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Written by Myles Ringel ‘25
Edited by Parsa Lajmiri ‘26
I believed that the sea star (Asterias forbesi) had been properly euthanized by the teaching assistants before my lab section. But as I made an incision along the sea star’s arm, carefully documenting its anatomy, it suddenly reacted, detaching its arm from its body. With horror, I realized that this dissection was in fact a vivisection. Leaving BioMed, I couldn’t shake the thought: had I hurt the sea star? Could sea stars even feel pain?
It seems obvious that a mammal, bird, or reptile might feel pain if it were to be poked or prodded, but it is less apparent with sea stars; after all, they can’t yelp in pain. Sea stars are invertebrates and have a less complex nervous system than their vertebrate counterparts, possibly limiting the extent of their perception [1]. Invertebrates are specifically excluded from animal welfare regulations [2] in part because of their perceived simplicity. But it is not clear whether or not this assumption is true. Cephalopods (i.e. octopi, squid, and cuttlefish) are invertebrates, but are being recognized as having higher-order cognition, leading some organizations to voluntarily include them under the same animal welfare regulations [3]. Without protections, professors and students can do almost anything with the billions of invertebrate models they work with [4], as long as it is in the name of research and education. It is paramount to consider the possibility that these other invertebrates might also feel pain.
Most animals—vertebrates or not—can perceive damaging stimuli, a process called nociception [5]. Nociception has no subjective experience; that is, it doesn’t require “feeling” in the way that we typically think of it. Nociception is a reflex that occurs when specific nerve groups are activated [5,6]. In response, the animal alters its behavior to minimize harm from the stimulus [7]. For example, when you touch a still-hot pot, your hand jumps away before you feel the pain from the heat [1]: you detected a harmful stimulus and altered your behavior with no need for subjective input. Pain, on the other hand, requires some level of cognition. It’s the other part of the example above where you have a negative reaction due to the harmful or imminently harmful stimulus [1,5].
The trouble with figuring out if a fruit fly or clam feels pain is that we can’t readily read their body language or understand how they might feel. Whereas we might recognize a dog sticking its tail between its legs, we are less attuned to the small behavioral cues of invertebrates. However, there are several somewhat straightforward benchmarks that might suggest an organism is capable of experiencing subjective pain rather than merely reacting through nociception (like in the hot stove example). Beyond the basics of an ability to sense a noxious stimulus and a centralized nervous system, the two most compelling include a responsiveness to sensation-dampening molecules and rapid learning to avoid noxious stimuli [1].
Animals that can feel pain have receptors in their nervous systems that bind to pain-relieving molecules called analgesics [8]. Opioids are one such chemical that vertebrates produce when they are injured so that they can still function without being overwhelmed by pain. While it might seem to be in every animal’s interest to release analgesics when it is injured, retaining the nociceptive stimulation is actually evolutionary advantageous. For animals that may not have a strong pain response—i.e. those that will not be overwhelmed with the subjective experience of their injury—the nociceptive stimulation primes the animal to be on the defensive and be ready to get away from predators [7]. If analgesics are not beneficial to the animal’s ability to avoid predators, it stands to reason that these chemical pathways would only develop if the experience of pain would otherwise overwhelm its ability to flee or fight. In other words, the animal would only evolve this system if they felt some type of pain [1].
A true pain experience would also likely manifest in quick learning by the organism to avoid the stimulus, rather than the immediate instinct to escape that is provided by nociception [1]. For example, imagine taking two animals—one that can experience pain and one that can only experience nociception—and shocking them when they press a button. The animal that can feel pain will have an intense experience and will quickly learn to avoid that button because it is motivated to not have such a negative experience again. The nociception-only animal, however, will reflexively move away from the shock but will have no emotional experience and will not feel as motivated to avoid the button. In an experimental setup, it would likely take multiple trials to condition the nociception-only animal to not press the button, whereas the animal that experiences pain may get the memo right away.
While neither of these low-bar benchmarks is definitive in determining if an animal feels pain or not, they can provide insight into which families of animals might be capable of experiencing pain. Of just these two experimental categories, a number of arthropods like crabs and fruit flies fit these criteria [1,5].
If we accept that some invertebrates may be capable of feeling pain, the question of regulation ensues. The primary regulations for vertebrate animals in research and education use stems from empathy and compassion. The implicit logic is that these vertebrates experience pain similarly to us, and we wouldn’t subject humans to these painful experiments, so we shouldn’t subject these similar animals to it either. Stemming from the same reasoning, some may argue that we regulate the use of invertebrates in the same way we aim to protect mice and chimpanzees in the lab. For example, they would suggest that we should have complex post-operative care for surgical interventions and strictly regulate how cold an enclosure is, or how much light it receives [9].This harm avoidance approach uses a thick brush to eliminate any potential suffering. But this approach will be difficult to enact. Educators, scientists, stakeholders, and policymakers will likely have difficulty buying into these broad restrictions because of the broad and sustained disruption this would cause to education and research [1].
Instead, I argue for a second, harm-reduction approach. While some invertebrates may experience pain, not all families of invertebrates do. It is not pragmatic to restrict research using low-risk invertebrates like sea sponges, which lack nervous systems and likely don’t feel pain [10]. Even if a pain response is established, it is also unclear how similarly an invertebrate processes pain to a human. Differing brain structures and sensory anatomy may make it so that a fruit fly’s most painful experience is as intense as our suffering after getting a paper cut. With these considerations, the use of invertebrates should depend both on their phylogeny and quality of their observed pain response. Institutions and governmental agencies should also encourage investigators to rethink experiments that cause bodily injury or exposure to noxious stimuli.
While this framework will minimize potential inflicted pain without crippling biomedical research and education, it’s not a perfect solution. Not all pain will be avoided, but we can—and should—take strides to limit it. With even the slightest evidence that a specific invertebrate can feel pain, it is our duty to ensure that no fly, crab, snail, or clam meets the same awful demise as the sea star in the basement of BioMed.
References
Elwood RW. Pain and Suffering in Invertebrates? ILAR J. 2011 Jan 1;52(2):175–84.
Office of Laboratory Animal Welfare. Public Health Service Policy on Humane Care and Use of Laboratory Animals [Internet]. National Institutes of Health; 2015. Report No.: 15–8013. Available from: https://olaw.nih.gov/sites/default/files/PHSPolicyLabAnimals.pdf
Office of the Chief Health & Medical Officer. Care and Use of Animals [Internet]. National Aeronautics and Space Administration; 2022. Policy Directive No.: 8910.1D. Available from: https://nodis3.gsfc.nasa.gov/displayDir.cfm?t=NPD&c=8910&s=1D .
abrahamrowe. The scale of direct human impact on invertebrates [Internet]. 2020. Available from: https://forum.effectivealtruism.org/posts/9drbh8sKzzykaX38P/the-scale-of-direct-human-impact-on-invertebrates
Burrell BD. Comparative biology of pain: What invertebrates can tell us about how nociception works. J Neurophysiol. 2017 Apr;117(4):1461–73.
Gibbons M, Sarlak S, Chittka L. Descending control of nociception in insects? Proc R Soc B Biol Sci. 2022 Jul 6;289(1978):20220599.
Crook RJ, Dickson K, Hanlon RT, Walters ET. Nociceptive Sensitization Reduces Predation Risk. Curr Biol. 2014 May 19;24(10):1121–5.
Che T, Roth BL. Molecular basis of opioid receptor signaling. Cell. 2023 Nov 22;186(24):5203–19.
National Research Council (U.S.), Institute for Laboratory Animal Research (U.S.), National Academies Press (U.S.), editors. Guide for the care and use of laboratory animals. 8th ed. Washington, D.C: National Academies Press; 2011. 220 p.
Ludeman DA, Farrar N, Riesgo A, Paps J, Leys SP. Evolutionary origins of sensation in metazoans: functional evidence for a new sensory organ in sponges. BMC Evol Biol. 2014 Jan 13;14(1):3.
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