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Crustaceans
Many large crustaceans, including crabs and lobsters, prey on shellfish. They can open the shell by crushing it or prying it open with their claws. Because many crustaceans are bottom feeders, bottom-dwelling shellfish such as oysters and clams represent a valuable food source. This is especially important because crustaceans carry their own heavy shells, and cannot rely on their ability to chase down more agile prey such as fish.
Mollusks and Echinoderms
Many shellfish are able to prey on other shellfish. Whelks pry open the shells of other bivalves using the sharp edge of their own shell, forcing it between a clam's shell using their muscular feet. Some snails can drill through shells using their radula, a rasping appendage similar to a rough tongue. Octopi and squids commonly pry open bivalve shells with their powerful tentacles, exposing the shellfish to their sharp beaks. Starfish prey on shellfish by slowly prying open the shell with their arms. The starfish then inserts its stomach through the narrow gap, digesting the shellfish inside its own shell.
Fish
Many bottom-feeding fish can crack open the shells of shellfish in their jaws. Stingrays, skates and groupers dig up shellfish from the ocean floor, and rely on them for a significant portion of their diet. Beaked fish that feed primarily on coral, including the parrot fish, are also able to prey on shellfish. Even small sharks are known to devour shellfish when the opportunity presents itself.
Mammals
A few marine mammals rely on shellfish as a major part of their diet. Walruses use their tusks to probe for shellfish buried on the seafloor, cracking the shells in their powerful jaws. Sea otters also prey on shellfish, but lack the ability to bite through the shell. Instead, they float on their backs, hold the shellfish on their stomachs and open the shellfish by hitting it repeatedly with a stone.
Birds
Some waterfowl are able to prey on shellfish. Herons and cranes are major predators of shellfish. They spend much of their time hunting in shallow water, and are able to crack open small shellfish with their beaks. Seagulls search for shellfish along shorelines at low tide, locating shellfish that have buried themselves in the sand. Once they find a shellfish, they crack open the shells by carrying them into the air and dropping them onto rocks.