The question of whether or not crustaceans can experience pain is unresolved. Because of the ambiguous nature of pain, most people who contend that crustaceans do have this capacity approach the issue using 'argument by analogy' – that is, they hold that certain similarities between crustacean and human biology or behaviour warrant an assumption that crustaceans can feel pain.[1]
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 "at present no certain conclusion can be drawn",[2] some have interpreted their presence as an indication that crustaceans may be able to experience pain.[1][2] Lobsters' opioids may "mediate pain in the same way" as in vertebrates.[1]
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The idea that animals might not feel pain as human beings feel it traces back to the 17th-century French philosopher, René Descartes, who argued that animals do not experience pain, because they lack consciousness.[3][4][5] Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain.[6] In his interactions with scientists and other veterinarians, Bernard Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain.[6] Carbone writes that the view that animals feel pain differently is now a minority view. Academic reviews of the topic are more equivocal, noting that although the argument that animals have at least simple conscious thoughts and feelings has strong support,[7] some critics continue to question how reliably animal mental states can be determined.[4][8]
The presence of pain in an animal, or another human for that matter, cannot be known for sure, but it can be inferred through physical and behavioral reactions.[9] Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too.[10][11] Nociceptors, the neurones required for the sensation of pain, have been found in Nematodes, Annelids, Molluscs[12] and in the arthropod, Drosophila,[13] despite earlier claims that nociceptors were absent in insects.[14]
As for other animals, plants, or other entities, their ability to feel physical pain is at present a question beyond scientific reach, since no mechanism is known by which they could have such a feeling. In particular, there are no known nociceptors in groups such as plants and fungi.
Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics as used in humans.[15]
In February 2005, a review of the literature by the Norwegian Scientific Committee for Food Safety tentatively concluded that "it is unlikely that [lobsters] can feel pain," though they note that "there is apparently a paucity of exact knowledge on sentience in crustaceans, and more research is needed." This conclusion is based on the lobster's simple nervous system. The report assumes that the violent reaction of lobsters to boiling water is a reflex to noxious stimuli.[2]
However, review by the Scottish animal rights group Advocate for Animals released in the same year reported that "scientific evidence ... strongly suggests that there is a potential for [lobsters] to experience pain and suffering," primarily because lobsters (and other decapod crustaceans) "have opioid receptors and respond to opioids (analgesics such as morphine) in a similar way to vertebrates," indicating that lobsters' reaction to injury changes when painkillers are applied. The similarities in lobsters' and vertebrates' stress systems and behavioral responses to noxious stimuli were given as additional evidence for their capacity for pain.[1]
A 2007 study at Queen's University, Belfast, suggested that crustaceans do feel pain.[16] In the experiment, when the antennae of prawns were rubbed with sodium hydroxide or acetic acid, the animals showed increased grooming of the afflicted area and rubbed it more against the side of the tank. Moreover, this reaction was inhibited by a local anesthetic, even though control prawns treated with only anesthetic did not show reduced activity. Robert Elwood, who headed the study, argues that sensing pain is crucial to prawn survival, because it encourages them to avoid damaging behaviors. Some scientists responded, saying the rubbing may reflect an attempt to clean the affected area.[17]
In a subsequent 2009 study, Elwood and Mirjam Appel showed that hermit crabs make motivational tradeoffs between electric shocks and the quality of the shells they inhabit.[18] In particular, as hermit crabs are shocked more intensely, they become increasingly willing to leave their current shells for new shells, and they spend less time deciding whether to enter those new shells. Moreover, because the researchers did not offer the new shells until after the electrical stimulation had ended, the change in motivational behavior was the result of memory of the noxious event, not an immediate reflex.
Morphine, an analgesic, and naloxone, an opioid receptor antagonist, may affect the estuarine crab Neohelice granulata in much the same way they affect vertebrates: injections of morphine into crabs produced a dose-dependent reduction of their defensive response to an electric shock.[19] (However, the attenuated defensive response could originate from either the analgesic or sedative properties of morphine, or both.)[20] These findings have been replicated for other invertebrate species,[20] but similar data is not yet available for lobsters.