Bivalves including mussels, oysters and clams are farmed in enormous numbers with minimal welfare consideration, but the question of their sentience is genuinely scientifically debated.
Bivalve welfare occupies genuinely uncertain scientific territory. Unlike crustaceans where evidence for nociception is clear, the evidence for bivalve pain is much weaker and contested. The precautionary principle would suggest some degree of welfare consideration even given uncertainty, but the practical implications are limited given weak evidence. Environmentally, bivalve aquaculture is generally positive - minimal feed inputs, water quality improvement and low carbon footprint. For those seeking to eat animal products with minimal welfare harm, sustainably farmed bivalves are often recommended as the most defensible choice.