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Pain in cephalopods

From Wikipedia, the free encyclopedia

Examples of cephalopds
The common octopus
(Octopus vulgaris)
The common cuttlefish
(Sepia officinalis)
The common squid
(Loligo vulgaris)
The palau nautilus
(Nautilus belauensis)

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.[1]

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,[2] 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, 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.

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Transcription

From the adorable platypus to snakes with one inch fangs, here are 12 of the most venomous animals to humans. 12. Platypus Adorable and goofy looking, the duck-billed platypus looks like it would be more at home in a cartoon rather than real life. However, this waddling chimera of animals has a rather painful surprise: the males of the species have venomous spurs on their hind legs. Deciding how venomous an animal is isn’t an exact science. However, scientists do use a standard known as LD50. LD50 refers to how much venom is needed to kill 50 percent of a test population of lab mice. The platypus’ poison glands are located in their thighs and they use spurs near their heels to inject the poison. While not fatal to humans, the toxins create excruciating pain that can last for weeks, and causes swelling and increased temperature. The pain, though, really is the worst part because no painkillers, not even morphine, can touch it. Why would such an adorable creature need poison glands? Scientists aren’t actually sure, although it’s most likely used to defend their burrows and their young. 11. Brown Recluse Also called violin spiders or fiddleback spiders, the brown recluse carries venom that can cause serious injury and death. These spiders are mainly found in the Southeastern and Midwestern states of the United States, but they can be accidentally transported inside boxes and packages. This is a rare phenomenon and there have only been 10 confirmed cases of brown recluses being found outside their native habitats. Most sightings are due to these spiders being confused with similar looking cousins. The venom of the brown recluse is hemotoxic, meaning that it affects red blood cells. The bite of a recluse causes nausea, vomiting, necrosis, muscle and joint pain, the bursting of red blood cells, organ damage, and death. Death, however, only occurs in young children or the elderly with weak immune systems. Luckily, these spiders are not aggressive and only bite when disturbed. There is no antivenom for the brown recluse bite. Medical treatment is often slow and carries physical complications. Many times, the term “venomous” and “poisonous” are used interchangeably. Technically they aren’t the same. In a poisonous animal, the toxin is deployed passively like the toxins excreted by the poison dart frog. An animal that injects you with toxin, like the brown recluse, is venomous. 10. Gila Monster One of the most venomous lizards to humans, the Gila monster is a member of an exclusive club. Out of over 4,600 species of lizards, only a handful are venomous. In 1952, the Gila monster became the first venomous animal in North America to be granted legal protection. Getting bitten by a Gila monster is nasty business. It makes its venom in a row of glands in its lower jaw. When the lizard bites, small grooves in the teeth deliver the toxin into the wounds. Gila monsters bite and chew their victims and can take up to fifteen minutes before releasing it. While extremely painful, the venom of the Gila monster is not fatal in healthy humans. Victims can experience swelling, nausea, vomiting, high blood pressure, weakness, faintness, excessive perspiration, chills and fever. There have also been cases of severe reactions resulting in breathing difficulties. Most reported bites occur on the hands, suggesting the lizard defended itself after being handled or prodded. There is a good side to Gila monster venom. In the early 1990s, John Eng, an endocrinologist, discovered the venom contained a peptide that increased the production of insulin. Humans release a similar hormone but this peptide is longer lasting. Eng used his discovery to create a medication for those with Type 2 diabetes. 9. Eastern Diamondback Rattlesnake The eastern diamondback is not only the largest rattlesnake in North America, but it is also the most venomous. It can inject between 400 to 1,000 mg of venom in one bite. To give that some perspective, it only takes 100 to 105 mg to be lethal to humans. They can also strike up to one-third of their body length and their fangs can grow to over an inch long. Found in the Southeastern US, the diamondback rattlesnake is not as aggressive as portrayed. In fact, they actively avoid humans. Like the brown recluse, their venom is hemotoxic, causing tissue damage and killing red blood cells. The good news is that the antivenom is widely available. Therefore, deaths rarely occur. 8. Stonefish Stonefish live mostly in the tropic of Capricorn. They live in the shallow tropical waters of the Pacific and Indian oceans, ranging from the Red Sea to the Great Barrier Reef off the Australian coast. Acknowledged as the world’s most venomous fish, its dorsal area is lined with 13 spines. These spines release venom from two sacs attached to each spine. Usually, this happens when a person accidentally steps on a stonefish. The venom causes severe pain, possible shock, paralysis, and tissue death depending on the depth of the penetration. If left untreated, it can cause heart failure. Hot water can cause temporary relief but the anti-venom is essential. 7. Cone Snail The cone snail looks innocent enough, but it doesn’t take much to be lethal to humans. One drop of their venom is enough to kill 20 people. This little cone snail can be as deadly as any other animal on this list. The cone snail, which lives in tropical waters, has specialized teeth called radula. They work like a combination between a hypodermic needle and harpoon, allowing it to both skewer and poison its prey. The venom works on the nervous system, paralyzing its victim. Because of their ornate shells, they pose a risk for unsuspecting divers and tourists who may want to take home a souvenir. Larger species of cone snails have radulae large enough to pierce a wetsuit. If medical treatment isn’t sought immediately after being punctured, 65 percent of people stung die. While we don’t have an anti-venom, doctors can treat the symptoms and keep the victim alive until the venom wears off. Symptoms include intense pain, numbness, muscle paralysis, double vision, and respiratory paralysis. However, like with the Gila monster, the poison of a cone snail has proven to be very useful. Scientists believe that it could be used as an effective painkiller more powerful than morphine but less addictive. 6. Deathstalker Scorpion Deathstalker scorpions live in North Africa and the Middle East. A healthy adult doesn’t have to worry about dying from a scorpion sting. However, that doesn’t make them harmless. Children, the elderly, and anyone with a heart condition could potentially die from the sting of the deathstalker. Just the name alone is scary. Deathstalker scorpion venom is an unusual cocktail of neurotoxins and cardiotoxins. Neurotoxins attack the nervous system while cardiotoxins attack the cardiovascular and respiratory systems. A deathstalker sting causes intense and unbearable pain. It can also cause convulsions, coma, paralysis, and death. Unlike other venomous creatures who would rather not interact with people, the deathstalker is also very aggressive. Treatment after a sting needs to be immediate. The venom resists treatment, so a large dose of antivenom is required. On a positive note, deathstalker scorpion venom has components that could be useful in treating cancer tumors and diabetes. 5. Brazilian Wandering Spider Also called the banana spider, the Brazilian wandering spider has the dubious honor of being awarded in 2007 by the Guinness Book of World Records for being the spider responsible for the most human deaths. They also dubbed it “the most venomous spider”. Scientists believe the Brazilian wandering spider has the most potent neurotoxic venom of any living spider. It only takes 0.006mg (0.00000021oz) of its venom to kill a mouse. True to their name, the Brazilian wandering spiders like to wander! They have no nest or web to call home. During the day, they can hide in highly populated areas inside houses, clothes, boots, and cars. If they’re in the forest, they hide in crevices and logs. During the night, they actively hunt prey, killing by both ambush and direct attack. If only a little venom is introduced into a human, they may experience some pain. However, if it’s a more serious bite, then the victim experiences more intense pain, sweating, increased or slowed heartbeat, nausea, hypothermia, vertigo, blurred vision, convulsions, sweating, and widespread tissue necrosis. Also men listen up! You guys should be extra careful with this spider because they can also cause a syndrome known as priapism in men. Priapism is a painful erection that lasts for many hours and can lead to impotence. 4. King Cobra The King Cobra can grow up to 18 feet (5 meters) long, making it the world’s longest venomous snake. Even though this cannibal mostly eats other snakes, the King Cobra still poses a threat to humans. One bite can cause death in a healthy adult within 15 minutes and in a full-grown Asian elephant within 3 hours. This doesn’t mean that its venom is especially toxic. In fact, milligram to milligram, there are other more toxic venoms. What sets the King Cobra apart is volume: this snake is capable of injecting more venom into its victim than other snakes. The King Cobra’s venom is neurotoxic, like many others on this list, and can cause extreme dizziness, blurred vision, and paralysis. Unless the antivenom is administered quickly, you will most likely die. 3. Blue-Ringed Octopus Although, the Blue-Ringed Octopus is only the size of a golf ball, its venom is powerful enough to kill a human. And there is no antidote. It venom is 1,200 times more toxic than cyanide and it is considered one of the deadliest animals in the ocean. This octopus gets its name from the iridescent blue rings that appear whenever it is alarmed. It’s most eye-catching feature, and the one that would certainly attract a person’s notice, is really a warning signal. If you don’t leave it alone, the octopus will deliver its venom through a tiny bite. Many times you might not even know if you’ve been bitten until it’s too late. Its neurotoxic venom causes muscular weakness, numbness, difficulty breathing and ultimately death.This beautiful but deadly cephalopod can be found in tide pools in the Pacific Ocean, from Japan to Australia so if you are splashing around there, be careful!! 2. Inland Taipan Here we have “The World’s Most Venomous Snake”, the inland taipan of Australia. One bite from this snake contains enough venom to kill 100 human adults. That makes its venom at least 200 – 400 times more toxic than a common cobra. The inland taipan’s neurotoxic venom can kill an adult human in as little as 45 minutes. Luckily, there is an antivenom. Symptoms of the taipan’s venom include headache, nausea, vomiting, abdominal pain, and paralysis. As well as being strongly neurotoxic, the venom contains a ‘spreading factor’, the hyaluronidase enzyme, which increases the rate of absorption. Because of the taipan’s shy nature, only a handful of people have ever been bitten and in theory all have survived due to receiving immediate treatment. 1. Box Jellyfish The box jelly has been dubbed the most venomous animal in the world. It lives like most dangerous things in the waters surrounding Australia which only strengthens that country’s reputation of being the most dangerous place to live. There are at least 5,567 recorded deaths since 1954 credited to this jelly. It’s toxins attack the heart, nervous system, and skin cells. But that’s not the worst part. The agony caused by its venom is so overwhelming that it sends people into shock, causing them to either drown or die of heart failure before being able to get help. Anyone who survives a bite experiences pain weeks after the sting. People are advised to apply vinegar for thirty seconds to the stung area right after an attack. The acetic acid in the vinegar helps disable some of the venom’s properties. Unfortunately, it doesn’t do anything for the pain. Swimmers are advised to wear special suits or leggings to prevent the box jelly from stinging a swimmer’s leg. Thanks for watching! Beware of venomous creatures!! Remember to subscribe and see you next time!!! Byeeeee

Contents

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,[3] and are considered by some to be "advanced invertebrates" or an "exceptional invertebrate class".[4] About 700 living species of cephalopods have been identified.

The nervous system of cephalopods is the most complex of all the invertebrates[5] and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates.[6] 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

René Descartes
René Descartes

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.[7][8][9] 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?"[10]

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.[11]

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.[12] 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.[12]

Continuing into the 1990s, discussions were further developed on the roles that philosophy and science had in understanding animal cognition and mentality.[13] 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[14] and that the view animals feel pain differently to humans is now a minority view.[7]

Scientific investigation

Cambridge Declaration on Consciousness (2012)
The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses [which are cephalopods], also possess these neurological substrates.[15]

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.[16] 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..."[17] 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.[18]

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.[19] In 2005, it was written "Avian pain is likely analogous to pain experienced by most mammals"[20] and in 2014, "...it is accepted that birds perceive and respond to noxious stimuli and that birds feel pain".[21]

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.[22][23]

Reptiles and amphibians

Veterinary articles have been published stating both reptiles[24][25][26] and amphibians[27][28][29] 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.[22]

Argument by analogy[22]
Property
Fish Amphibians Reptiles Birds Mammals
Has nociceptors Green tickY Green tickY Green tickY Green tickY Green tickY
Has brain Green tickY Green tickY Green tickY Green tickY Green tickY
Nociceptors and brain linked Green tickY ?[a] / Green tickY ?[b] / Green tickY ? / Green tickY Green tickY
Has endogenous opioids Green tickY Green tickY Green tickY Green tickY Green tickY
Analgesics affect responses Green tickY ?[c] ?[d] Green tickY Green tickY
Response to damaging stimuli similar to humans Green tickY Green tickY Green tickY Green tickY Green tickY

Notes

  1. ^ But see[30]
  2. ^ But see[31]
  3. ^ But see[32]
  4. ^ But see[33]

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.[22][34]

The experience of pain

Although there are numerous definitions of pain, almost all involve two key components.

First, nociception is required.[35] 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".[23]

Nociception

Nociception: The reflex arc of a dog with a pin in her paw.  Note there is no communication to the brain, but the paw is withdrawn by nervous impulses generated by the spinal cord.  There is no conscious interpretation of the stimulus by the dog.
Nociception: The reflex arc of a dog with a pin in her paw. Note there is no communication to the brain, but the paw is withdrawn by nervous impulses generated by the spinal cord. There is no conscious interpretation of the stimulus by the dog.

Nociception has been defined as "the detection of stimuli that are injurious or would be if sustained or repeated".[36] 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.[35] 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.[11]

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".[36] 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.[23][37] Some criteria that may indicate the potential of another species, including cephalopods, to feel pain include:[37]

  1. Has a suitable nervous system and sensory receptors
  2. Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics
  3. Physiological changes to noxious stimuli
  4. Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy
  5. Shows avoidance learning
  6. Shows trade-offs between noxious stimulus avoidance and other motivational requirements
  7. High cognitive ability and sentience

Adaptive value

The adaptive value of nociception is obvious; an organism detecting a noxious stimulus immediately withdraws the limb, appendage or entire body from the noxious stimulus and thereby avoids further (potential) injury. However, a characteristic of pain (in mammals at least) is that pain can result in hyperalgesia (a heightened sensitivity to noxious stimuli) and allodynia (a heightened sensitivity to non-noxious stimuli). When this heightened sensitisation occurs, the adaptive value is less clear. First, the pain arising from the heightened sensitisation can be disproportionate to the actual tissue damage caused. Second, the heightened sensitisation may also become chronic, persisting well beyond the tissues healing. This can mean that rather than the actual tissue damage causing pain, it is the pain due to the heightened sensitisation that becomes the concern. This means the sensitisation process is sometimes termed maladaptive. It is often suggested hyperalgesia and allodynia assist organisms to protect themselves during healing, but experimental evidence to support this has been lacking.[38][39]

In 2014, the adaptive value of sensitisation due to injury was tested using the predatory interactions between longfin inshore squid (Doryteuthis pealeii) and black sea bass (Centropristis striata) which are natural predators of this squid. If injured squid are targeted by a bass, they began their defensive behaviours sooner (indicated by greater alert distances and longer flight initiation distances) than uninjured squid. If anaesthetic (1% ethanol and MgCl2) is administered prior to the injury, this prevents the sensitisation and blocks the behavioural effect. The authors claim this study is the first experimental evidence to support the argument that nociceptive sensitisation is actually an adaptive response to injuries.[40]

Research findings

Peripheral nervous system

A moving octopus. Note the co-ordination of the arms.

A science-based report from the University of British Columbia to the Canadian Federal Government has been quoted as stating "The cephalopods, including octopus and squid, have a remarkably well developed nervous system and may well be capable of experiencing pain and suffering."[41]

Nociceptors

The discovery of nociceptors in cephalopods has occurred relatively recently. In 2011, it was written that nociceptors had yet to be described in any cephalopod.[36] However, in 2013, nociceptors responsive to mechanical and electrical stimuli, but not thermal stimuli, were described in the longfin inshore squid (Doryteuthis pealeii)[42] (note – it is highly unlikely that squid encounter temperatures greater than 30 °C making it very improbable that the nervous system will have evolved nociceptors to detect such high temperatures.[43]) This study also provided evidence that these receptors, as in vertebrates, undergo both short-term and long-term sensitization (30 min and 24 h, respectively).[4][44] Similarly, low-threshold mechanoreceptors and cells considered to be nociceptors in the algae octopus (Abdopus aculeatus) are sensitised for at least 24 hrs after a crushing injury.[45]

Nerve fibres

Both the arms and the mantle contain nervous tissue that conduct nociceptive information to the higher processing areas of the CNS.[45]

Numerous studies have described the existence of neural tissue paths that connects the peripheral areas of cephalopods to their CNS. However it is unclear if specific pain pathways are among these.[44]

In octopuses, the large optic lobes and the arms' nervous system are located outside the brain complex. The optic lobes contain 120 to 180 million neurons and the nervous system of the arms contains two-thirds of the total 500 million neurons in the nervous system.[36][46]

Brain

The octopus central brain contains 40 to 45 million cells. The brain-to-body mass ratio of the octopus is the highest of all the invertebrates and larger than that of most fish and reptiles (i.e. vertebrates). However, scientists have noted that brain size is not necessarily related to the complexity of its function.[47][48]

Octopuses have centralized brains located inside a cartilaginous capsule surrounding the oesophagus. It is divided into approximately 40 specialized areas and lobes that are arranged hierarchically; these include the sub- and supra-oesophageal masses, and the magnocellular, buccal, inferior frontal, vertical, basal, optic, peduncle, and olfactory lobes. The lobe's functions include learning, memory, processing information from the various sensory modalities, control of motor responses and the blood system. The vertical and frontal lobe complexes, unique among invertebrates, have vertebrate-like properties and are dedicated to learning and memory.[36][44][46][49][50][51] It has been suggested the vertical lobe system processes information related to pain.[6]

The nautilus brain lacks the vertical lobe complex and is therefore simpler than that of the coleoids,[52] however, they still exhibit rapid learning (within 10 trials), and have both short- and long-term memory (as found in operant studies of cuttlefish).[52]

In 2011, it was written that it was not known where in the brain cephalopods process nociceptive information meaning that evidence for nociception is exclusively behavioural.[36]

Opioid system

The four main opioid receptor types (delta, kappa, mu, and NOP) are found in vertebrates; they are highly conserved in this taxon and are found even in primitive jawless fishes. The endogenous system of opioid receptors is well known for its analgesic potential in vertebrates. Enkephalins come in two forms, met-enkephalin and leu-enkephalin, which are involved in regulating nociception in the vertebrate body as they bind to the body's opioid receptors.

Enkephalin-like peptides have been found in neurones of the palliovisceral lobe of the brain in the common octopus, and met-enkephalin receptors as well as delta opioid receptors in the mantle, arms, gut and vena cava of various octopus species. Leu-enkephalin and delta receptors have been found in the mantle, arms and other tissues in Amphioctopus fangsiao.[53][54]

Although cephalopods appear to have an opioid system, it has been noted that in vertebrates, the opioid system has a wide range of complex functions meaning the presence of an opioid system in cephalopods (and other invertebrates) is not necessarily evidence of this having an antinociceptive function.[44]

Effects of naloxone

Naloxone is an μ-opioid receptor antagonist which, in vertebrates and some other invertebrates, negates the effects of opioids. The substance has a similar reversal effect in the California two-spot octopus (Octopus bimaculatus).[55]

Effects of analgesics and anaesthetics

Cephalopod veterinary medicine sometimes uses the same analgesics and anaesthetics used in mammals and other vertebrates, although in 2015, it was stated "there are no specific studies about the effect of analgesics in cephalopods".[44]

If anaesthetic (1% ethanol and MgCl2) is administered prior to a crushing injury, this prevents nociceptive sensitisation.[40]

General anaesthesia in cephalopods has been achieved with a large range of substances, including isoflurane.[4][56] Benzocaine is considered to be an effective anaesthetic for the giant Pacific octopus (Enteroctopus dofleini).[57] Magnesium hydrochloride, clove oil, carbon dioxide and ethanol are among the substances used for anaesthesia of cephalopods, however, their effectiveness and potency are still under-researched.[44]

Physiological responses

The physiological responses of cephalopods to noxious stimuli include cardio-respiratory changes, defecation and vomiting.[44]

Behavioural responses

Protective

Many animals, including some octopuses, autotomise limbs when these are injured. This is considered to be a nociceptive behaviour. After receiving a crushing injury to an arm, algae octopuses autotomise the affected arm and show wound protective behaviours such as wrapping other arms around the wounded arm. These protective responses continue for at least 24 hours. In the long-term, they also show heightened sensitisation at the site of the injury and a reduced threshold to showing escape responses.[43][45] The curled octopus (Eledone cirrhosa) also shows protective responses to injury.[58][59] These long-term changes in behaviour suggest that "... some molluscs may be capable not only of nociception and nociceptive sensitization but also of neural states that have some functional similarities to emotional states associated with pain in humans."[36]

Other immediate defensive behaviours that might indicate a perception of pain include inking, jetting locomotion and dymantic display.[59]

In one study, squid did not appear to show increased attention to areas of their body that have been injured.[23]

Avoidance learning

Avoidance learning in octopuses has been known since 1905.[60] Noxious stimuli, for example electric shocks, have been used as "negative reinforcers" for training octpuses, squid and cuttlefish in discrimination studies and other learning paradigms.[36][61] Repeated exposure to noxious stimuli can have long-term effects on behaviour. It has been shown that in octopuses, electric shocks can be used to develop a passive avoidance response leading to the cessation of attacking a red ball.[60]

As in vertebrates, longfin inshore squid show sensitization of avoidance responses to tactile and visual stimuli associated with a peripheral noxious stimulus. This persists for at least 48 hours after injury, indicating that behavioural responses to injury in cephalopods can be similar to those in vertebrates.[36]

Trade-offs in motivation

Octopuses show trade-offs in their motivation to avoid being stung by sea anemones. Octopuses frequently predate hermit crabs, however, they change their hunting strategy when the crabs place an anemone on their shell as protection. Octopuses attempt various different methods such as using only a single arm, moving below the anemone or blowing jets of water at it. The trade-off is that they attempt to avoid the anemone stings by using methods that are less effective than they would usually use for predating the hermit crab.[48]

Injured squid show trade-offs in motivation due to injury, for example, they use crypsis rather than escape behaviour when reacting to a visual threat. The same study showed that injured squid begin escape responses earlier and continue these for longer for up to 48 hours after injury.[62]

In 2014, the adaptive value of sensitisation due to injury was tested using the predatory interactions between longfin inshore squid and black sea bass (Centropristis striata) which are natural predators of this squid. If injured squid are targeted by a bass, they began their defensive behaviours sooner (indicated by greater alert distances and longer flight initiation distances) than uninjured squid. If anaesthetic (1% ethanol and MgCl2) is administered prior to the injury, this prevents the nociceptive hypersensitisation and blocks the effect. This study has wide implications because both long-term sensitisation and pain are often considered to be maladaptive rather than adaptive; the authors claim this study is the first evidence to support the argument that nociceptive sensitisation is actually an adaptive response to injuries.[40]

Cognitive ability and sentience

It has been argued that although a higher cognitive capacity in some animals may indicate a greater likelihood of them being able to perceive pain, it also gives these animals a greater ability to deal with this, leaving animals with a lower cognitive ability a greater problem in coping with pain.[63][64]

Cephalopods can demonstrably benefit from environmental enrichment[65] indicating behavioural and neuronal plasticity not exhibited by many other invertebrates.

Tool use

A veiled octopus travelling with shells it has collected for protection
A veiled octopus travelling with shells it has collected for protection

Octopuses are widely reported as examples of an invertebrate that exhibits flexibility in tool use. For example, veined octopuses (Amphioctopus marginatus) retrieve discarded coconut shells, manipulate them, transport them some distance, and then re-assemble them to use as a shelter.[66]

Learning

The learning abilities of cephalopods demonstrated in a range of studies indicate advanced cognitive abilities.

Octopuses are capable of reversal learning, a form of advanced learning demonstrated by vertebrates such as rats.[67] Giant Pacific octopuses are able to recognise individual humans[68] and common octopuses can recognise other octopus individuals for at least one day.[69]

In a study on social learning, common octopuses (observers) were allowed to watch other octopuses (demonstrators) select one of two objects that differed only in colour. Subsequently, the observers consistently selected the same object as did the demonstrators.[70]

Both octopuses and nautiluses are capable of vertebrate-like spatial learning.[36]

Pavlovian conditioning has been used to train chambered nautiluses (Nautilus pompilius) to expect being given food when a bright blue light flashed. The research revealed that nautiluses had memory capabilities akin to the "short-term" and "long-term memories" of the Coleoidea. This is despite very different brain structures. However, the long-term memory capability of nautiluses is much shorter than that of Coleoidea. Nautiluses appear to completely forget training they received 24 hours later, whereas octopuses remain conditioned for several weeks.[67][71][72][73]

Criteria for pain perception

Scientists have proposed that in conjunction with argument-by-analogy, criteria of physiology or behavioural responses can be used to assess the possibility that non-human animals can perceive pain.[23] In 2015, Lynne Sneddon, Director of Bioveterinary Science at the University of Liverpool, published a review of the evidence gathered investigating the suggestion that cephalopods can experience pain.[43] The review included the following summary table -

Criteria for pain perception in non-human animals[43]
Terrestrial

mammals

Fish

(teleosts)

Molluscs

(cephalopods)

Crustaceans

(decapods)

Nociceptors Green tickY Green tickY Green tickY Green tickY
Pathways to CNS Green tickY Green tickY Green tickY Green tickY
Central processing to CNS Green tickY Green tickY Green tickY Green tickY
Receptors for analgesic drugs Green tickY Green tickY Green tickY Green tickY
Physiological responses Green tickY Green tickY Green tickY Green tickY
Movement away from noxious stimuli Green tickY Green tickY Green tickY Green tickY
Abnormal behavioural changes Green tickY Green tickY Green tickY Green tickY
Protective behaviour Green tickY Green tickY Green tickY Green tickY
Responses reduced by analgesic drugs Green tickY Green tickY Green tickY Green tickY
Self-administration of analgesia Green tickY Green tickY not yet not yet
Responses with high priority over other stimuli Green tickY Green tickY Green tickY Green tickY
Paying a cost to access analgesia Green tickY Green tickY not yet not yet
Altered behavioural choices/preferences Green tickY Green tickY Green tickY Green tickY
Rubbing, limping or guarding Green tickY Green tickY Green tickY Green tickY
Paying a cost to avoid stimulus Green tickY Green tickY not yet Green tickY
Trade-offs with other requirements Green tickY Green tickY not yet Green tickY

In the table, Green tickY indicates positive evidence and not yet denotes it has not been tested or there is insufficient evidence.

Note: recent evidence[40][48][62] indicates that cephalopods exhibit "trade-offs with other requirements" which Sneddon might not have been aware of.

Societal implications

Sannakji is a dish of live baby octopuses eaten while still squirming on the plate.
Sannakji is a dish of live baby octopuses eaten while still squirming on the plate.

Some cephalopods are widely used food sources. In some countries, octopus is eaten live. Sannakji is a type of hoe, or raw dish, in Korea. It consists of live baby octopuses (nakji), either whole, or cut into small pieces and immediately served. The dish is eaten while the octopuses are still squirming on the plate.[74]

Cephalopods are caught by nets, pots, traps, trawling and hand jigging. Sometimes, the devices are left in situ for several days thereby preventing feeding and provoking the trapped animals to fight with each other, potentially causing suffering from discomfort and stress.

Other societal implications of cephalopods being able to perceive pain include acute and chronic exposure to pollutants, aquaculture, removal from water for routine husbandry, pain during slaughter and during scientific research.

Given the possibility that cephalopods can perceive pain, it has been suggested that precautionary principles should be applied during their interactions with humans and the consequences of our actions.[59]

Protective legislation

In addition to vertebrate animals including cyclostomes, cephalopods should also be included in the scope of this Directive, as there is scientific evidence of their ability to experience pain, suffering, distress and lasting harm. (emphasis added)

In most legislation to protect animals, only vertebrates are protected. However, cephalopods have a special position among invertebrates in terms of their perceived ability to experience pain, which is reflected in some national and international legislation.[75]

  • In the UK, the legislation protecting animals during scientific research, the "Animals (Scientific Procedures) Act 1986" (ASPA), protects cephalopods from the moment they become capable of independent feeding.[76] The legislation protecting animals in most other circumstances in the UK is "The Animal Welfare Act, 2006" which states that "...animal means a vertebrate other than man..." thereby excluding cephalopods.[77]
  • The Canadian Council on Animal Care (CCAC) differentiates between "invertebrates" which are classified on a tier of lowest concern with respect to invasive procedures and "cephalopods and other higher invertebrates".[78]
  • New Zealand's "Animal Welfare Act 1999", as of November 2015 [1] protects octopus and squid (but apparently not cuttlefish and nautiluses).
  • The EU's laboratory animal welfare EU Directive 2010/63/EU[79] implemented in 2013 protects all cephalopods, but stops short of protecting any other invertebrates, despite initially considering crustaceans.[78]

In the US, the legislation protecting animals during scientific research is the "Animal Welfare Act of 1966". This Act excludes protection of "cold-blooded" animals, thereby also excluding cephalopods.[80] Protection in Australia and the US is not national and instead is limited to institution specific guidelines.[81] The 1974 Norwegian Animal Rights Law states it relates to mammals, birds, frogs, salamander, reptiles, fish, and crustaceans, i.e. it does not include cephalopods.[82]

Controversy

There is controversy about whether cephalopods have the capability to experience pain. This mainly relates to differences between the nervous systems of different taxa. Reviews have been published arguing that fish cannot feel pain because they lack a neocortex in the brain.[83][84] If true, this would also rule out pain perception in most mammals, all birds, reptiles[85] and cephalopods. However, the Cambridge Declaration on Consciousness published in 2012, states that the absence of a neocortex does not appear to preclude an organism from experiencing affective states.[15]

In 1991, it was stated that "Although the evidence for pain perception is equivocal..." "...the evidence certainly does not preclude the possibility of pain in these animals [cephalopods] and, moreover, suggests that pain is more likely in cephalopods than in the other invertebrates with less ‘complex’ nervous organizations...".[86]

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Further reading

  • Zarrella I.; Ponte G.; Baldascino E.; Fiorito G. (2015). "Learning and memory in Octopus vulgaris: a case of biological plasticity". Current Opinion in Neurobiology. 35: 74–79. doi:10.1016/j.conb.2015.06.012. PMID 26186237.
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