The philosophical and scientific question of which animals are sentient - capable of subjective experience - underpins the entire framework of animal welfare ethics.
The sentience question matters enormously for welfare: only sentient beings have welfare interests, and only welfare interests create moral obligations. The consistent pattern in welfare science has been expansion of the sentience circle as evidence accumulates - fish, crustaceans, and insects have all shifted from 'probably not sentient' to 'probably sentient' within recent decades. This means that decisions made today about which animals to protect affect billions of sentient lives. The precautionary principle - extending protection where sentience is plausible rather than proven - is increasingly accepted as the appropriate ethical position.