There is a scientific debate which questions whether crustaceans experience pain. It 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 an animal, 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.
Definitions of pain vary, but most involve the ability of the nervous system to detect and reflexively react to harmful stimuli by avoiding it, and the ability to subjectively experience suffering. Suffering cannot be directly measured in other animals. 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 sometimes used.
Crustaceans fulfill several criteria proposed as indicating that non-human animals may experience pain. These fulfilled criteria include 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.
In vertebrates, endogenous opioids are neurochemicals that moderate pain by interacting with opioid receptors. Opioid peptides and opioid receptors occur naturally in crustaceans, and although it was concluded in 2005 "at present no certain conclusion can be drawn", more recent considerations suggest their presence along with related physiological and behavioural responses as indicating that crustaceans may experience pain. Opioids may moderate pain in crustaceans in a similar way to that in vertebrates. If crustaceans feel pain, there are ethical and animal welfare implications including the consequences of exposure to pollutants, and practices involving commercial and recreational fishing, aquaculture, food preparation and for crustaceans used in scientific research.
Background
The question of whether non-human animals experience pain has a long history. Initially, this was rooted in theoretical and philosophical argument, but more recently, it has also become a topic of 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. Argument by analogy is sometimes used to assess the capacity of other animals to experience pain. This is based on the principle that if a non-human animal's responses to noxious stimuli are similar to those of humans, they are likely to have had an analogous experience. For example, if a pin is stuck in a chimpanzee's finger and it rapidly withdraws its hand, then argument by analogy indicates that like humans, it felt pain.
In 2012 the American philosopher Gary Varner reviewed the research literature on pain in animals. His findings are summarised in the following table. Table 5.2, page 113. 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, including fish, probably experience pain, but invertebrates (e.g. crustaceans) apart from cephalopods probably do not experience pain.
Hexapods (insects and entognathans) are cladistically crustaceans, with the two groups now combined in the monophyletic clade Pancrustacea.
Vertebrates 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...". Birds with gait abnormalities self-select for a diet that contains carprofen, a human analgesic. 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." 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 Property Invertebrates Vertebrates Earthworms Insects Leeches/snails Cephalopods 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 ?
In the table - denotes Varner believes reliable research indicates the taxon has the attribute denotes Varner believes reliable research indicates the taxon does not have the attribute ? denotes Varner believes reliable research has not ascertained whether the taxon has the attribute or varies between species within the taxa. ? / denotes that later studies suggest the taxon has the attribute
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.
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.
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. Nociceptive reflexes act to immediately remove the animal or part of the body from a (potentially) damaging stimulus. However, without learning from this experience, the animal would likely expose itself to the damaging stimulus repeatedly. Pain has the adaptive advantage that it invokes a level of learning, thereby preventing the animal from repeatedly exposing itself to potential injury. 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.
Nociception
In vertebrates, nociceptive responses involve the transmission of a signal along a chain of nerve fibres from the site of a noxious stimulus at the periphery, to the spinal cord. This process evokes a reflex arc response such as flinching or immediate withdrawal of a limb, generated at the spinal cord and not involving the brain. Nociception is found, in one form or another, across all major animal taxa. Nociception can be observed using modern imaging techniques and both physiological and behavioural responses to nociception can be detected. Many crustacean species, including the rockpool prawn (Palaemon elegans), exhibit the caridoid escape reaction – an immediate, nociceptive, reflex tail-flick response to noxious stimuli (see here).
Emotional pain