Nociception is the ability to encode and perceive harmful stimuli and allows for a rapid reflexive withdrawal. Insome species, nociception might be accompanied by a pain experience, which is a negative feeling that allows forlonger-term changes in behaviour. Different types of stimuli may affect nociceptors, but in crustaceans there isconflicting evidence about the ability to respond to chemical stimuli. This study attempts to resolve this situationby testing behavioural responses of the common shore crab,Carcinus maenas,to two chemical irritantsfrequently used in vertebrate pain studies (acetic acid and capsaicin). In ourfirst experiment acetic acid, water,capsaicin or mineral oil were applied by brush to the mouth, and in a second experiment treatments were appliedto the eyes. Application of acetic acid had a marked effect on behaviour that included vigorous movement ofmouth parts, scratching at the mouth with the claws and attempts to escape from the enclosure. Acetic acid alsocaused holding down of the acid-treated eye in the socket. By contrast, capsaicin had no effect and was nodifferent to the control treatment of mineral oil and water. These results demonstrate responsiveness to aceticacid and thus nociceptive capacity for at least some chemicals. Further, the responses that persist afterapplication were consistent with the idea of pain, however, proof of pain is not possible in any animal.
- Acetic acid