Pisidium

a genus of freshwater bivalves of the Sphaeridae family. The shell is oval (length 2–10 mm) with its apexes directed toward the rear. The animals are found in fresh bodies of water on all continents. In the USSR there are a few dozen genera in a great variety of waters, including rivers, lakes, ponds, pools, and underground waters. They feed on bacteria and organic residues suspended in the water; they are eaten by certain fish.