I'm not sure of the muscles and oysters capacity for suffering, but then I never ate seafood anyway.
After a v.quick google... "However, as oysters do not have a central nervous system, they are not believed to be sentient and so don't feel pain....."
Yup, ergo the purists who don't eat them are perhaps being silly (obvs not liking seafood in general aside), doesn't seem to be too different to eating a plant.