It’s long been thought that the animals only respond to stimuli, but do not sense “pain” due to a simpler nervous system than those of mammals and other larger creatures.

But the belief is being challenged by an experiment that revealed at least crabs can learn to avoid uncomfortable sensations.

The response was discovered when a test group of the crustaceans were given the opportunity to choose between two identical shelters in which one discharged a mild electric shock.

Writing in the Journal of Experimental Biology, researchers say that nearly all of the crabs chose the shelter without the electricity.

The ability to interpret stimuli as negative and adapt behavior to avoid it is what some scientists, like report co-author Bob Elwood, see as a hallmark of feeling pain.

Other scientists, like Magnus Johnson of Hull University, remain unconvinced. “It looks to me ... that crabs move away from a potentially damaging stimulus. That doesn't necessarily mean that they sense pain," Johnson said.

While expert opinion remains polarized, Elwood warns: “You have lobsters being processed ... the head and the thorax being torn off. And the head with the brain will carry on being a viable nervous system and will continue to go on like that for an hour or so.”

The food industry currently kills billions of crustaceans each year. This leads some to advocate that even if there is a possibility they feel pain, methods of harvesting need to be changed to err on the side of caution in the same way the food industry treats poultry and livestock.

Photo: Egon - Flickr