Growing scientific evidence of nociception and pain-like behaviour in spiders, crabs, and other invertebrates is expanding the boundaries of welfare consideration to include animals traditionally assumed to be outside the circle of moral concern.
Invertebrate welfare sits at the frontier of welfare science and philosophy, where the question of which animals experience suffering is genuinely unresolved. The precautionary welfare principle — that where there is significant uncertainty about sentience, we should err toward preventing potential suffering — argues for extending consideration to invertebrates where evidence of nociception and avoidance learning exists. This does not require certainty of subjective experience, only recognition that the probability of morally relevant experience is non-trivial. The expanding circle of welfare concern has historically proven to be on the right side of ethics as scientific understanding grows.