Pain in crustaceans

Pain in crustaceans is a scientific debate which questions whether they experience pain or not. Pain is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in an animal, or another human for that matter, cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness which is deduced from comparative brain physiology as well as physical and behavioural reactions. Definitions of pain vary, but most involve the ability of the nervous system to detect and reflexively react to harmful stimuli by avoiding it, and the ability to subjectively experience suffering. Suffering cannot be directly measured in other animals. Responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument by analogy is sometimes used. Crustaceans fulfill several criteria proposed as indicating that non-human animals may experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors; opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics; physiological changes to noxious stimuli; displaying protective motor reactions; exhibiting avoidance learning; and making trade-offs between noxious stimulus avoidance and other motivational requirements. In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opioid receptors. Opioid peptides and opioid receptors occur naturally in crustaceans, and although it was concluded in 2005 "at present no certain conclusion can be drawn", more recent considerations suggest their presence along with related physiological and behavioural responses as indicating that crustaceans may experience pain. Opioids may moderate pain in crustaceans in a similar way to that in vertebrates.