Do Crustaceans Feel Pain?
As a kid, one of my favorite activities was searching for crayfish along the rocky shores of lakes in the Boundary Waters region in the Upper Midwest. I walked barefoot over the rocks, gently turning them to peek underneath and hopefully find a big ol’ crayfish, then gleefully admire its tenacity upon being captured. Eventually I would release it and admire the speed at which it zoomed backwards under the nearest crevice. Later in life when I moved to Louisiana, it was tough for me to see a cooler (ice chest) full of squirming crayfish, with their fierce little claws (known as chelipeds) and beady eyes destined for a boiling pot – a stark contrast to the sunny days of my catch and release experiences with them. To me, it begged the question, do these little organisms feel anything when plunged into the boiling pot?
Now, if you are busy and just want to know the answer: it is inconclusive. (Don’t you hate when authors make you read all the way to the end for it?) But please read on if you have time, because the details are pretty interesting and might make you rethink your relationship with crustaceans.
Pain or Reflex
One reason the answer is inconclusive is that pain is difficult to measure, especially when your study subject is non-human. What is pain anyway? In terms of responses, it can be broken into two fundamental components: 1) the physiological detection of pain by the nervous system leading to a response, i.e., a reflex; and 2) the experience of suffering – conscious, emotional, or sensational (Andrews et al. 2013). As you probably already know, the first component is capable of occurring without the second, like when the doctor strikes your knee with a reflex hammer. But with the addition of the second component, the waters become muddy – measuring the internal suffering of an organism is no straightforward task.
Pain is in the Brain
Crustaceans have long been viewed as maintaining reflexes that do not cause internal suffering, which would mean they do not truly feel pain (as noted by Elwood 2019). A reflex involves the firing of relatively few neurons resulting in a very fast response to stimuli. Conversely, the neurotransmitters involved in what we know as pain are slower and lag behind the reflex reaction. Anyone who has stubbed their toe and enjoyed the few pain-free seconds before the wave of agony hits is familiar with this lag time. In some animals the ‘pain’ signal never reaches the brain and therefore the reflex reaction is kept separate from a pain-induced reaction. The main distinction here is that unlike a reflex, pain is located in the brain, not in the body.
One method to distinguish pain from reflex is to measure changes in animal behavior as a result of a painful experience. Scientists tested motivation change in hermit crabs by evaluating their behavior after being shocked with a low intensity, and then offered a new shell of equivalent value (Elwood and Stewart 1985). Crabs that were shocked displayed a much higher probability of abandoning their original shell for the new shell compared to crabs that were not shocked. This study demonstrated that the crabs receiving the shocks valued that shell less and changed their behavior due to the shocks.
Another measurement of pain is how animals treat an injured body part. Pain can be expressed by grooming, guarding, or rubbing a wound, which implies awareness of an afflicted location. To test this response in crustaceans, researchers applied sodium hydroxides, which is painful to mammals, to a single antenna of glass prawns and observed the animals’ reactions. The prawns exhibited significantly prolonged grooming of the wounded antenna compared to the other antenna, indicating awareness of the inflicted site (Barr et al. 2008). Another study involving shrimp showed that using a topical anesthetic prior to wounding the animal’s eye reduced the stress response exhibited by the shrimp (Taylor et al. 2004). Other measurable behaviors consistent with pain include increased anxiety in crayfish due to electric shock (Fossat et al. 2014), and avoidance learning in shore crabs (Magee and Elwood 2013).
Although these studies provide evidence that crustaceans experience more than a reflex when experiencing something ‘painful’, it is important to note that the presence of pain is not proven, nor can it ever be until the day we get the crustaceans to tell us for themselves. Moreover, there remains strong opposition to the hypothesis that crustaceans feel pain. Critics have pointed to the limited number of species studied, the “low bar” for evaluating pain criteria, and several non-replicable results on what was deemed ‘pain’ in crustaceans (Diggles 2019). For instance, in the study testing responses to wounded antenna, researchers also used an anesthetic to examine if it reduced the stress response like it did for wounded shrimp, but their findings on this were inconsistent with the shrimp study (Taylor et al. 2004, Barr et al, 2008).
Where researchers do agree, is that crustaceans can recognize physical damage and harmful conditions. But again, this is not the same as pain.
Absence of evidence is not evidence of absence1
In general, very few people care whether crustaceans experience pain. Crustaceans are invertebrates (meaning they lack a backbone), and invertebrates make up over 90 percent of living animals. Yet when it comes to legal regulation invertebrates are essentially unprotected in comparison to vertebrate species (e.g., birds, mammals, amphibians, reptiles, and fish). However, as the debate over whether crustaceans feel pain presses on, discussion has bubbled up as to how we manage these animals. One option is to apply the precautionary principle, which basically means when evidence is inconclusive, we should give animals the benefit of the doubt (Birch 2017). (Interestingly, the precautionary principle originated in environmental policy – erring on the side of taking effective precautions when uncertainty exists between human action and environmental effects.) Still, even this suggestion, though soundly based, has credible critics (Woodruff 2017, Diggles 2019).
Shellfish production is a billion-dollar industry worldwide, responsible for numerous jobs, sources of food, and is deeply imbedded in the cultures of many nations. The estimated value of shellfish produced by U.S. farmers amounted to $340 million in 2016 (NOAA 2017). In Louisiana, the crawfish industry alone adds an estimated $120 million to the state economy each year. Adding crustaceans to animal welfare laws may lead to constraints on research institutions and food industries, and some scientists argue that more reliable, robust evidence is yet needed prior to enacting the precautionary principle (Diggles 2019).
However, other momentum is growing to the benefit of the crustaceans. In March 2018 the Swiss government added lobsters to their animal protection laws, no longer permitting them to be boiled alive. They must instead be stunned or killed first. In other counties, such as the United Kingdom, petitions for similar motions are underway.
What to do?
Indeed, gathering adequate scientific evidence to change animal welfare laws is a daunting task, but an even more mammoth task is getting people to care. Should we even care? Even if these animals experience pain, is it our duty to change our behavior for this one reason? These are fair questions that could be applied not only to other invertebrates, but to vertebrate animals as well. It is worth considering, but that lies far beyond the scope of this article.
If you do want to enact your own precautionary principle, some options to reduce possible pain experienced by your future food include chilling or freezing the animal, stunning it, or swift mechanical killing prior to cooking. More details can be found here.
As for me, I am happy to keep crayfish-hunting at the lake, catching and releasing my little friends. Thanks to being raised in a landlocked region with my own mild shellfish allergy, I have never craved the flavor of crustacean cuisine. I still very much enjoy a good crawfish boil now and then, but I mainly stick to the corn, potatoes, and other fixin’s… and try not to notice the cooler of crayfish beforehand.
Andrews, P.L.R. A.S. Darmaillacq, N. Dennison, I.G. Gleadall, P. Hawkins, J.B. Messenger, D. Osorio, V.J. Smith, and J.A. Smith. 2013. The identification and management of pain, suffering and distress in cephalopods, including anaesthesia, analgesia and humane killing. Journal of Experimental Marine Biology and Ecology 447, 46-64.
Barr, S., P.R. Laming, J.T.A. Dick, and R.W. Elwood. 2008. Nociception or pain in a decapod crustacean? Animal Behavior 75, 745–751. (doi:10.1016/j.anbehav.2007.07.004)
Birch, J. 2017. Animal sentience and the precautionary principle. Animal Sentience 16(1).
Diggles, B.K. 2019. Review of some scientific issues related to crustacean welfare. ICES Journal of Marine Science 76(1), 66-81. (doi:10.1093/icesjms/fsy058)
Elwood, R.W. 2019. Discrimination between nociceptive reflexes and more complex responses consistent with pain in crustaceans. Philosophical Transactions of the Royal Society B 374: 20190368. (http://dx.doi.org/10.1098/rstb.2019.0368)
Elwood, R.W., and A. Stewart. 1985. The timing of decisions during shell investigation by the hermit crab, Pagurus bernhardus. Animal Behavior 33, 620–627. (doi:10.1016/S0003-3472(85)80086-5)
Fossat, P., J. Bacque-Cazenave, P. De Deurwaerdere, J.P. Delbecque, D. Cattaert. 2014 Anxiety-like behavior in crayfish is controlled by serotonin. Science 344, 1293–1297. (doi:10.1126/science.1248811)
Magee, B., and R.W. Elwood. 2013 Shock avoidance by discrimination learning in the shore crab (Carcinus maenas) is consistent with a key criterion for pain. Journal of Experimental Biology 216, 353–358. (doi:10.1242/jeb. 072041)
NOAA. 2017. https://www.fisheries.noaa.gov/insight/understanding-shellfish-aquaculture
Taylor, J., L. Vinatea, R. Ozorio, R. Schuweitzer, and E.R. Andreatta. 2004. Minimizing the effects of stress during eyestalk ablation of Litopenaeus vannamei females with topical anesthetic and a coagulating agent. Aquaculture 233, 173-179.
Woodruff, M.L. 2017. Scientific uncertainty and the animal sentience precautionary principle. Animal Sentience 16(11).