Oyster Mud Blisters
A common condition affecting oysters from Massachusetts to the Gulf of Mexico is the appearance of black areas along the perimeter of the inner sides of the shells. These areas, commonly referred to as "Mud Blisters" are caused by the occurrence of a small worm, Polydora websteri.
This worm, which is typically less than 0.75 inches in length, lives between the mantle (outer most oyster tissue) and the inner shell surface in order to be protected from predators. The worms form blister-like chambers on the oyster shells, which often fill with mud creating the appearance of blisters.
The worms and "blisters" do not effect oyster health, quality or taste and they have no detrimental effects on consumers