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.

Shell small, up to about 3 mm long, relatively high (H/L 0.82-1.00), greatly inflated (W/L 0.80-0.95), ovate, and rather heavy. Beaks large, swollen, and placed far back on the shell. Dorsal margin smoothly curved and passing imperceptibly into anterior margin; anterior margin sloped, broadly curved above, and roundly curved centrally or in an area just below the center; ventral margin long, evenly rounded, and continuing in an uninterrupted curve into the posterior margin; posterior margin flatly curved and undercut. Hinge plate heavy and relatively short, that is less than 3/4 the length of the shell. Surface finely striate (more than 30 per mm) and with prominent growth rests in many specimens. Periostracum yellowish brown to grayish, and glossy. Hinge teeth similar to those in P. rotundatum but heavier, and the hinge plate between the cardinal teeth and A2 is relatively wide, not thin. This small species is well characterized by its greatly inflated heavy shell and prominent, swollen, posteriorly-placed beaks (Clarke 1981).

The species is found in perennial-water: ponds, lakes, rivers, and streams of all sizes, typically in muddy bottoms among aquatic vegetation (Clarke 1981).

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.