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 short (less that 3/4 shell length); cusp of P II distal or on distal side of center; anterior end joining dorsal margin at an angle (La Rocque 1967). La Rocque (1967) cites dimensions in the range: L. 3.8-1.7, H. 3.2-1.4, D. 2.5-0.9 mm.

The species prefers lakes but is also found in rivers, but not in small creeks or ponds. Has been collected in bottoms consisting of sand, mud, sand-gravel, gravel, boulders, marly clay, black mud, sand and clay, and sand and mud; at depths of 2-15 m (La Rocque 1967). Clarke (1981) indicates the species is common and can be found in all permanent-water habitats, especially lakes. It is found in mud, clay, sand, and gravel.

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.

Adult specimens containing mature young have been found only in summer, and litter sizes of up to 13 have been recorded (Clarke 1981).