Mussels are farmed at enormous scale; while sentience is uncertain, some evidence of basic sensory integration suggests precautionary welfare consideration is appropriate.
Mussel welfare represents a genuine frontier of moral uncertainty. Unlike vertebrates where sentience evidence is strong, or decapod crustaceans where evidence is suggestive and growing, bivalve molluscs have very limited neurological complexity and limited behavioral evidence of pain-relevant experience. However, uncertainty is not evidence of absence, and the enormous scale of bivalve farming (hundreds of billions annually) means that if any significant welfare-relevant experience is occurring, the magnitude of the harm would be enormous. A precautionary approach that avoids obviously aversive conditions where it costs nothing to do so is appropriate given this uncertainty.