Mud is a great place for all the Nassau mud snails and the softshelled clam
Mya arenaria.  I've never found edible oysters, Crassostrea virginica,
anywhere but in mud.  And angel wings, Cyrtopleura costata, are always in
about 3 feet of mud so you are armpit deep in mud with your nose in it if
and when you succeed in getting one of them live. The marsh periwinkle,
Littorina irrorata, and the Melampus shells love the spartina mud flats and
the mangrove swamps are home to lots of stuff.  The crown conch, Melongena
corona, and the marginellas come to mind.  The sunray venus, Macrocallista
nimbosa, loves mud or sandy mud.  Mud's much better than sand as a littoral
habitat.