Spiders and most invertebrates occupy a contested territory in animal welfare science. Growing evidence of nociception and simple learning challenges the assumption that their welfare is irrelevant.
The welfare status of spiders and other invertebrates sits at the frontier of moral circle expansion in animal welfare. The traditional view that invertebrates lack the neural complexity for morally relevant experience is increasingly challenged by behavioural and neurobiological evidence. Spiders modify their behaviour after injury, show associative learning, and demonstrate escape responses that go beyond simple reflexes. Whether these constitute subjective experiences in any morally relevant sense remains genuinely uncertain. The appropriate response to this uncertainty is precautionary: avoiding gratuitous harm, supporting research into invertebrate sentience, and incorporating invertebrate welfare into the broader conversation about which animals deserve moral consideration. Even modest welfare improvements across the enormous number of invertebrate individuals in agriculture, research, and pest control could have significant aggregate impact if sentience is confirmed.