There are many predators to oysters. Here are a few:
Crabs: Crabs can attack oysters from seed size (6 to 12mm) to market size.
Otters & Raccoons: These animals will use oysters as a food source.
Whelks & Oyster Drills: These gastropods will attack all sizes of oysters.
Worms: Polydora, while not consuming the oyster, can cause mud blisters on the shell interior. These blisters are unsightly and can break during the shucking process, releasing mud into the oyster flesh and liquor.
Other Vertebrates: Other vertebrates (rays, fish and birds) will attack and consume oysters in the wild. Few will bother the oyster gardener, because the oysters generally are grown in cages that protect them.
Fungus: A fungus, which causes a condition known as “foot disease,” can form hard shell protrusions into the base of the adductor muscle. It is unclear if his can directly cause mortalities in Virginia oysters, but it will definitely cause difficulties during the shucking process.