Spent a couple of hours on the Connecticut coast this weekend. A vast swath of Common Slipper Shells had washed up during winter storms, a couple of feet deep. People don't eat this mollusc.
(These limpets are different from the European Common Limpet.)
They attach themselves to rocks and other hard objects. Also, to eachother. They move, extremely slowly, if they want to. I tried to figure out whether they have any predators. Supposedly some crabs can eat them, and some of those little snails can bore holes into them. I wonder whether diving ducks can pry them off their attachments, as they do with mussels.