page_title: Pain in crustaceans
section_title: Trade-offs in motivation
hierarchical_section_title: Pain in crustaceans / Research findings / Trade-offs in motivation
caption_attribution_description: English: Image title: A hermit crab emerges from its shell Image from Public domain images website, http://www.public-domain-image.com/full-image/fauna-animals-public-domain-images-pictures/crabs-and-lobsters-public-domain-images-pictures/a-hermit-crab-emerges-from-its-shell.jpg.html
context_page_description: The question of whether crustaceans experience pain is a matter of scientific debate. 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.
context_section_description: Nociceptive responses are reflexes that do not change regardless of motivational priorities.  In contrast, a painful experience may change the motivation for normal behavioural responses, thereby indicating a plastic response to an aversive stimulus, rather than a simple reflex response.
  In 2009, Elwood and Mirjam Appel showed that hermit crabs make motivational trade-offs 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.  It was also shown that hermit crabs experiencing increasing electric shocks left their shell at a reduced intensity when the shell was from a less preferred species than did those in shells of a more desirable species. This demonstrates that hermit crabs are willing to risk predator attack by evacuating their shells to avoid a noxious stimulus and that this is dependent upon how valuable the shell is.
  A similar study shows that hermit crabs are less likely to leave their shells after electric shock if they are in an environment that contains the odour of predators.  This shows the crabs trade-off the motivation to avoid electric shocks and predator avoidance.
  Shore crabs (Carcinus maenas) also show motivational trade-offs; 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.
  A 2014 study on crayfish (Procambarus clarkii) tested their responses in a fear paradigm, the elevated plus maze in which animals choose to walk on an elevated cross which offers both aversive and preferable conditions (in this case, two arms were lit and two were dark). Crayfish which experienced an electric shock displayed enhanced fearfulness or anxiety as demonstrated by their preference for the dark arms more than the light. Furthermore, shocked crayfish had relatively higher brain serotonin concentrations coupled with elevated blood glucose, which suggests a stress response.  A follow-up study using the same species showed the intensity of the anxiety-like behaviour, presumably resulting from the pain, was dependent on the intensity of the electric shock until reaching a plateau. Such a quantitative relationship between stress and anxiety is also a very common feature of human and vertebrate anxiety.
caption: A hermit crab
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