Whether invertebrates can feel pain is a contentious issue. 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 evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not necessarily imply any adverse, subjective feeling; it is a reflex action. The second component is the experience of "pain" itself, or sufferingi.e., the internal, emotional interpretation of the nociceptive experience. 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 a non-human animal's responses to stimuli are similar to those of humans, it is likely to have had an analogous experience. It has been argued that if a pin is stuck in a chimpanzee's finger and they rapidly withdraw their hand, then argument-by-analogy implies that like humans, they felt pain. It has been questioned why the inference does not then follow that a cockroach experiences pain when it writhes after being stuck with a pin.Sherwin, C.M., (2001). Can invertebrates suffer? Or, how robust is argument-by-analogy? Animal Welfare, 10 (supplement): S103-S118 This argument-by-analogy approach to the concept of pain in invertebrates has been followed by others.
The ability to experience nociception has been subject to natural selection and offers the advantage of reducing further harm to the organism. While it might be expected therefore that nociception is widespread and robust, nociception varies across species. For example, the chemical capsaicin is commonly used as a noxious stimulus in experiments with mammals. However, Heterocephalus glaber, the African naked mole-rat, is an unusual rodent species as it shows a remarkable lack of pain-related behaviours to capsaicin. Birds, for instance, are unaffected by capsaicin because their capsaicin receptor proteins do not respond to it, but what makes the African naked mole rat unique is that, while it is indeed a mammal, and capsaicin receptor proteins in mammals are indeed very sensitive to capsaicin, the African naked mole rat lacks pain-related neuropeptides such as substance P in its cutaneous sensory fibres. Similarly, capsaicin does trigger nociceptors in some invertebrates, but this substance is not noxious to Drosophila melanogaster (the common fruit fly). Criteria that may indicate a potential for experiencing pain include: Has a suitable nervous system and receptors Physiological changes to noxious stimuli Displays protective motor reactions that might include reduced use of an affected area such as limping, rubbing, holding or autotomy Has opioid receptors and shows reduced responses to noxious stimuli when given analgesics and local anaesthetics Shows trade-offs between stimulus avoidance and other motivational requirements Shows avoidance learning Exhibits high cognitive ability
Suitable nervous system
Central nervous system Brain size does not necessarily equate to complexity of function. Moreover, weight for body-weight, the cephalopod brain is in the same size bracket as the vertebrate brain, smaller than that of birds and mammals, but as big or bigger than most fish brains.
Invertebrate nervous systems are very unlike those of vertebrates and this dissimilarity has sometimes been used to reject the possibility of a pain experience in invertebrates. In humans, the neocortex of the brain has a central role in pain and it has been argued that any species lacking this structure will therefore be incapable of feeling pain. However, it is possible that different structures may be involved in the pain experience of other animals in the way that, for example, crustacean decapods have vision despite lacking a human visual cortex.
Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others) and modern cephalopods (octopuses, squid, cuttlefish) and other molluscs. The brains of arthropods and cephalopods arise from twin parallel nerve cords that extend through the body of the animal. Arthropods have a central brain with three divisions and large optic lobes behind each eye for visual processing. The brains of the modern cephalopods in particular are highly developed, comparable in complexity to the brains of some vertebrates . Emerging results suggest that a convergent evolutionary process has led to the selection of vertebrate-like neural organization and activity-dependent long-term synaptic plasticity in these invertebrates. Cephalopods stand out by having a central nervous system that shares prime electrophysiological and neuroanatomical features with vertebrates like no other invertebrate taxon.
Nociceptors
Nociceptors are sensory receptors that respond to potentially damaging stimuli by sending nerve signals to the brain. Although these neurons in invertebrates may have different pathways and relationships to the central nervous system than mammalian nociceptors, nociceptive neurons in invertebrates often fire in response to similar stimuli as mammals, such as high temperature (40°C or more), low pH, capsaicin, and tissue damage. The first invertebrate in which a nociceptive cell was identified was the medicinal leech, Hirudo medicinalis, which has the characteristic segmented body of an Annelida, each segment possessing a ganglion containing the T (touch), P (pressure) and N (noxious) cells. Later studies on the responses of leech neurones to mechanical, chemical and thermal stimulation motivated researchers to write "These properties are typical of mammalian polymodal nociceptors".
There have been numerous studies of learning and memory using nociceptors in the sea hare, Aplysia. Many of these have focused on mechanosensory neurons innervating the siphon and having their somata (bulbous end) in the abdominal ganglion (LE cells). These LE cells display increasing discharge to increasing pressures, with maximal activation by crushing or tearing stimuli that cause tissue injury. Therefore, they satisfy accepted definitions of nociceptors. They also show similarities to vertebrate Aδ nociceptors, including a property apparently unique (among primary afferents) to nociceptors—sensitization by noxious stimulation. Either pinching or pinning the siphon decreased the threshold of the LE cells firing and enhanced soma excitability.
Nociceptors have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods.
Physiological changes In vertebrates, potentially painful stimuli typically produce vegetative modifications such as tachycardia, pupil dilation, defecation, arteriole blood gases, fluid and electrolyte imbalance, and changes in blood flow, respiratory patterns, and endocrine excretions.
At the cellular level, injury or wounding of invertebrates leads to the directed migration and accumulation of haematocytes (defence cells) and neuronal plasticity, much the same as the responses of human patients undergoing surgery or after injury. In one study, heart rate in the crayfish, Procambarus clarkii, decreased following claw autotomy during an aggressive encounter.
Recording physiological changes in invertebrates in response to noxious stimuli will enhance the findings of behavioural observations and such studies should be encouraged. However, careful control is required because physiological changes can occur due to noxious, but non-pain related events, e.g. cardiac and respiratory activity in crustaceans is highly sensitive and responds to changes in water level, various chemicals and activity during aggressive encounters.
Protective motor reactions Invertebrates show a wide range of protective reactions to putatively painful stimuli. However, even unicellular animals will show protective responses to, for example, extremes of temperature. Many invertebrate protective reactions appear stereotyped and reflexive in action, perhaps indicating a nociceptive response rather than one of pain, but other responses are more plastic, especially when competing with other motivational systems (see section below), indicating a pain response analogous to that of vertebrates.
Mechanical stimulation
Rather than a simple withdrawal reflex, the flatworm, Notoplana aticola, displays a locomotory escape behaviour following pin pricks to the posterior end. Touching the larvae of fruit flies, Drosophila melanogaster, with a probe causes them to pause and move away from the stimulus; however, stronger mechanical stimulation evokes a more complex corkscrew-like rolling behaviour, i.e. the response is plastic. When a weak tactile stimulus is applied to the siphon of the sea-hare Aplysia californica, the animal rapidly withdraws the siphon between the parapodia. It is sometimes claimed this response is an involuntary reflex ; however, the complex learning associated with this response suggests this view might be overly simplistic.
In 2001, Walters and colleagues published a report that described the escape responses of the tobacco hornworm caterpillar (Manduca sexta) to mechanical stimulation. These responses, particularly their plasticity, were remarkably similar to vertebrate escape responses.
Autotomy
Over 200 species of invertebrates are capable of using autotomy (self amputation) as an avoidance or protective behaviour including: land slugs (Prophysaon)McDonnel, R.J., Paine, T.D. and Gormally, M.J., (2009). Slugs: A Guide to the Invasive and Native Fauna of California . 21 pp., , page 9. sea snails (Oxynoe panamensis) crickets spiders crabs lobsters octopuses
These animals can voluntarily shed appendages when necessary for survival. Autotomy can occur in response to chemical, thermal and electrical stimulation, but is perhaps most frequently a response to mechanical stimulation during capture by a predator. Autotomy serves either to improve the chances of escape or to reduce further damage occurring to the remainder of the animal such as the spread of a chemical toxin after being stung, but the 'decision' to shed a limb or part of a body and the considerable costs incurred by this suggests a pain response rather than simply a nociceptive reflex.
Thermal stimulation A heated probe (») evokes a complex, corkscrew-like rolling avoidance behaviour in Drosophila larvae which occurs in as little as 0.4seconds; a non-heated probe does not cause this avoidance behaviour. In contrast, cold stimuli (≤14°C or 57.2°F) primarily elicit a bilateral full-body contraction along the head-to-tail axis; larvae might also respond by lifting their head and/or tail, but these responses occur less frequently with decreasing temperatures. Land snails show an avoidance response to being placed on a hotplate (») by lifting the anterior portion of the extended foot. A 2015 study found that crayfish (Procambarus clarkii) respond adversely to high temperatures, but not to low temperatures.
Chemical stimulation