AbstractMud blister worms bore into oyster shells; and oysters respond to shell penetration by secreting new layers of shell, resulting in mud blisters on inner surfaces of oyster shells. We conducted two experiments in off-bottom oyster farms along Alabama's coast in summer 2017 to explore the dynamics of worm infestation, blister formation, and shell repair. Results support our hypothesis that only a small proportion of worms that bore into oysters cause blisters. Triploid oysters had fewer blisters than diploids, likely because of faster growth and shell repair. We treated oysters to remove mud blister worms, redeployed them at intertidal and subtidal sites for nine weeks, and found that reinfestation by worms occurred only in subtidal oysters. Intertidally deployed oysters showed no visible blister coverage, indicating recovery, whereas blister coverage increased in subtidal oysters. Reinfestation of subtidal oysters was correlated with previous burrow damage, visualized with X-ray images, thus supporting our hypothesis that worms preferentially settle in previously infested shells. Forces required to break blisters, measured with a custom-built shucking knife with an integrated force sensor, were low relative to forces required to shuck oysters, possibly because our experiment was conducted when worm infestation was increasing. Higher forces were required to break smaller, lighter-colored blisters, consistent with blister recovery; but results were highly variable and not consistent across sites and sampling times, suggesting that size and color of blisters alone did not explain shell strength. Our results indicate that oysters repair shells slowly relative to more dynamic patterns of worm infestation.