Marsh periwinkle, littorina irorata, is the ubiquitous marsh snail you see on the marsh grasses and across the mud flats sometimes.  They often move up and down the marsh grasses with the tides. Both blue crabs and diamondback terrapin eat periwinkles, and when they retreat to the highest parts of the grasses, perhaps they are avoiding these predators.  Periwinkles rasp the edges of the grass to rough it up and provide a more irregular surface to collect detritus, a film of decaying matter borne on the tides, and then feed on those deposits left by the tide.