No discrimination shock avoidance with sequential presentation of stimuli but shore crabs still reduce shock exposure

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

The suggestion that decapod crustaceans might experience pain has been dismissed by some authors who claim decapods only respond to noxious stimuli by nociceptive reflexes. Because reflexes do not require complex neuronal processing, but pain does, demonstrating reflex responses to noxious stimuli would not support the case for pain. Here, we report an experiment in which shore crabs are repeatedly placed in a light area (20 trials), but the animals can avoid the light by moving to a dark shelter. However, some crabs received an electric shock of 6 or 12 volts each time they entered the shelter. Those receiving either level of shock swiftly reduced their use of shelters and remained in the light. However, the magnitude of shelter avoidance was influenced by the brightness of the arena and the intensity of the shock. Shelter use was subsequently reduced to a greater extent if the shock level was high and the light intensity low. That is, crabs traded their avoidance of shock for their avoidance of bright light. Further, these animals showed avoidance learning and demonstrated activities suggesting anxiety, such as contact with the tank wall in the light area and increased latency to enter shelters when making the decision to enter the shelter if they had received shock in earlier trials. These results fulfil three key behavioural criteria for pain and, thus, are consistent with the idea that decapods can experience pain.

Behavioural Indicators of Pain and Suffering in Arthropods and Might Pain Bite Back?

Pain in response to tissue damage functions to change behaviour so that further damage is minimised whereas healing and survival are promoted. This paper focuses on the behavioural criteria that match the function to ask if pain is likely in the main taxa of arthropods. There is evidence consistent with the idea of pain in crustaceans, insects and, to a lesser extent, spiders. There is little evidence of pain in millipedes, centipedes, scorpions, and horseshoe crabs but there have been few investigations of these groups. Alternative approaches in the study of pain are explored and it is suggested that studies on traumatic mating, agonistic interactions, and defensive venoms might provide clues about pain. The evolution of high cognitive ability, sensory systems, and flexible decision-making is discussed as well as how these might influence the evolution of pain-like states.

Potential Pain in Fish and Decapods: Similar Experimental Approaches and Similar Results

I review studies that examined the possibility of pain experience in fish and note how they provided guidance on general methods that could be applied to other animals such as decapod crustaceans. The fish studies initially reported the occurrence of prolonged rocking movements in trout and rubbing of their lips if they were injected with acetic acid. Subsequent studies examined the role of morphine in reducing these activities and examined shifts in attention when responding to noxious stimuli. Various studies take up these themes in decapods. The results reported for the two taxonomic groups are remarkably similar and indicate that responses of both go beyond those expected of mere nociceptive reflex. Thus, the idea of pain cannot be dismissed by the argument that fish and decapods respond only by reflex. The responses of both clearly involve central processing, and pain experience, although not proven for either, is a distinct possibility. These studies have been the subjects of highly critical opinion pieces and these are examined and rebutted. The conclusion is that both fish and decapods should be awarded consideration for their welfare.

Discrimination between nociceptive reflexes and more complex responses consistent with pain in crustaceans

Animals have quick-acting nociceptive reflexes that protect them from tissue damage. Some taxa have also evolved the capacity for pain. Pain appears to be linked to long-term changes in motivation brought about by the aversive nature of the experience. Pain presumably enhances long-term protection through behaviour modification based, in part, on memory. However, crustaceans have long been viewed as responding purely by reflex and thus not experiencing pain. This paper considers behavioural and physiological criteria that distinguish nociception from potential pain in this taxon. These include trade-offs with other motivational systems and prolonged motivational change. Complex, prolonged grooming or rubbing demonstrate the perception of the specific site of stimulus application. Recent evidence of fitness-enhancing, anxiety-like states is also consistent with the idea of pain. Physiological changes in response to noxious stimuli mediate some of the behavioural change. Rapid avoidance learning and prolonged memory indicate central processing rather than mere reflexes. Thus, available data go beyond the idea of just nociception. However, the impossibility of total proof of pain described in ways appropriate for our own species means that pain in crustaceans is still disputed. Pain in animals should be defined in ways that do not depend on human pain experience. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Aversive responses by shore crabs to acetic acid but not to capsaicin

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