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Invertebrates form a huge part of the human diet across the globe; from lobsters and crabs to squid and shrimp. Billions are farmed each year for the food industry, but unlike vertebrates, virtually no laws exist to ensure humane treatment of these animals prior to human consumption. This means that these animals are subjected to harsh methods of killing - like being boiled alive. But if they can't feel pain, who cares - right? This attitude has spurred research into whether certain invertebrates, in particular cephalopods (a class of animals including squid and octopuses, which have the most complex brains amongst invertebrates) and decapods (crustaceans including crabs, lobsters and prawns) can feel pain. If this can be proven, it may induce a change in the care taken towards these animals, thus greatly enhancing their welfare.

Evolutionary biologist and behavioural neuroscientist Robyn Crook set out to investigate whether squid possess a certain type of nerve cell endings which initiate pain sensation, called nociceptors. Without this knowledge, it is difficult to judge whether animals such as these can feel pain; avoidance of a certain stimulus, for example by reflex, does not necessarily indicate that pain is felt. In a paper published last year in The Journal of Neuroscience, and reported by the New Scientist, Crook discovered that squid do indeed possess nociceptors, which inform the nervous system of injury. It was also discovered that these nociceptors do not respond to heat stimuli, but do respond to mechanical stimuli. When the scientists crushed the squid's fin, they found that that nociceptors were activated in a widespread manner, reaching as far as the opposite fin, as oppose to localised activation at the site of injury. This is in contrast to mammals, where nociceptor activation is assumed to be localised to the area experiencing injury, promoting defensive behaviours aimed at the site of pain. Crook believes that the widespread activation of nociceptors may serve to increase vigilance. It should be of note, however, that this study cannot definitively prove that cephalopods experience pain; rather, this study addresses a grey area of whether cephalopods possess pain sensing machinery, and fills certain criterion for pain.

Robert Elwood, from Queen's University Belfast, also has interest in this field and has investigated numerous invertebrates in an attempt to gain insight into pain reception. In a study investigating responses of the prawn to different noxious stimuli applied to the antenna, he found that not only were immediate reflexes evoked, but longer term behaviors also such as rubbing of the affected area. This indicated that the animal was at least aware of the location of the stimulus. Later studies involved two different types of crab. Through the use of small electric shocks, he found that despite only the fact that the majority of crabs investigated did not leave their shells, only the crabs that experienced shocks evacuated their shells. Furthermore, crabs were less likely to leave their homes, and tolerated larger shocks, if they were residing in a desirable shell, indicating a trade-off at play. Next, he investigated whether shore crabs could learn to avoid 1 of 2 dark shelters placed within a brightly lit chamber, when entering these chambers resulted in an electric shock. Indeed, crabs that experienced shocks were more likely to switch shelters, even after only two trials, indicating learning is taking place.

Elmwood believes that results such as these should be an important consideration in the treatment of these animals. Whether research such as this is sufficient to evoke wider belief that invertebrates such as these do feel pain, and subsequently instigate large-scale changes in the ways these animals are treated, is uncertain at present. 

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