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 long (more that 3/4 shell length); anterior end rounded; shell without heavy ridges or with ridges only on the beaks; cardinals near anterior cusps; cusps of A II shaped like a toothpick, i.e., with nearly vertical and parallel sides (La Rocque 1967). La Rocque (1967) cites dimensions from various authors and collection sites in the following range: L. 2.8-1.3, H. 2.5-1.1, D. 2.2-0.9 mm.

The species prefers cool climates and exhibits some variation in form related to the environmental conditions in which it lives. "When found on a sandy bottom the striae are prominent and the beaks more or less tubercular. Those specimens obtained from lakes that are filling up with marl or developing a mucky bottom are smoother, have a greater diameter, and the beaks do not have the tubercular appearance. The varieties are much more common than the typical form with its strange tubercular beaks." Previous records are mainly for lakes but the species is also found in creeks and rivers. Has been collected in mud, marly clay, and sand bottoms, at depths of 1 to 3 m, usually among vegetation, with pH levels of 7.23 to 8.14, and fixed carbon dioxide levels of 10.8 to 22.5 ppm (La Rocque 1967). Clarke (1981) indicates the species is found in ponds, lakes, streams and rivers, usually on a sandy or muddy bottom, and among vegetation.

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.