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.
Fingernail clams comprise the family Sphaeriidae. According to ecologist Elizabeth A. Colburn (2004), five species are commonly found in vernal pools. Herrington’s fingernail clam (Sphaerium occidentale) is a vernal pool specialist, restricted to vernal pools that dry out annually. Four other species, swamp fingernail clam (Musculium partumeium), pond fingernail clam (Musculium securis), ubiquitous peaclam (Euglesa casertana), and fat peaclam (Pisidium rotundatum) are found in a variety of freshwater bodies, including vernal pools.
Montana Field Guide
University of Wisconsin