A Question of Pain in Invertebrates
Jane A. Smith, Ph.D.

Jane A. Smith, Ph.D., is a lecturer in the Department of Biomedical Science and Biomedical Ethics at the University of Birmingham Medical School in Birmingham, England.

INTRODUCTION

Quite apart from philosophical considerations, practical and scientific evidence may lead us to assume that all mammals can experience something analogous to (though most likely qualitatively and quantitatively different from) the human experience of pain. Humans, after' all, are mammals; and although the details may differ, we share our basic physiology with other mammalian species. There is also a reasonableness, it seems, in extending this view to include other members of the Vertebrata. The further we move away from the mammalian plan, the more difficult it becomes to infer pain in other species. But vertebrates, at least, have similarities in basic anatomy and physiology, including similarities in nervous organization, which are especially important in this context.

What, however, of the 95 percent of species in the Animal Kingdom that do not possess a backbone--the heterogeneous assemblage of animals, organized very differently from the vertebrate plan, which we call "invertebrates"?

Invertebrates are used in many disciplines in biomedical research and also in toxicity testing. Sometimes, invertebrate species are regarded as "replacement" alternatives for vertebrates (Office of Technology Assessment, 1986), presumably because they are thought to be insentient, or at least less sentient than vertebrate animals.

Is this contention true? Can any of the invertebrates experience pain in anything like the human, or the more general vertebrate, sense? Can these animals, in this sense, "suffer"?

This short paper approaches such questions i:rom a practical, rather than a philosophical, point of view. Some biological evidence relating to the possibility of pain in invertebrates is reviewed, and some practical implications are raised.

There are unlikely to be any easy answers to the difficult question of assessing pain in invertebrates. It is hoped, however, that the following discussion will provide some food for thought and help to stimulate further debate.

EVIDENCE OF NOCICEPTION IN INVERTEBRATES

Most, if not all, invertebrates have the capacity to detect and respond to noxious or aversive stimuli. That is, like vertebrates, they are capable of "nociception." Examples of aversive stimuli include changes in temperature beyond the animal's normal range, contact with noxious chemicals, mechanical interference, or electric shock. Under certain conditions, all of these might be expected to cause pain in humans. In general, invertebrates, like vertebrates, respond to such stimuli by withdrawing or escaping so as to reduce the likelihood that they will be damaged by the noxious conditions.

Behavioral Responses of Invertebrates to Noxious Stimuli

Even the "simplest" invertebrates, the single-celled Protozoa, exhibit nociceptive-type responses. The ciliated protozoon Paramecium, for example, changes the rate and form of its ciliary beat in response to aversive stimulation (such as a poke with a fine needle) so as to effect typical avoidance and escape reactions. The animals, of course, have no nervous systems to coordinate such responses; rather, these changes in behavior are triggered by changes in the electrical activity of the cell surface membrane (Naitoh, 1974).

As a detailed review by Kavaliers (1988) describes, all other invertebrates (with the possible exception of the Porifera, or sponges), can also exhibit coordinated responses to aversive stimuli. Thus, to take a few examples: * sea anemones show protective withdrawal responses by retracting their tentacles and oral disc. Some may even detach from the substrate in response to a variety of i aversive mechanical, electrical, or chemical stimuli (Pantin, 1935; Ross, 1968); * earthworms show rapid withdrawal reflexes mediated by giant nerve fibers when subjected to unfavorable stimuli; * medicinal leeches show pronounced writhing and coiling responses when their skin is pinched or damaged (Nicholls and Baylor, 1968); * insects have a variety of avoidance and escape responses (Eisemann et al., 1984), and appear also to exhibit physiological changes to aversive stimuli (Angioy et al., 1987). They may be more responsive to some stimuli than to others. Thus, most insects "do not flinch or run," when the cuticle is cut, but high temperature (such as a heated needle brought close to the antennae) can produce violent escape responses (Wigglesworth, 1980); * gastropod snails of the species Cepaea nemoralis show foot-lifting responses when placed on a surface wanned to temperatures approaching 40º C, which is above their normal range (Kavaliers and Hirst, 1983); and * cephalopod mollusks, such as octopuses, may respond to noxious stimuli by withdrawing, sometimes producing a cloud of ink from the ink sac, and usually changing color.

Invertebrate Nociceptors

Some invertebrates, like vertebrates, also have special sensory receptors called nociceptors, which respond specifically to noxious stimulation. Such nociceptive nerve cells have been found in the segmental ganglia of the medicinal leech, Hirudo medicinalis (Nicholls and Baylot, 1968). Nerve impulses are generated in these cells, which Nicholls and Baylor called N cells, specifically in response to noxious mechanical stimulation, such as pinching, squeezing, or cutting of the body wall.

Modulation of Nociceptive Responses in Invertebrates

In mammals and other vertebrates, opioid substances (including enkephalins and endorphins) manufactured in the body can modify nervous transmission in nociception, producing analgesic effects. Administration of substances that mimic the effects of these endogenous opioids (i.e., administration of opiate agonists, such as morphine), also produces analgesia and thus may reduce or abolish behavioral responses to noxious stimuli. Furthermore, opiate antagonists such as naloxone may suppress these analgesic effects. Recent investigations have shown that similar opiate systems may have a functional role in invertebrate nociception (Fiorito, 1986; Kavaliers, 1988).

Enkephalin and b-endorphin-like substances have been found in earthworms (Alumets et al., 1979), and injections of nalaxone have been shown to inhibit the worms' touch-induced escape responses (Gesser and Larsson, 1986), suggesting that the opioid substances may play a role in sensory modulation. Opiate binding sites, with properties similar to those of mammalian opiate receptors, have been shown to be present in the neural tissue of the marine mollusk Mytilus edulis (Kavaliers et al., 1985). Kavaliers et al. (1983, 1985) have shown that administration of low doses of the opioid peptides methionine-enkephalin and b-endorphin produces "analgesic" effects in terrestrial snails of the species Cepaea nemoralis and that morphine has a similar effect. All three substances increase the time taken for the snails to respond by foot-lifting when placed on a 40º C hot plate. Furthermore, naloxone has been found to abolish the effect of morphine, and all of the effects were dose-dependent.

Enkephalin-like substances and their receptors have also been found in insects (Stefano and Scharrer, 1981; EI-Salhy et al., 1983), and opiate agonists and antagonists have been shown to modulate nociceptive-type responses in several species of arthropod, including mantis shrimps (Squilla mantis) (Maldonado and Miralto, 1982), honeybees (Nfinez et al., 1983), and praying mantes (Zabala et al., 1984.

EVIDENCE OF PAIN IN INVERTEBRATES

Invertebrates, it seems, exhibit nociceptive responses analogous to those shown by vertebrates. They can detect and respond to noxious stimuli, and in some cases, these responses can be modified by opioid substances. However, in humans, at least, there is a distinction to be made between the "registering" of a noxious stimulus and the "experience" of pain. In humans, pain "may be seen as the response of the whole awake conscious organism to noxious stimuli, seated.., at the highest levels in the central nervous system, involving emotional and other psychological components" (Iggo, 1984). Experiments on decorticate mammals have shown that complex, though stereotyped, motor responses to noxious stimuli may occur in the absence of consciousness and, therefore, of pain (Iggo, 1984). Thus, it is possible that invertebrates' responses to noxious stimuli (and modifications of these responses) could be simple reflexes, occurring without the animals being aware of experiencing something unpleasant, that is, without "suffering" something akin to what humans call pain.

Leaving aside the conceptual questions which arise in trying to understand what other individuals (of our own or other species) might experience, it might be asked whether there is any practical, physiological, or behavioral evidence that could lead us to infer that particular invertebrates might or might not experience pain. What evidence might help in distinguishing between nociceptive "responsiveness" and the perception of pain?

At the outset, it should be pointed out that because pain is a subjective experience, it is highly unlikely that any clear-cut, definitive criteria will ever be found to decide this question. However, certain evidence might lead to judgements that pain is more or less likely to occur in one particular kind of invertebrate than in another kind. In particular, it can be argued that evidence might come from further examination of an animal's behavioral responses to noxious stimulation and from consideration of the complexity of its nervous organization (Smith and Boyd, 1991).

In mammals, responses to painful stimuli often persist beyond a simple reflex withdrawal, so that, for example, the animals may become immobile, limp or "guard" the affected part, show aggression when approached, reduce or stop feeding and drinking, and show decreased sexual activity (Morton and Griffiths, 1985). The animals may also learn in the future to avoid situations similar to the one in which the pain occurred. Such responses, while not proof that the animals have experienced pain, can indicate that something more than a simple nociceptive reflex is involved. Together, they may help the animal to recover from damage caused by the painful event and avoid being harmed in the future.

Insects and "Less Complex" Invertebrates

In the majority of examples of invertebrate nociception noted above, there seems to be little, if any, evidence that the animals' responses persist in anything akin to the manner described for mammals. As Eisemann et al. (1984) have described in a review of the "biological evidence" concerning pain in insects, "No example is known to us of an insect showing protective behavior towards injured parts, such as by limping after leg injury or declining to feed or mate because of general abdominal injuries. On the contrary, our experience has been that insects will continue with normal activities even after severe injury or removal of body parts."

Eisemann et al. (1984) use a variety of examples to support this contention, including: * an insect walking with a crushed tarsus continues "applying it to the substrate with undiminished force"; * a locust carries on feeding while being eaten by a mantis; * a tsetse fly, although half-dissected, flies in to feed. Although some insect behavior, such as the writhing of insects poisoned by insecticides, or the struggling of restrained living insects, resembles that of "higher animals responding to painful stimuli," Eisemann et al. conclude that the resemblance is superficial and that it "no more requires the presence of a pain sense than do reflexive withdrawal responses." Similarly, although it has been shown that fruit flies can be trained to avoid certain odors and colored lights when these are associated with impending electric shock (Quinn et al., 1974), such learning is open to explanation in terms of neural mechanisms, without the need to postulate subjective experience on the part of flies.

The "relatively simple organization" of the insect central nervous system, Elsemann et al. argue, "raises the question of whether any experience akin to human pain could be generated" in these animals (and by implication in other invertebrates with a similar or less complex nervous organization). On the analysis of Gould and Gould (1982), the answer to such a question would be "no," for these authors can find no evidence for conscious experience in insects. Certainly, on the limited amount of evidence presented here, it seems very difficult to imagine that insects and the other simpler invertebrates mentioned above can "suffer" pain in anything like the vertebrate sense. Nevertheless, the issue certainly is not closed, and further questions should be asked.

Perhaps such a view simply reflects a paucity of (human) imagination. Griffin (1984) surely would urge us to maintain an open mind on the issue, having provided behavioral evidence which, he argues, should challenge "the widespread belief' that an insect, for example, "is too small and its central nervous system too differently organized from ours to be capable of conscious thinking and planning or subjective feelings." Indeed, to take a more radical view, perhaps "it is presumptuous for us to assume that because our suffering involves self-awareness, this should also be true of other species" (McFarland, 1989).

Alternatively, perhaps, as Mather (1989) suggests, we should simply accept that these animals "are different from us, and wait for more data."

Cephalopods

Perhaps the question of pain in invertebrates could be more easily settled where the "most highly organized of all invertebrates" (Russell-Hunter, 1979), the cephalopods, are concerned.

These animals, which include cuttlefish, squid, and octopuses, "have the largest brains of all invertebrates" (Wells, 1962). The ratio of brain-weight to body-weight of many cephalopods also exceeds that of most fish and reptiles (Packard, 1972). The cephalopod brain has a "hierarchical" organization (see Boycott, 1961), the "higher" centers of the brain being concerned with sensory analysis, memory, learning, and decision-making. It has been suggested that these areas of the cephalopod brain might be regarded as analogous to the cerebral cortex of higher vertebrates (Russell-Hunter, 1979). According to Wells (1978), since in Octopus the brain "represents only the more specialized sensory integrative, higher movement control and learning parts of a rather diffuse nervous system...it becomes clear that one is dealing with an animal that might well be expected to possess a central nervous capability approaching that or exceeding that of many birds and mammals."

Cephalopods show a remarkable ability to learn and to be trained. In learning experiments, Octopus vulgaris "clearly generalizes on the basis of its own past experience,'' and the animals may show marked individual preferences (Wells, 1978). Anecdotal reports suggest considerable individuality of behavior. Dews (1959) describes how one of the octopuses he was using in training experiments "spent much time with eyes above the surface of the water, directing a jet of water at any individual who approached the tank."

The evidence seems to suggest that at least some of the cephalopods might have a nervous organization that would allow them to experience something like pain. It is unclear, however, whether cephalopods are able to "suffer" pain.

Certainly, noxious stimuli such as electric shocks are effective "negative reinforcers" when used for training cephalopods in discrimination learning experiments. It seems also that repeated noxious stimulation can have long-term effects on behavior. Wells (1978) describes the adverse effects of repeated electric shocks given to blinded octopuses. He describes how blinded octopuses will normally be found sitting in their tanks with their arms outstretched, allowing for a "very standardized presentation'' of test objects in tactile discrimination experiments. However cases in which "the animal has become withdrawn as a result of making large numbers of errors and receiving many shocks in the course of training, it is not so easy. Considerable practice may then be required to recognize which arm is in which tangle, and some delicacy may be needed in presenting test objects since the suckers are very sensitive and the animal may shy away from contacts."

Both Young's (1965) and Wells' (1978) models of learning in the octopus include a "pain" pathway leading into the "highest" center (the vertical lobe) of the brain. Wells, however, notes that although this nervous path is "generally assumed to signal 'pain'...there is no proof of this and it might well carry 'pleasure' or any other 'signal' ".

Although the evidence for pain perception is equivocal, it seems that cephalopods might exhibit body postures, color patterns and behaviors that the human observer can interpret as signaling, at least, whether or not "something is wrong" with those animals. An anecdotal example is quoted by Lane (1974), from Joseph Sinel (1906), who handled hundreds of common octopuses. "When highly content, as after a meal, and perched, as it is fond of perching at times, upon an eminence, the papillae [pimple-like projections on the skin] are erected and these are always of an orange color. Oftentimes the whole body will be marked off in irregular, honeycomb-like patches, or more like crocodile-skin. First some of the patches are purple, others orange, then these colors are reversed. When danger threatens, or even when the hand is raised towards it as if to strike, the animal winces, and turns to ashy grey."

Mather (1989) describes how "...when I recently had two octopuses in one aquarium and one of the animals changed from its known daytime activity in a hidden location and took on a coloration previously associated with aversire stimuli, I surmised before I saw the other animal attack it that there was a problem."

Wells (1978), however, is more guarded about human abilities to empathize with these animals. He agrees that it is very easy for humans "to identify with Octopus vulgaris, even with individuals, because they respond in a very 'human' way." Nevertheless, he argues, the octopus has a very different way of life, and "...we should not fall into the trap of supposing that we can interpret its behavior in terms of concepts derived from birds or mammals [since] the animal lives in a very different world from our own."

It is important to bear in mind this caveat when considering evidence concerning pain in cephalopods. Nevertheless, the evidence certainly does not preclude the possibility of pain in these animals and, moreover, suggests that pain is more likely in cephalopods than in the other invertebrates with less "complex" nervous organizations, considered in this review.

Comparisons between cephalopods and fish could lend weight to this conclusion. Packard (1972) has provided a comprehensive review of the similarities between cephalopods and fish. He concludes that "it might reasonably be argued that the similarities between fish and cephalopods are greater than between any other two major groups belonging to different phyla. The remarkable fact that cephalopods are like fish in almost every other feature except in their basic anatomical plan--that is, in its simplest expression, cephalopods functionally are fish--seems to have passed largely unnoticed." Thus, if it can be agreed that fish have the ability to perceive pain, this would suggest that the possibility that cephalopods also feel pain should be taken all the more seriously.

CONCLUSION: SOME PRACTICAL "WAYS FORWARD"

Clearly, in all this, there is the danger of adopting an uncritical anthropomorphic (or, in this context, perhaps a "vertebromorphic") approach, which could lead to incorrect conclusions about the experiences of invertebrates (see Morton et al., 1990). Thus, it might be inferred, incorrectly, that certain invertebrates experience pain simply because they bear a (superficial) resemblance to vertebrates-the animals with which humans can identify with most clearly. Equally, pain might incorrectly be denied in certain invertebrates simply because they are so different from us and because we cannot imagine pain experienced in anything other than the vertebrate or, specifically, human sense.

This limitation should be borne in mind when considering the practical implications of the tentative conclusions drawn from the evidence presented above. Although pain might seem less likely in the more "simple" invertebrates, than in the most "complex" invertebrates, such as the cephalopod mollusks (and, perhaps, decapod crustaceans such as crabs and lobsters, not considered here), this certainly does not mean that the more "simple" invertebrates ought not to be afforded respect.

A principle of respect should lead those who use invertebrates in research (or display them in zoos, rear them for food, and so on) to try to maintain the highest possible standards of husbandry and care, so as to promote the animals' general "well-being" and, whenever practicable, to give the animals the benefit of the doubt where questions of pain and suffering are concerned.

The well-being of invertebrates used for research is being taken increasingly seriously. Wigglesworth (1980), for example, has suggested that for practical purposes it should be assumed that insects feel pain and that they should, therefore, be narcotized in procedures that have the potential to cause pain. Cooper (1990) has identified several practical ways in which the well-being of invertebrates might be promoted. These include: * providing husbandry conditions that match, as closely as possible, those preferred by the species in the wild; * assuring high standards of care, provided by staff with an interest in invertebrates; * avoiding unnecessary or insensitive handling or restraint; * narcotizing the animals for any invasive or disruptive procedures and during prolonged restraint (some methods of anesthesia are described by Cooper, 1990) and; * where possible, avoiding the use of the more "complex'' species.

To this list might be added: * attempting to kill invertebrates by the most humane methods possible and; * providing suitable guidance and training for all involved in the care and use of these animals.

Recently, the Canadian Council on Animal Care (CCAC) established a Committee on Invertebrates (CCAC, 1988a), and in the U.K., the Universities Federation for Animal Welfare (UFAW) has published a handbook on the care of cephalopods in the laboratory (Boyle, 1991). Guidance on caring for invertebrates might also come from groups that keep these animals for purposes other than research. In 1987, the National Federation of Zoological Gardens of Great Britain and Ireland convened a Working Group on Invertebrates. Among other activities, this group is producing codes of practice for those who keep invertebrates. The second of these codes gives guidance on methods of humane killing (National Federation of Zoos, 1990).

Consideration is also being given to including invertebrates, especially the cephalopods, under some systems of control of animal experiments. Most such statutory or non-statutory systems cover only vertebrate species. However, the CCAC's list of "Categories of Invasiveness in Animal Experiments" recognizes that "cephalopods and some other higher invertebrates have nervous systems as well developed as some vertebrates" and so might be included in categories in which pain and distress (including "severe pain") is caused. Protocols involying these "higher" invertebrates must be evaluated and approved by an Animal Care Committee before the work can commence (CCAC, 1988b). It has been suggested in the U.K. also that there might be a case for including cephalopods under the terms of the Animals (Scientific Procedures) Act 1986 (Report of a Working Party of the Institute of Medical Ethics) (Smith and Boyd, 1991). Currently, the Act protects only vertebrate animals, but its terms provide for protection to be extended to cover "invertebrates of any description" if, in the future, this is thought appropriate. Whether the inclusion of invertebrates species under laboratory animal protection laws is indeed the way forward is open to further discussion. Such a debate, nevertheless, helps to promote careful consideration of the use of all animals in research, not simply the use of animals which possess backbones.

The question of pain in invertebrates will be extremely difficult to resolve--if, indeed, it is resolvable. In the meantime, perhaps it can be agreed that it is most appropriate to concentrate efforts on maintaining and improving the general well-being of invertebrates used in research, that is, to ensure that these animals are kept in the best and most appropriate conditions during their lives in the laboratory; given the benefit of the doubt in procedures which have the potential to cause pain and distress; and, when the time comes, killed in the most humane manner possible.

ACKNOWLEDGEMENT

I am very grateful to the Institute of Medical Ethics and the Leverhulme Trust for their support in the preparation of this paper, which was written while working for the Institute of Medical Ethics' Working Party on the Ethics of Using Animals in Biomedical Research.

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