So how do we answer Stein's question? Elwood has been looking for ways to do so since running into Stein eight years ago. For a start, arguments by analogy are silly, he says. "Denying that crabs feel pain because they don't have the same biology is like denying they can see because they don't have a visual cortex."
Elwood and his colleagues at Queen's University Belfast are instead tackling the question by looking at how these animals behave. Most organisms can respond to a stimulus that signals a potentially harmful event. Special receptors called nociceptors - which sense excessive temperatures, noxious chemicals or mechanical injuries such as crushing or tearing - are found throughout the animal world, from humans to fruit flies. When a parasitic wasp jabs its egg-laying ovipositor into a fruit fly larva, for example, the larva senses the needle and curls up, which can make the wasp pull out.
But when an animal responds to something we would consider painful, it does not necessarily mean the animal is in pain. The response might be a simple reflex, where signals do not travel all the way to the brain, bypassing the parts of the nervous system connected with the conscious perception of pain. When we scald our hand, for example, we immediately - and involuntarily - pull it away. Pain is the conscious experience that follows, once the signals have reached the brain. The key for Elwood was to look for responses that went beyond reflex, the crustacean equivalents of limping or nursing a wound.
He started with prawns. After so many years of working with them, he thought he knew what to expect, which was that he would see nothing more than reflex reactions. But to his surprise, when he brushed acetic acid on their antennae, they began grooming the treated antennae with complex, prolonged movements of both front legs. What's more, the grooming diminished when local anesthetic was applied beforehand.