During a recent canoe trip on the Upper Delaware, while navigating clear of the occasional eel rack, I spotted numerous bright white oblongs scattered on the river bed. These were the emptied shells of freshwater mussels, most likely Elliptio complanata. I may have paddled past as many or more live mussels, but as you can see from the photo, the closed shells would be much harder to spot against the riverbed.
Their predators include fish, waterfowl, and small mammals, but what about people? A staffer at the canoe-rental company stated that earlier in the year, someone on the river had gone shellfishing with thoughts of a meal. Granting for the story's sake that they're fit for human consumption, the staffer could offer no follow-up, however, on whether the mussels were tasty or merely tolerable.
From the Upper Delaware River, vicinity of Minisink Ford, New York