shore crab
Study on shore crabs suggests they feel pain D. Hazerli/CC

Crabs appear to feel pain when administered electric shocks, revising current thinking on pain perception in invertebrates. Scientists from Queen's University Belfast were looking to find out if stress responses displayed by crabs are the result of pain or because of escape behaviour.

The team, publishing their findings in the Royal Society journal Biology Letters, note that in the UK, vertebrates are protected in scientific investigations and that this protection has recently been extended to cephalopod molluscs.

"However, the vast bulk of invertebrates are considered not to experience pain and receive no protection, their responses regarded as purely nociceptive reflexes," they wrote. "Indeed, a recent review dismisses the idea that any invertebrate (or fish) experience pain because they lack the specific brain areas implicated in human pain experience."

They said that because we cannot establish what any animal feels when exposed to a stimuli, guides are provided to define criteria in relation to possible pain experience. They added: "In particular, we expect activities that go beyond mere reflex response and instead indicate central processing and long-term motivational change that protects the animal from further damage, and physiological changes in response to aversive stimuli."

For the study, the scientists had 20 crabs in a control group and 20 crabs in an electric shock group. They were placed in plastic tanks and while one group was given electric shocks from wires at intervals for two minutes, the other had wires attached but no shocks. Researchers observed the crabs looking for certain behaviours – no response, walking but no extreme response, and the extreme response where they tried to climb the walls of the tank and a threatened posture.

Findings showed those in the shock group showed far more extreme responses, suggesting they perceived pain and fulfil the criteria set out to indicate pain in animals. The team wrote: "Shocked crabs showed more vigorous behaviour than 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."