Found year-round buried in the substrate of freshwater ecosystems, fingernail clams are indicators of vernal pools even when no water is present. These small (< 2cm) bivalves are also known as pea clams.
The family Sphaeriidae consist of three genera: Musculium, Pisidium, and Sphaerium. According to ecologist Elizabeth A. Colburn (2004), species from all three genera are commonly found in northeastern woodland vernal pools. Herrington’s fingernail clam (Sphaerium occidentale) is a vernal pool specialist, restricted to vernal pools that dry out annually. Three other species, swamp fingernail clam (Musculium partumeium), pond fingernail clam (Musculium securis), and ubiquitous peaclam (Pisidium casertanum) are found in a variety of water bodies, including vernal pools.
Montana Field Guide
University of Wisconsin