Decapod crustaceans are widely used as experimental models, due to their biology, their sensitivity to pollutants and/or their convenience of collection and use. Decapods have been viewed as being non-sentient, and are not covered by current legislation from the European Parliament. However, recent studies suggest it is likely that they experience pain and may have the capacity to suffer. Accordingly, there is ethical concern regarding their continued use in research in the absence of protective measures. We argue that their welfare should be taken into account and included in ethical review processes that include the assessment of welfare and the minimization or alleviation of potential pain. We review the current use of these animals in research and the recent experiments that suggest sentience in this group. We also review recent changes in the views of scientists, veterinary scientists and animal charity groups, and their conclusion that these animals are likely to be sentient, and that changes in legislation are needed to protect them. A precautionary approach should be adopted to safeguard these animals from possible pain and suffering. Finally, we recommend that decapods be included in the European legislation concerning the welfare of animals used in experimentation. Our current understanding of the nervous systems and behavior of decapods suggests an urgent need to amend and update all relevant legislation. This paper examines recent experiments that suggest sentience and how that work has changed current opinion. It reflects on the use of decapods as models in biomedical research and in ecotoxicology, and it recommends that these animals should be included in the European protection legislation. Abstract Decapod crustaceans are widely used as experimental models, due to their biology, their sensitivity to pollutants and/or their convenience of collection and use. This requires an evaluation of sentience so that an animal with “higher” sentience is replaced by one that is “lower” (often unprotected by legislation, i.e., embryos of vertebrates before of the last third of their normal development or invertebrates except live cephalopods). To this end, the EU Directive 63/2010 [ 8 ] states that animals “ with the lowest capacity to experience pain, suffering, distress or lasting harm” , should be used. Invertebrates vary enormously in neural and behavioral complexity [ 9 , 10 ]. Some have diffusely organized nervous systems with relatively small numbers of neurons (such as nematodes), and for these there is little concern for their welfare. The taxa that give rise to most welfare concerns are the arthropods (e.g., crustaceans, insects) and some molluscs (e.g., cephalopods). These invertebrates have large, complex nervous systems containing a very high number of neurons, and varied behavior, which may be modified by early social conditions [ 11 ]. The cephalopods receive protection [ 8 ], but this is not extended to other invertebrate groups. Recently, decapod crustaceans have become important models for biochemical, physiological, and ecological research [ 12 , 13 , 14 , 15 , 16 ], due to their biological characteristics and/or their convenience of collection/use. The focus here is on aspects that can be measured within appropriate experimental studies. Non-reflexive responses are important because, for many years, the idea of pain in decapods was dismissed given that it was thought that all responses to noxious stimuli were reflexive, and there was no need to invoke ideas of pain. The rationale is that virtually all animals have neurons (nociceptors) that respond to tissue damage, mechanical stimuli, and noxious chemicals. In many cases these allow for a withdrawal response via a reflex loop, and without signals going to the centralized neuronal masses or brain. Thus, there is no feeling or noxious experience. For example, the UK Government, together with several organizations, are producing a detailed animal welfare manifesto for protecting and improving crustacean welfare following Brexit [ 101 , 103 , 104 ]. The British Veterinary Association policy document on the slaughter of animals for food [ 105 ] states “ Evidence indicates that decapods (e.g., lobsters, crabs) and cephalopods (e.g., octopus, squid) are sentient, and experience pain and distress. We therefore support the principle that commercially caught decapods and cephalopods should be stunned before slaughter ”. Further, the Royal Society for the Protection from Cruelty to Animals (UK) (RSPCA) [ 106 ] states “There is currently debate about whether species like decapod crustaceans (crabs, lobsters etc.) and cephalopods (octopus, squid etc.) are sentient. The RSPCA and many others believe that there is sufficient scientific evidence to indicate that these animals should be considered as sentient, and therefore protected appropriately by legislation. This would help ensure they are no longer subjected to some of the current practices, like boiling crabs and lobster alive, that cause serious pain and distress”. Criteria Species (1) Nociceptors M, B, A, F, C, D, I (2) Pathways to central nervous system M, B, A, F, C, D, I (3) Central processing in brain M, B, A, F, C, D, I (4) Receptors for analgesic drugs M, B, A, F, C, D (5) Physiological responses M, B, A, F, C, D (6) Movement away from noxious stimuli M, B, A, F, C, D, I (7) Behavioral changes from norm M, B, A, F, C, D, I (8) Protective behavior M, B, A, F, C, D (9) Responses reduced by analgesic drugs M, B, A, F, C, D, I (10) Self-administration of analgesia M, B, F (11) Responses with high priority over other stimuli M, F, C, D (12) Pay cost to access analgesia M, B, I (13) Altered behavioral choices/preferences M, B, A, F, C, D, I (14) Relief learning M, B, I (15) Rubbing, limping or guarding M, B, F, C, D (16) Paying cost to avoid stimuli M, B, F, D (17) Trade-offs with other requirements M, B, F, D M = Mammals, B = Birds, A = Amphibians/reptiles, F =

necessarily imply any adverse, subjective feeling; it is a reflex action. The second component is the experience of "pain" itself, or suffering‍— i.e., the internal 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 neces 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. 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 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 (see: Invertebrate brain). 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 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". land slugs (Prophysaon) 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. Slugs and snails have an opioid receptor system. In experiments on different terrestrial snails, morphine prolonged the latency of the snails' raising their foot in response to being placed on a hot (40 °C) surface. The analgesic effects of the morphine were eliminated by naloxone as is seen in humans and other vertebrates. There was also habituation to morphine. Snails administered with morphine for four days did not differ from the control ones in tests on pain sensitivity and analgesia was achieved only at a higher dose. Donald Griffin's 1984 Animal Thinking defends the idea that invertebrate behavior is complex, intelligent, and somewhat general. He points to examples in W. S. Bristowe's 1976 The World of Spiders detailing how spiders respond adaptively to novel conditions. For instance, a spider can eat a fly held in front of it by an experimenter, bypassing the usual step of moving toward an insect caught on its web. A spider may adapt the shape of its web to abnormal circumstances, suggesting that the web is not just built with a fixed template. Griffin also considers leaf-cutter ants, with central nervous systems "less than a millimeter in diameter", and asks: "Can the genetic instructions stored in such a diminutive central nervous system prescribe all of

physical and behavioural reactions. Definitions of pain vary, but most involve the ability of the nervous system to detect and reflexively react to harmful 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 ani 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 a 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 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.