Effects of Acetic Acid and Morphine in Shore Crabs, Carcinus maenas: Implications for the Possibility of Pain in Decapods

Simple Summary Injection of acetic acid into one leg of a crab caused rubbing of that leg and holding the leg off the floor of a tank. Such activities directed towards the site of a noxious stimulus are consistent with the idea of pain. Some crabs also cast off the leg injected with acid. Because that occurred in association with possible pain-related behaviour, it too might be caused by pain. Prior morphine injection caused various behavioural changes, but morphine did not ameliorate the responses to acetic acid. Therefore, morphine does not act as an analgesic, and this agrees with previous work. Nevertheless, the directed behaviour that follows injection into a leg agrees with other studies and provides additional evidence suggesting that these animals experience pain. Abstract Noxious chemicals, coupled with morphine treatment, are often used in studies on pain in vertebrates. Here we show that injection of morphine caused several behavioural changes in the crab, Carcinus maenas, including reduced pressing against the sides of the enclosure and more rubbing and picking at the mouth parts and, at least for a short time, more defensive displays. Subsequent injection of acetic acid into one rear leg caused rubbing of the injected leg and the injected leg was held vertically off the ground. These activities directed at or involving the specific leg are consistent with previous observations of directed behaviour following noxious stimuli and are consistent with the idea that decapods experience pain. Further, acetic acid but not injection of water induced autotomy of the injected leg in these animals. Because autotomy is temporally associated with directed behaviour, it is possible that the autotomy is a pain-related response. Acetic acid is clearly a noxious substance when applied to decapods. However, morphine had no effect on the activities associated with acetic acid injection and thus there is no evidence for an analgesic effect. Further, the injection of acetic acid did not interfere with behavioural effects of morphine. The activities directed towards the site of injection are like those observed with injection, or with external application, of various noxious substances and the present study adds to a growing body of knowledge about possible pain in decapods.