The Effects of Caustic Soda and Benzocaine on Directed Grooming to the Eyestalk in the Glass Prawn, Palaemon elegans, Are Consistent with the Idea of Pain in Decapods

Abstract

Acceptance of the possibility of pain in animals usually requires that various criteria are fulfilled. One such criterion is that a noxious stimulus or wound would elicit directed rubbing or grooming at the site of the stimulus. There is also an expectation that local anaesthetics would reduce these responses to damage. These expectations have been fulfilled in decapod crustaceans but there has been criticism of a lack of replication. Here, we report an experiment on the effects of a noxious chemical, sodium hydroxide, applied to one eyestalk of the glass prawn. This caused an immediate escape tail-flick response. It then caused nipping and picking with the chelipeds at the treated eyestalk but much less so at the alternative eyestalk. Prior treatment with benzocaine also caused an immediate tail-flick and directed behaviour, suggesting that this agent is aversive. Subsequently, however, it reduced the directed behaviour caused by caustic soda. We thus demonstrated responses that are consistent with the idea of pain in decapod crustaceans.

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

“…It was also like a study on the shore crab, H. sanguineus , in which claw-bearing limbs were injected with formalin, and shaking, rubbing and unusual postures of that limb occurred [ 9 ]. These studies have demonstrated that both acetic acid and formalin causes decapods to show distinct activities that are consistent with the idea of pain, and similar effects have been noted with caustic soda brushed onto an eye in glass prawns [ 8 ].…”

“…Morphine has analgesic effects in vertebrates [ 1 , 2 , 3 ], but the evidence for such effects in crustaceans is weak or lacking [ 35 ]. However, local anaesthetics, such as benzocaine, are effective in reducing responses to noxious stimuli in decapods [ 5 , 8 ]. Benzocaine blocks the transmission of action potentials along axons, whereas opiates block transmission across synapses by inhibiting specific neurotransmitters [ 35 ].…”

“…Several following studies questioned that conclusion. Shore crabs, Carcinus maenas , showed directed behaviour when their mouthparts or an eye was brushed with acetic acid [ 7 ], and glass prawns ( P. elegans ) used their chelipeds to rub an eye if the eye had been brushed with caustic soda [ 8 ]. Crabs ( Hemigrapsus sanguineus ) responded when formalin, another substance that causes pain in humans, was injected into a claw and the animal reduced the use of that appendage when walking and pressed that claw against its carapace [ 9 ].…”

Trade-Offs between Avoidance of Noxious Electric Shock and Avoidance of Bright Light in Shore Crabs Are Consistent with Predictions of Pain

“…This is like a previous study that seemed to show some hesitation to enter a shelter in which a shock was delivered [ 20 ], but, as with that previous study, we cannot exclude the possibility that a shock toward the fifth leg reduces the ability to walk. We note, however, that noxious chemicals applied to the eyes or antennae of two species of prawns also increased the latency in the initiation of walking [ 25 , 26 ], and this is unlikely to be caused by reduced walking ability. We also examined contact with the walls of the arena as a percentage of the time the crabs were in the arena.…”

“…Nevertheless, the present observations, and those of numerous other experiments, add to the idea of pain in decapods. For example, we saw a directed rubbing of the site of noxious stimulus application [ 25 , 30 , 31 , 32 ], swift avoidance learning [ 17 , 33 ], long-term changes in behaviour [ 13 , 31 ], anxiety [ 18 , 34 ], giving up vital resources to escape [ 20 ], physiological changes [ 18 , 34 , 35 , 36 ], in addition to trade-offs noted above. With the increasing number of studies that provide data consistent with expectations of pain, there was an increasing acceptance that pain is possible or even likely.…”