Lottia pelta has a fair number of predators.  In defense the large foot acts like a suction cup and the shell clamps down when touched.  

However fish will eat limpets if they fall off a rock or release their grip.  Some kinds of crabs and seabirds as well as humans can pry limpets off the rocks.   Several kinds of  sea stars glide along and will eat limpets if they can catch them.  However,  shield  limpets will run away from many predatory sea stars. The limpets seem to be able to sense the sea stars through either touch or some form of smell.

Photo from Cannings, R., & S.  Cannings.  1996.  British Columbia. A Natural History.  Greystone Books, Douglas & McIntyre, Vancouver, BC