Our understanding of which animals are sentient - capable of subjective experience - has expanded dramatically in recent decades, with profound implications for how we treat them.
The history of moral progress shows a consistent pattern: as evidence of sentience and suffering accumulates, moral consideration expands. Fish were widely considered insensate within living memory; the science now strongly supports their sentience. Invertebrate welfare is the current frontier. The precautionary principle - treating animals as potentially sentient when evidence is uncertain - is increasingly accepted as the ethical standard. This has practical implications for fisheries, pest control, and aquaculture at enormous scale.