The question of whether invertebrates feel pain is difficult to assess as it is a subjective experience perceived by sentient beings (neurobiologists generally exclude all invertebrates from this group). As such, it is not surprising that pain-like states have received little to no attention in the invertebrate scientific literature. In contrast, nociception, which in mammals is responsible for the long-lasting pain sensation following injury, has been investigated in several species of invertebrates including Drosophila, Caeonorhabditis, Aplysia and now recently a cephalopod.
New research published in the Journal of Neuroscience by Robyn Crook and colleagues at The Woods Hole Marine Biological lab, USA, has presented convincing evidence that squid (Doryteuthis pealeii) possess nociceptors. They report for the first time the presence of nociceptor activity in the fins of squid. They showed that when the fin was crushed, neuronal afferents (nerves that relay sensory information to the brain) displayed long-lasting sensitization and spontaneous activity after injury, which resembles the activity associated with painful stimuli in mammals. More interesting was their finding that spontaneous activity recorded in neuronal afferents relaying stimuli information was not just restricted to the site of injury but also observed on the un-injured fin on the opposite side of the body. This suggests that however the squid interprets this sensory information, the response to it is not localized to the site of injury, but spreads across the body. In mammals, such an increase in spontaneous afferent activity at the site of injury promotes defensive limb withdrawal and directs the individual's attention specifically to the site of injury.
Their findings do not directly address the question of whether pain-like states and pain-induced suffering are experienced by cephalopods. As it is understood in humans, pain is a highly variable, subjective experience incorporating many kinds of sensory input (mostly nociceptors) that culminates in a negative feeling and subsequent aversive reaction to avoid the unpleasant sensation. It is strongly influenced by previous experiences, including conditioning from past personal experiences and witnessing painful responses to stimuli on others, suggesting that a portion of its perception is learned. Physiological and behavioural changes that occur in response to pain may include the following: reflexive limb withdrawal, increased heart and breathing rates, vasoconstriction/blood pressure increase, vocalization, aggression and subsequent inflammation. When we observe these behavioural correlates in other mammals we conclude that the animal is experiencing pain and suffering. However, this presumption becomes harder to defend as one examines animals from phylogenetically more removed species such as invertebrates.
An unfortunate mistake made by many people is the personification of invertebrate behaviour. When one witnesses an insect recoil in response to a poke by a stick or observes the tail-flips of a lobster as it is placed in boiling water, one may erroneously think the animal is experiencing pain. The scientific basis for this assumption is questionable. For the lobster tail-flip example (caridoid escape response), a very simple (just a few neurons) neuronal circuit coordinates this behaviour whereas perhaps many hundreds of thousands of neurons participate in pain cognition in mammals. However, the question of nociceptive sensation in complex invertebrate species such as cephalopods is worthy of further scientific inquiry as these animals possess the most complex nervous systems of invertebrates, which rivals those of similarly sized vertebrates and allows them to interact socially, possess an extensive repertoire of behaviours and have an impressive capacity to learn (unlike most other invertebrate species). Clearly, with this evidence in hand one should be mindful and err on the side of caution when interacting with these animals.