True Arca attach by a byssus to hard surfaces, but Anadara and some other genera of Arcidae are often found in sand or mud. Sometimes they are in mud under a rock, and also attached to the underside of the rock by a byssus. But other times they are in sediment, not attached to any hard surface. Also, some species seem able to live either way. I have collected Anadara ovalis and Anadara transversa in muddy sand, not attached to anything. And I have collected those same species attached navigation buoys. Paul M.