An important new study suggests octopuses are likely to feel and respond to pain in a similar way to mammals - the first strong evidence for this capacity in any invertebrate.
The experience of pain is far more than a simple reflex to harmful stimuli or injury; it's a complex emotional state, leading to distress or suffering.
While vertebrates are generally thought to experience both the physical and emotional aspects of pain, it remains unresolved whether or not invertebrates, which generally have much simpler nervous systems, are capable of something similar.
Octopuses are the most neurologically complex invertebrates on Earth, and yet surprisingly few experiments have focused on their potential for experiencing pain.
Neurobiologist Robyn Crook from San Francisco State University has been investigating this issue for years, and the latest work from her lab has now used the same protocols for testing pain in laboratory rodents on cephalopods - specifically, the octopus.
Using detailed measurements of spontaneous pain-associated behaviors and neural activity, Crook has identified three lines of evidence that all indicate octopuses are capable of feeling negative emotional states when confronted with pain.
These are the same characteristics that mammals show, despite the fact that the octopus nervous system is organized in a fundamentally different way to vertebrates.
Of course, it's really difficult for scientists to interpret a subjective feeling or emotional state in an animal - especially one so different from us - but Crook argues the behavior shown by octopuses in these experiments suggests they are probably experiencing the physical and emotional components of pain in a way not so different to rodents, including lasting changes in their affective state (what we would call, in humans, our mood, feelings and attitudes).
"Even in the absence of proof on conscious awareness or sentience in cephalopods, it remains clear that the responses demonstrated by octopuses in this study are so similar to those that would be expressed by mammals experiencing pain, that a reasonable, cautionary argument can be made that internal state of these disparate species is likely also similar," Crook concludes.
Previous studies by Crook and others have shown octopuses can reflexively respond to noxious stimuli, learning to avoid those harmful contexts. The new research goes several steps further. After a single training session in a three-chambered box, octopuses that received an injection of acetic acid into one arm showed clear avoidance of the chamber in which they received that shot.
Those injected with non-harmful saline, on the other hand, showed no such avoidance.
Furthermore, when the octopuses that had been given a painful injection were then administered lidocaine (an analgesic), they tended to prefer the chamber in which they experienced immediate pain relief. Those that received saline only couldn't care less about the chamber where they had analgesic applied.
This preference for place, free from harm, is considered strong evidence of an affective pain experience in vertebrates. Nor is that the only parallel.
Crook also found evidence that octopuses could discriminate between different qualities and intensities of pain in different locations on their bodies.
All of the octopuses injected with the acid showed grooming behaviors at that injection site for the full 20-minute training trial, removing a small area of their skin with their beak.
This differs to other studies on peripheral pain responses, where octopuses had their arms crushed or cut off, and suggests the acid injection is producing some sort of centralized response. In mammals, ongoing pain is caused by sustained activity in the periphery, which then drives long-term changes in the brain or spinal cord.
Cephalopods, on the other hand, rely heavily on their peripheral nervous system and it's unclear how much of that information makes its way to their central circuits.
Using electrophysiological recordings, Crook has indirectly shown a prolonged peripheral response in the pathway to the octopus' brain, which appears to represent the intensity of pain experienced from the acid injections. What's more, these messages are rapidly silenced and reversed with an analgesic - a strong sign of pain experience.
"Together, these data provide strong support for the existence of a lasting, negative affective state in octopuses," the paper concludes.
To date, ongoing pain, as opposed to transient pain, has only been demonstrated in mammals, so it's pretty incredible that scientists have noticed something similar in an invertebrate.
Earlier this week, scientists also showed that cuttlefish are capable of passing the marshmallow test - a cognitive test designed to measure children's self regulation.
Such knowledge raises ethical questions about how we care for and study cephalopods, while also providing a novel evolutionary origin for the experience of pain in the animal kingdom.
"Our goal with this study was to move the question of invertebrate pain beyond reasonable doubt," reads a press release from Crook's lab, "so that efforts to better regulate their humane use can proceed with a strong evidentiary foundation that until now, has been lacking."
The study was published in iScience.