Robert Elwood, of Queen's University, Belfast, has announced research which, he argues, demonstrates that prawns, and other crustaceans such as lobsters, can feel pain. Surprisingly, this research, to be published in Animal Behaviour, didn't involve a detailed analysis of the neurology of crustaceans, but, rather, involved daubing an irritant, acetic acid, on to one of the two antennae belonging to each in a collection of 144 prawns. Immediately, the creatures began grooming and rubbing the affected antenna for up to 5 minutes. Elwood argues that "the prolonged, specifically directed rubbing and grooming is consistent with an interpretation of pain experience."
Elwood, however, is conflating a controlled response to a potentially damaging stimulus, with the conscious experience or feeling of pain.
Elwood's use of the phrase "consistent with an interpretation of pain experience" is crucial here. This is a much weaker assertion than the claim that something has been observed which provides evidence in favour of a pain experience. A controlled response to a potentially damaging stimulus does not entail that pain is experienced. Even a single cell can respond to a potentially damaging stimulus, hence the observations made by Elwood and his colleagues are also consistent with the absence of experienced pain. If the observed behaviour is consistent with both the presence and absence of experienced pain, then it cannot constitute evidence to support the hypothesis that crustaceans are capable of experiencing pain.