Mussels and other bivalves are farmed in enormous quantities, and the question of whether they have morally significant welfare interests is scientifically unresolved.
The welfare status of bivalves is genuinely uncertain in a way that makes clear policy recommendations difficult. Current scientific consensus suggests they are probably not sentient, but the probability is not zero. For consumers making ethical choices, bivalves represent a relatively low-risk option compared to vertebrates and crustaceans. The welfare science continues to develop and positions may evolve as evidence accumulates.