Some seastars are voracious predators that will prey on many different animals. Corallivorous crown-of-thorns starfish will wipe out entire areas of scleractinian corals. Others will eat mussels and other shellfish as seen in the below video.
Watch as seastars advance up a pier piling probing the encrusted mussels for a weakness. Once a vulnerable mussel is found, the seastar sets to work exuding its stomach inside the mussel and digests it while it is still alive. In the below footage, videographers embedded a tiny video camera inside the mussel to watch the drama unfold: