Starfishes do not seem particularly cruel at first sight. However, this is very misleading as most are actually vicious predators that live on other invertebrates, especially bivalves.
They kill the bivalve by attaching suction feet, which are numerous under their arms, to both shells. Then they gradually open up the bivalve. The grip is not very strong but constant and eventually the bivalve gets tired, starts to lack oxygen and opens up. The starfish then ejects its stomach into the bivalve and begins to digest the shellfish from outside.
The most common starfish species in the North Atlantic is the aptly named common starfish. It is found all around Iceland at various depths and is large and conspicuous. It is considered a great pest where scallops and mussels are harvested because it is very efficient predator on these bivalves.