The question of which invertebrates can suffer is one of the most important unresolved questions in animal welfare science. Recent research is expanding our understanding of pain capacity across the animal kingdom.
Invertebrate welfare science is advancing rapidly and has profound implications for how we think about animal suffering at scale. Evidence for nociception in insects, wound-sensitisation in flies, and learning responses disrupted by analgesics in molluscs collectively suggest that the absence of vertebrate neural architecture does not necessarily indicate absence of pain experience. The uncertainty is genuine and significant: if even a small fraction of invertebrates are sentient, the scale of human-caused invertebrate suffering from agriculture, pest control, and environmental destruction is orders of magnitude larger than suffering in vertebrates. The precautionary principle suggests that this uncertainty should be taken seriously in research, policy, and individual decision-making.