Euthanasia Methods in Invertebrates: A Critical Narrative Review of Methodological and Welfare Standards.

Invertebrates are increasingly consumed and kept as pets, research models, and in zoological exhibits, creating a growing need to better understand their clinical management and welfare. However, the knowledge regarding nociception, pain perception, and euthanasia in invertebrates remains limited, and standardized protocols are largely absent. Current guidelines are incomplete, often anecdotal, and omit several major invertebrate phyla. To address this gap, we conducted a narrative review of the literature, aiming to critically evaluate existing euthanasia methods, associated welfare implications, and opportunities for refinement. The amount of peer-reviewed species-specific literature is limited and scattered. In addition, most described methods are insufficiently studied and/or do not align with our definition of euthanasia. Based on the available literature, and to provide practical guidance despite these limitations, we propose a two-step approach to invertebrate euthanasia. The first step consists of inducing anesthesia to achieve loss of responsiveness, followed by a second step; a terminal procedure involving physical or chemical destruction of the brain or major ganglia. Our review focuses on the application of this two-step approach. The effectiveness and humaneness of euthanasia techniques vary considerably across taxa and life stages. Substantial further research is required to validate and optimize humane end-of-life procedures for diverse invertebrate species.