Electric shock causes physiological stress responses in shore crabs, consistent with prediction of pain

Abstract

Animal pain is defined by a series of expectations or criteria, one of which is that there should be a physiological stress response associated with noxious stimuli. While crustacean stress responses have been demonstrated they are typically preceded by escape behaviour and thus the physiological change might be attributed to the behaviour rather than a pain experience. We found higher levels of stress as measured by lactate in shore crabs exposed to brief electric shock than non-shocked controls. However, shocked crabs showed more vigorous behaviour than controls. We then matched crabs with the same level of behaviour and still found that shocked crabs had stronger stress response compared with controls. The finding of the stress response, coupled with previous findings of long-term motivational change and avoidance learning, fulfils the criteria expected of a pain experience.

Do insects feel pain? A question at the intersection of animal behaviour, philosophy and robotics

“…Without an understanding of the neural architecture or minimum brain size required to support subjective experience, we are left with no neurobiological method of determining whether insects experience pain (also see Elwood, 2011). Merely pointing out that insect brains have a different neuroanatomical structure than mammals does not demonstrate that they are incapable of experiencing an emotional response to pain; insects could use different neurobiological mechanisms (Sherwin, 2001).…”

Animal Consciousness

“…Other scientists have suggested however, that the presence of a centralised nervous system that is protected by a blood-brain interface, and particularly, the rather complex behavioural reactions in octopuses and in mollusc cephalopods in general to various situations, including injury, would suggest at least the possibility of the presence of some degree of a negative affective experience in response to a nociceptive stimulation (Barr et al, 2008). Similarly, the behaviours of crustaceans present a certain complexity and some scientists interpret their reactions to nociceptive stimuli as indicative of at least some degree of a negative affective experience associated with nociception (Magee and Elwood, 2013;Elwood and Adams, 2015).…”

Stress was never said to be pain: response to Stevens et al. (2016)

“…The shocked crabs nevertheless had greater physiological signs of stress than did controls. The paper [2] considered if that might be due to greater muscular activity but concluded that there were no overt signs of such activity. The critique suggests that heart and 'breathing' rates should have been monitored.…”

“…The paper [2] provides a way of largely overcoming this problem by comparing subgroups of shocked and non-shocked crabs that showed a similar level of behaviour. The shocked crabs nevertheless had greater physiological signs of stress than did controls.…”

“…That crustaceans tend to show positive results has been a surprise to the present author. They have been shown not to respond by mere reflex [8], rather they show long-term motivational change [11], swift avoidance learning [12], giving up of valuable resources to avoid noxious stimuli [12,13], trade-offs with other motivational requirements indicating central processing [13], inhibition of responses by local anaesthetics [8] and stress responses [2]. That is, many studies to date are indisputably consistent with the idea of pain, but as stated above, being consistent with something is not the same as absolute proof.…”