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, less than 8 mm long; posterior end nearly at right angles with dorsal margin; anterior ventral margin of shell slopes sharply upward; surface dull; striae coarser than in S. partumeium. Dimensions from several authors in the following ranges: L. 6.3-5.5, H. 5.2-4.4, D. 3.2-3.0 mm (La Rocque 1967).

The species is found in ponds, lakes, and rivers; frequently in fine sand. Collections have been made in the following habitats: a swale in water a few inches deep with a muddy bottom; a lake in water plants, 3-5 m deep; an old stone quarry; trenches; a small lake; a creek; in a small permanent or nearly permanent pond in sandy bottom among dense vegetation in shallow water (La Rocque 1967). Clarke (1981) indicates the species occurs in ponds, lakes, rivers and streams of all sizes, and in both vernal and perennial-water habitats. It is usually found in mud among abundant 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.

A few young in up to three size classes are found in most adult specimens (Clarke 1981).