Have you ever wondered, whilst tucking into a tasty prawn cocktail, whether those delicious little crustaceans ever felt pain? When Professor Robert Elwood was asked this very same question by a well-respected British seafood chef, Elwood found he didn't have a definitive answer. Having previously studied many aspects of crustaceans' lives Elwood pondered this question for a few weeks before beginning his investigations, using the common shore crab as his subjects to tackle the question (p. 353).

From the outset, Elwood and his student, Barry Magee, wanted to carefully design an experiment that would distinguish between pain and a phenomenon known as nociception. Explaining the difference, Elwood says, ‘The function of pain is to aid future avoidance of the [painful] stimulus, whereas nociception enables a reflex response that provides immediate protection but no awareness or long-term behaviour [change]’. While nociception is generally accepted to exist in all animals, the same is not true of pain. Whether pain exists in all animals, but especially in crustaceans, remains widely debated.

Pain results in a learned avoidance of potentially harmful and painful stimuli, but as Elwood notes, it can also lead animals to make trade-offs between keeping a valuable resource and avoiding a painful stimulus. But what resource would a crab value and need convincing to give up? Despite their intimidating appearance with their fearsome pincers, it appears that crabs value dark hideaways beneath rocks where they hunker down to avoid hungry predators. Exploiting the crabs' preference for seclusion, Elwood and Magee tested whether the crustaceans experienced pain by seeing if they could learn to give up a perfect dark hiding place in order to avoid a mild, but potentially painful electric shock.

Capturing 90 crabs at a nearby beach, the duo transported the animals back to their lab at Queen's University, Belfast, UK, ready to test the animals' responses to a gentle electric jolt. Introducing the crabs individually to a tank equipped with two attractive dark shelters, the duo allowed the crustaceans to select their shelter of choice. However, for some of the wannabe lodgers their chosen shelter had an unpleasant surprise in store, dispensing mild electric shocks. Returning the crabs to the tank after a short breather, Elwood and Magee gave them a second chance to sample the shelters, and found that the crabs mostly stuck with what they knew best, returning to shelter that they had chosen first time around, even if it meant receiving another shock. However, when the crustaceans were introduced to the tank for a third time, most of the shocked crabs began exploring alternative options. After just two rounds of shocks the crabs were learning to avoid the shelter where they got zapped – although some crabs (perhaps the unadventurous ones that hadn't found the other welcoming shelter) continued to return to the unfavourable shelter, but this time they were much more likely to scuttle away and give up their precious hideaway to avoid the electric shocks.

Elwood and Magee explain that the crab's swift shock avoidance and discrimination learning (between the two shelters) clearly shows that shock affects their choice of shelter and is consistent with the definition of pain used for other species. The evidence collected from their study thus strongly suggests that crustaceans do indeed feel pain.

R. W.
Shock avoidance by discrimination learning in the shore crab (Carcinus maenas) is consistent with a key criterion for pain
J. Exp. Biol.