Do crabs and other crustaceans feel pain, or do they react to that boiling pot of water purely out of reflex? Well, according to new findings published in Biology Letters last week, that’s pain they’re experiencing. 

Most invertebrates, except cephalopods like octopuses, are thought to be unable to feel pain because they lack the specific brain areas implicated in our own pain experience. But that reasoning ignores how neuron structures with the same function can be located in different regions of the brain in different species. Processing visual cues, for example, occurs in very different brain areas of vertebrates and invertebrates.

But exactly how do you measure pain in crabs anyway? Recent work defined animal pain using a series of expectations or criteria: There should be a physiological stress response associated with noxious stimuli, as well as changes in behavior to protect themselves from further damage, such as increased wariness. While stress responses of crustaceans have been demonstrated in shore crabs, hermit crabs, and crayfish, these are typically preceded by escape behavior. So it’s hard to say if the physiological change is because of pain. 

To investigate, Robert Elwood and Laura Adams of Queen's University collected 40 European shore crabs (Carcinus maenas) from Portaferry in Northern Ireland. A copper wire was placed around a joint on their fifth walking legs, and for 20 of them in the “shocked” group, that wire was attached to an electric stimulator. Then the team delivered 10-volt, 180-hertz shocks for 200 milliseconds every 10 seconds for two minutes. The other 20, “non-shocked” crabs served as controls. The crabs were categorized into three types: no movement throughout, walking but no extreme response, and extreme response, which includes threat postures and attempts to climb the walls of the tank. Four minutes later, the team collected a sample of their vital fluid (called hemolymph) and measured the levels of lactate, which indicate a stress response. 

They found higher levels of lactate in shore crabs exposed to brief electric shock than in non-shocked controls. But, since shocked crabs showed more vigorous behavior, the team then matched the crabs with the same level of behavior: Walking shocked crabs were compared to walking non-shocked crabs. Even then, they still found that shocked crabs had stronger stress responses.

These results, together with previous findings on behavioral changes and avoidance learning, suggest that crabs fulfill the criteria expected of a pain experience. In the U.K., vertebrates are protected in scientific investigations, and this protection was recently extended to cephalopods. The vast majority of invertebrates have no such protection.