Laterally compressed mollusks with a bivalved shell with a dorsal hinge. The head is poorly developed. The mantle cavity is capacious, the gills are large and serve also in feeding.
Marine forms normally have external fertilization and free-swimming trochophore and veliger larvae, freshwater forms usually shed the sperm into the water, but the eggs are fertilized and brooded within the gill cavity. The freshwater fingernail clams (Sphaeriidae) are hermaphroditic and self-fertile, and they have direct development with young clams shed from the gills. Most species complete the live cycle in a month to a year. Freshwater mussels (Unionidae) are mostly dioecious. Their eggs are brooded until the glochidia larval stage is achieved. The glochidia are parasitic on fish or aquatic amphibians. The glochidia transforms to a young mussel in a few days to few weeks and drops to the bottom to begin feeding as an adult. Breeding may be limited to the spring or throughout most of the year. Adults may hibernate or remain active throughout the winter. It may take several years to reach maturity. Adults may live for many years.
Uniformly "collectors", using the large gills to filter organic particles from the water. Under many conditions, most of the food is obtained from the substrate. They are able to take very small particles, and use dissolved organic compounds. They are sensitive to siltation, and they may bio-accumulate toxins.