Mudworm
The mudworm (Polydora sp.) is a segmented worm that infects oysters grown on, or near, the ground.
Mudworms do not directly harm oysters, but they compete with the oyster for food. Infected oysters not only endure reduced food but they spend a large amount of energy secreting shell to isolate the invader, which gives them less energy to grow. The major problem with infected oysters is that their market value drops because of unsightly brown blisters.
To prevent infestation with mudworm, oysters are placed above the mid-tide level. Although this reduces feeding time for the oysters, it also reduces the chance of mudworm larvae entering them.