Invertebrates make up 97% of animal species and the vast majority of individual animals on Earth. The question of whether they can suffer is scientifically contested but morally important given their scale.
The welfare status of invertebrates is deeply uncertain but morally pressing given their numbers. If even a small fraction of invertebrate species have morally relevant experiences, the scale of their suffering — in agricultural settings, from pesticide exposure, from habitat destruction — is enormous. Precautionary inclusion of invertebrates in welfare consideration — particularly for species with stronger evidence like decapod crustaceans and some insects — is both scientifically justified and ethically prudent. The cost of being wrong about invertebrate sentience in a permissive direction is potentially the greatest overlooked welfare catastrophe in history.