Humans are capable of experiencing a wide range of pains, from bereavement to hunger and – perhaps worst of all – stubbing your toe on a cold winter’s morning. For invertebrates, pain was thought to be a far simpler concept, as a means of trying to correct something that’s damaging the body through reflex. A new study, however, paints a different picture for pain perception among one of Earth’s most charismatic and intelligent invertebrates: the fish-punching octopus.
A Journal pre-proof that's to appear in the journal iScience addresses the pain experience of octopuses in three parts. The first involved an octopus receiving different treatments in three chambers of a tank, after which they observed which chamber the octopus showed a preference for. Octopuses given a painful injection of acetic acid, or AA, would avoid the chamber in which they were given it – even if that chamber had previously been a preferred hangout.
A control used painless saline injections, which didn’t appear to influence the octopuses’ chamber preferences. Octopuses experiencing ongoing pain could have their least favorite chamber transformed into their favorite if they were given pain relief in the form of a lidocaine injection while inside. This avoidance or attraction to chambers – associated with pain and pain relief respectively – could demonstrate that octopuses have negative emotional feelings towards pain, information which they can use to navigate their environment as a means of avoiding that pain.
The next part explored if the octopuses had a discriminatory approach to pain, asking whether they could isolate the site of discomfort, or whether pain response behaviors were systemic regardless of where the boo-boo was. Some octopuses were again injected into one arm with AA, while others were injected with lidocaine and saline. The AA grouped differed, as they performed a directed and sustained pain response in grooming the injected area with their beak. This grooming lasted up to twenty minutes and saw the removal of skin surrounding the injection site in some animals. Those injected with the other two treatments either didn’t react, or only briefly groomed the injection site.
The third and final part of the research drew interesting parallels between mammals and cephalopods. When mammals injure a limb, say, their paw, the neural activity associated with the pain takes place in the central brain. This remote activity from the site of injury is also seen among cephalopods and – so far – they are the only invertebrate taxon known to replicate this mammalian-like pattern. A confusing concept in the context that octopuses have unique brains that extend into their limbs.
To test if this way of processing pain could be observed in octopuses, the researchers took electrophysiological recordings to ascertain where neural activity was sparked following the injection of AA, and if this activity remained until the pain was undone with lidocaine. Sure enough, pain in the arm triggered prolonged activity which was swiftly silenced by pain relief being injected into the same site as the AA.
A lot of octopuses with bruised arms, it seems, but what does it all mean?
“Together, these data provide strong support for the existence of a lasting, negative affective state in octopuses; the first evidence for pain experience in this neurologically complex invertebrate clade,” wrote the study authors in the paper, whose conclusions raise worrying questions about proposed ideas for farming octopus triggered by a spike in demand for their meat. “The presence of lasting pain experience in cephalopods (and possibly other invertebrate species) as a result of tissue injury raises both significant concerns for their welfare and interesting new questions about the evolutionary origins of pain experience.”