Pain in cephalopods is a contentious issue. Pain is a complex mental state, with a distinct perceptual quality but also associated with suffering, which is an emotional state. Because of this complexity, the presence of pain in non-human animals, or another human for that matter, cannot be determined unambiguously using observational methods, but the conclusion that animals experience pain is often inferred on the basis of likely presence of phenomenal consciousness which is deduced from comparative brain physiology as well as physical and behavioural reactions.
Cephalopods are complex invertebrates, often considered to be more "advanced" than other invertebrates. They fulfill several criteria proposed as indicating that non-human animals may be capable of perceiving pain. These fulfilled criteria include having a suitable nervous system and sensory receptors, opioid receptors, reduced responses to noxious stimuli when given analgesics and local anaesthetics used for vertebrates, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements. Furthermore, it has been argued that pain may be only one component of suffering in cephalopods; others potentially include fear, anxiety, stress and distress.
Most animal welfare legislation protects only vertebrates. However, cephalopods have a special position among invertebrates in terms of their perceived ability to experience pain, which is reflected by some national and international legislation protecting them during research.
If cephalopods feel pain, there are ethical and animal welfare implications including the consequences of exposure to pollutants, practices involving commercial fishing, aquaculture and for cephalopods used in scientific research or which are eaten. Because of the possibility that cephalopods are capable of perceiving pain, it has been suggested that "precautionary principles" should be followed with respect to human interactions and consideration of these invertebrates.
Background Extant cephalopods are divided into two subclasses, the Coleoidea (cuttlefish, squid, and octopus) and Nautiloidea (nautiluses). They are molluscs, meaning they are related to slugs, snails and bivalves. Cephalopods are widely regarded as the most intelligent of the invertebrates. They have well-developed senses and large brains, and are considered by some to be "advanced invertebrates" or an "exceptional invertebrate class". About 700 living species of cephalopods have been identified.
The nervous system of cephalopods is the most complex of all the invertebrates and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates. The brain is protected in a cartilaginous cranium.
The possibility that non-human animals may be capable of perceiving pain has a long history. Initially, this was based around theoretical and philosophical argument, but more recently has turned to scientific investigation.
Philosophy
The idea that non-human animals might not feel pain goes back to the 17th-century French philosopher, René Descartes, who argued that animals do not experience pain and suffering because they lack consciousness.
In 1789, the British philosopher and social reformist, Jeremy Bentham, addressed in his book An Introduction to the Principles of Morals and Legislation the issue of our treatment of animals with the following often quoted words: "The question is not, Can they reason? nor, can they talk? but, Can they suffer?"
Peter Singer, a bioethicist and author of Animal Liberation published in 1975, suggested that consciousness is not necessarily the key issue: just because animals have smaller brains, or are 'less conscious' than humans, does not mean that they are not capable of feeling pain. He goes on further to argue that we do not assume newborn infants, people suffering from neurodegenerative brain diseases or people with learning disabilities experience less pain than we would.
Bernard Rollin, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were taught to simply ignore animal pain. In his interactions with scientists and other veterinarians, Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain.
Continuing into the 1990s, discussions were further developed on the roles that philosophy and science had in understanding animal cognition and mentality. In subsequent years, it was argued there was strong support for the suggestion that some animals (most likely amniotes) have at least simple conscious thoughts and feelings and that the view animals feel pain differently to humans is now a minority view.
Scientific investigation
In the 20th and 21st centuries, there were many scientific investigations of pain in non-human animals.
Mammals
At the turn of the century, studies were published showing that arthritic rats self-select analgesic opiates. In 2014, the veterinary Journal of Small Animal Practice published an article on the recognition of pain which started – "The ability to experience pain is universally shared by all mammals..." and in 2015, it was reported in the science journal Pain, that several mammalian species (rat, mouse, rabbit, cat and horse) adopt a facial expression in response to a noxious stimulus that is consistent with the expression of humans in pain.
Birds At the same time as the investigations using arthritic rats, studies were published showing that birds with gait abnormalities self-select for a diet that contains the painkiller carprofen. In 2005, it was written "Avian pain is likely analogous to pain experienced by most mammals" and in 2014, "...it is accepted that birds perceive and respond to noxious stimuli and that birds feel pain".
Fish Whether fish are able to perceive pain is contentious. However, teleost fishes have a suitable nervous system and sensory receptors, opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics, physiological changes to noxious stimuli, displaying protective motor reactions, exhibiting avoidance learning and making trade-offs between noxious stimulus avoidance and other motivational requirements.
Reptiles and amphibians Veterinary articles have been published stating both reptiles and amphibians experience pain in a way analogous to humans, and that analgesics are effective in these two classes of vertebrates.
Argument by analogy In 2012, the American philosopher Gary Varner reviewed the research literature on pain in animals. His findings are summarised in the following table.Varner, Gary E. (2012) "Which Animals Are Sentient?" Chapter 5 in: Personhood, Ethics, and Animal Cognition: Situating Animals in Hare's Two Level Utilitarianism, Oxford University Press. . The table in the article is based on table 5.2, page 113.
Argument by analogy Property Fish Amphibians Reptiles Birds Mammals Has nociceptors Has brain Nociceptors and brain linked ? / ? / ? / Has endogenous opioids Analgesics affect responses ? ? Response to damaging stimuli similar to humans
Notes
Arguing by analogy, Varner claims that any animal which exhibits the properties listed in the table could be said to experience pain. On that basis, he concludes that all vertebrates probably experience pain, but invertebrates, apart from cephalopods probably do not experience pain.
The experience of pain Although there are numerous definitions of pain, almost all involve two key components.
First, nociception is required. This is the ability to detect noxious stimuli which evoke a reflex response that rapidly moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not imply any adverse, subjective "feeling" – it is a reflex action. An example in humans would be the rapid withdrawal of a finger that has touched something hot – the withdrawal occurs before any sensation of pain is actually experienced.
The second component is the experience of "pain" itself, or suffering – the internal, emotional interpretation of the nociceptive experience. Again in humans, this is when the withdrawn finger begins to hurt, moments after the withdrawal. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if an animal responds to a stimulus in a similar way to ourselves, it is likely to have had an analogous experience.
A definition of "pain" widely accepted by scientific investigators is "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage".
Nociception
Nociception has been defined as "the detection of stimuli that are injurious or would be if sustained or repeated". It initiates immediate withdrawal of limbs or appendages, or the entire body, and therefore has clear adaptive advantages. Nociception usually involves the transmission of a signal along a chain of nerve fibers from the site of a noxious stimulus at the periphery to the spinal cord and brain. In vertebrates, this process evokes a reflex arc response generated at the spinal cord and not involving the brain, such as flinching or withdrawal of a limb. Nociception is found, in one form or another, across all major animal taxa. Nociception can be observed using modern imaging techniques; and a physiological and behavioral response to nociception can be detected.
Emotional pain
Sometimes a distinction is made between "physical pain" and "emotional" or "psychological pain". Emotional pain is the pain experienced in the absence of physical trauma, e.g. the pain experienced by humans after the loss of a loved one, or the break-up of a relationship. It has been argued that only primates and humans can feel "emotional pain", because they are the only animals that have a neocortex – a part of the brain's cortex considered to be the "thinking area". However, research has provided evidence that monkeys, dogs, cats and birds can show signs of emotional pain and display behaviours associated with depression during painful experience, i.e. lack of motivation, lethargy, anorexia, unresponsiveness to other animals.
Physical pain
A definition of pain widely accepted and used by scientists is "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage". The nerve impulses of the nociception response may be conducted to the brain thereby registering the location, intensity, quality and unpleasantness of the stimulus. This subjective component of pain involves conscious awareness of both the sensation and the unpleasantness (the aversive, negative affect). The brain processes underlying conscious awareness of the unpleasantness (suffering), are not well understood.
There have been several published lists of criteria for establishing whether non-human animals are capable of perceiving pain, e.g. Some criteria that may indicate the potential of another species, including cephalopods, to feel pain include: Has a suitable nervous system and sensory receptors Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics Physiological changes to noxious stimuli