 Invertebrate predators include crabs , starfish and octopuses . Crabs crack the shells with their pincers and starfish use their water vascular system to force the valves apart and then insert part of their stomach between the valves to digest the bivalve 's body . It has been found experimentally that both crabs and starfish preferred molluscs that are attached by byssus threads to ones that are cemented to the substrate . This was probably because they could manipulate the shells and open them more easily when they could tackle them from different angles . Octopuses either pull bivalves apart by force , or they bore a hole into the shell and insert a digestive fluid before sucking out the liquified contents . 
