Pain in crustaceans
Most definitions of pain involve the two key components of nociception and suffering. Nociception is an ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not imply any adverse, subjective feeling - it is a simple reflex action, not processed by the brain. Suffering is the subjective experience of the pain; the internal, emotional interpretation of the nociceptive experience and therefore a private, emotional experience. Suffering cannot be directly measured in other animals, including other humans. 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 used.
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", some have interpreted their presence as an indication that crustaceans may experience pain. Opioids may moderate pain in lobsters similar to the way they moderate pain in vertebrates.
The belief that non-human animals might not feel pain in the same manner as humans dates back to at least the 17th century, when French philosopher René Descartes argued that animals do not experience pain because they lack consciousness. Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, wrote that researchers remained unsure into the 1980s as to whether animals experience pain. Veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain. In his interactions with scientists and other veterinarians, Rollin was regularly asked to prove that animals are conscious and to provide scientifically acceptable grounds for claiming that they feel pain. Veterinarian and author Larry Carbone has written 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, some critics continue to question how reliably animal mental states can be determined.
Pain is a private experience and therefore we cannot know with certainty whether other animals, or other humans, experience pain. However, the capacity for experiencing pain can be inferred using argument-by-analogy and physiological and behavioral reactions. Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too. Nociceptors, the neurones required for the sensation of pain, have been found in nematodes, annelids, molluscs and in the arthropod, Drosophila, despite earlier claims that nociceptors were absent in insects.
Nociception is the ability to perceive a noxious stimulus and react in a reflexive manner; it occurs across a wide range of taxa. The rockpool prawn (Palaemon elegans) reacts to noxious stimuli with an immediate reflex tail flick response
Research on decapod crustaceans
Prawns, crayfish and lobsters
In 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 response (i.e. does not involve conscious perception) to noxious stimuli.
Latter studies at Queen's University, Belfast, indicated that similar crustaceans do feel pain. When the antennae of rockpool prawns (Palaemon elegans) were rubbed with sodium hydroxide or acetic acid, the animals showed increased grooming and rubbing of the afflicted area 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. Elwood argues that sensing pain is crucial to prawn survival, because it encourages them to avoid damaging behaviors. Other scientists suggested the rubbing may reflect an attempt to clean the affected area as application of anesthetic alone caused an increase in grooming. Several key effects were not observed in a separate study which found no behavioural or neural changes in three different species (red swamp crayfish (Procambarus clarkii), white shrimp (Litopenaeus setiferus) and Palaemonetes sp.) in response to acids or bases.
In 2009, Elwood and Mirjam Appel showed that hermit crabs make motivational tradeoffs between electric shocks and the quality of the shells they inhabit. 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.
Shore crabs (Carcinus maenas) also show motivational tradeoffs; they will discard a valuable resource (a preferred shelter) to avoid future encounters with painful stimuli, thereby indicating avoidance learning - a key criterion of the ability to experience pain.
Morphine, an analgesic, and naloxone, an opioid receptor antagonist, affect the estuarine crab (Neohelice granulata) in a similar way to their effects on vertebrates: injections of morphine produced a dose-dependent reduction of their defensive response to an electric shock. However, it has been suggested the attenuated defensive response could originate from either the analgesic or sedative properties of morphine, or both. These findings have been replicated for other invertebrate species, but similar data are not yet available for lobsters.
Advocates for Animals, a Scottish animal welfare group, stated in 2005 that "scientific evidence ... strongly suggests that there is a potential for decapod crustaceans and cephalopods to experience pain and suffering". This is primarily due to "The likelihood that decapod crustaceans can feel pain [which] is supported by the fact that they have been shown to have opioid receptors and to respond to opioids (analgesics such as morphine) in a similar way to vertebrates." Similarities between decapod and vertebrate stress systems and behavioral responses to noxious stimuli were given as additional evidence for the capacity of decapods to experience pain.
- Animal cognition
- CrustaStun (device which administers a lethal electric shock to a crustacean)
- Declawing of crabs
- Emotion in animals
- Moral status of animals in the ancient world
- Pain in animals
- Pain in fish
- Pain in invertebrates
- Pain and suffering in laboratory animals
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