.

In each litter 2 to 7 young are ordinarily produced (Clarke 1981).