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Mikhalevich & Powell (2020) argue that it is wrong to dismiss the idea of sentience in invertebrates. Here, I expand on the evidence for crustaceans responding to noxious stimuli in ways that are not explained by mere reflexes, and that are consistent with pain. I consider the idea that male praying mantids must not feel pain because they may continue to mate whilst being consumed by the female. I finish with thoughts about the idea that because robots may be constructed to act as if they experience pain, the argument that animals might experience pain is diminished.

Jonathan Birch suggests that we should take one well-conducted study that produces results consistent with the idea of pain as being sufficient to invoke the animal sentience precautionary principle. Here, I consider how to balance negative and positive results from such studies using examples from my own work. I also consider which criteria of pain might provide strong inference about pain and which may prove to be weaker.

The aim of this commentary is to expand the discussion about subjective experience to other arthropods, notably crustaceans. Various species of crustaceans show responses consistent with their feeling pain. Hermit crabs also show prolonged investigation of new shells. They clearly attend to and integrate information from a wide variety of sources that enable them to evaluate the quality of the new shell relative to their current shell. These observations too are consistent with their having subjective experience.

Key (2016) describes the neural system involved in human pain experience in an excellent fashion but then suggests that only that complete system can generate the experience of pain. Thus animals without all components will not feel pain. This argument has been refuted in the past by analogy to vision where it is clear that a broad range of taxa, vertebrate and invertebrate, have good visual abilities albeit with completely different central nervous systems and receptors. This known counterargument to Key's main idea is not mentioned in the target article. Further criteria that might indicate pain and studies examining these criteria in fish and other animals also get scant attention. The function of pain in fitness terms is not considered, yet that might provide clues as to when and how often pain might have evolved.