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Crabs suffer, remember pain, study finds

March 27, 2009
Courtesy Queen's University Belfast
and World Science staff

Crabs not only suf­fer pain but re­mem­ber it, a study has found. The re­search calls in­to ques­tion a pop­u­lar view that when small an­i­mals re­coil from un­pleas­ant­ness, it’s just re­flex with­out ac­tu­al feel­ing.

The stu­dy, by Bob El­wood and Mir­jam Ap­pel at Queen’s Uni­ver­s­ity Bel­fast, is to ap­pear in an up­com­ing is­sue of the re­search jour­nal An­i­mal Be­hav­iour. “A po­ten­tially very large prob­lem is be­ing ig­nored,” said El­wood, not­ing that  the mil­lions of crus­ta­ceans caught daily for food re­ceive few or no safe­guards against pain.

A hermit crab (courtesy  Queen's University Belfast)


Her­mit crabs have no shell of their own so they in­hab­it oth­er, emp­ty shells that they find. They pre­fer some types of shells over oth­ers, and will switch shells if they see a bet­ter op­tion. 

El­wood and col­leagues sub­jected crabs with­in their shells to elec­tric shocks that were usu­ally too weak to prompt im­me­di­ate evacua­t­ion. Rath­er, El­wood said, shocked crabs simply were more likely than oth­ers to switch shells when they were lat­er of­fered a new one.

The out­come sug­gests crabs were con­sid­er­ing pain in their de­ci­sion­mak­ing, not just re­spond­ing in a knee-jerk fash­ion, said El­wood, whose pre­vi­ous re­search also found prawns en­dure pain.

“There has been a long de­bate about wheth­er crus­taceans in­clud­ing crabs, prawns and lob­sters feel pain,” he said. “We know from pre­vi­ous re­search that they can de­tect harm­ful stim­u­li and with­draw from the source of the stim­u­li but that could be a sim­ple re­flex with­out the in­ner ‘feel­ing’ of un­pleas­ant­ness that we as­so­ci­ate with pain. 

“This re­search demon­strates that it is not a sim­ple re­flex but that crabs trade off their need for a qual­ity shell with the need to avoid the harm­ful stim­u­lus. Such trade-offs are seen in ver­te­brates,” he went on. “Hu­mans, for ex­am­ple, may hold on a hot plate that con­tains food where­as they may drop an emp­ty plate.”


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Crabs not only suffer pain but remember it, a study has found. The research calls into question a popular view that when small animals recoil from unpleasantness, it’s just reflex without actual feeling. The study, by Bob Elwood and Mirjam Appel at Queen’s University Belfast, is to appear in an upcoming issue of the research journal Animal Behaviour. Hermit crabs have no shell of their own so they inhabit other, empty shells that they find. They prefer some types of shells over others, and will switch shells if they see a better option. Elwood and colleagues subjected crabs to small electric shocks within their shells. By design, the shocks were usually too weak to prompt immediate evacuation. Rather, Elwood said, shocked crabs simply were more likely than others to switch shells when they were later offered a new one. The outcome suggests crabs were considering pain in their decisionmaking, not just responding in a knee-jerk fashion, said Elwood, who did previous research indicating prawns endure pain. “There has been a long debate about whether crustaceans including crabs, prawns and lobsters feel pain,” he noted. “We know from previous research that they can detect harmful stimuli and withdraw from the source of the stimuli but that could be a simple reflex without the inner ‘feeling’ of unpleasantness that we associate with pain. “This research demonstrates that it is not a simple reflex but that crabs trade-off their need for a quality shell with the need to avoid the harmful stimulus. Such trade-offs are seen in vertebrates in which the response to pain is controlled with respect to other requirements. Humans, for example, may hold on a hot plate that contains food whereas they may drop an empty plate, showing that we take into account differing motivational requirements when responding to pain.” Elwood said that unlike mammals, the millions of crustaceans caught for food daily receive little protection against pain. “Potentially very large problem is being ignored,” he said. “Legislation to protect crustaceans has been proposed but it is likely to cover only scientific research.”