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.