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 introduced Asian clams of the family Corbiculidae, which have not been reported in Montana, yet.

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 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 not closed. Dimensions in the range: H. 3.4-1.8. L. 2.8-1.5, D. 1.8-0.9 mm (La Rocque 1967).

The species is found in streams, bays, lakes, and rivers. It is "very common in lakes, mainly in the littoral zone, but lives down to 25-30 m below the surface." The species prefers running water, and rivers with clear water, and is rarely found in pools. It has been collected at altitudes up to 1,300 m; in mud, sand, and gravel bottoms; at depths of 0.8-5.6 m (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.

Two litters of young are born each year, and life spans exceed 1 year (Clarke 1981).