Do Decapod Crustaceans Feel Pain?
Yes, an official government report put together by a team of expert scientists was published in November 2021 with a clear conclusion that animals such as crabs, lobsters, prawns & crayfish (decapod crustaceans) are capable of feeling pain. The authors’ central recommendation is:
“We recommend that all cephalopod molluscs and decapod crustaceans be regarded as sentient animals for the purposes of UK animal welfare law. They should be counted as “animals” for the purposes of the Animal Welfare Act 2006 and included in the scope of any future legislation relating to animal sentience.”
Below we have reviewed some key pieces of scientific evidence, and explored how the findings support that decapods can feel pain. Check out all the different factors that contribute to our understanding of sentience in these animals.
Why is it important to know if decapod crustaceans can feel pain?
The ability of decapod crustaceans to feel and experience pain is one indicator of their sentience, and one that is particularly important for their protection in animal welfare law. Sentience is the capacity to experience feelings that matter to the individual, such as feelings of pain, pleasure, hunger, thirst, warmth, joy, comfort and excitement, as well as other feelings that may be unimaginable to humans. It is not simply the capacity to feel pain. However, feelings of pain and suffering have a particular significance for animal welfare law because it is considered ethically relevant for their treatment; and because evidence of the capacity to feel pain is usually required before an invertebrate can be included in animal welfare legislation (for example, in the Animal Welfare Act 2006).
Note on Ethics: The experiments below involve some pain being inflicted on the animals concerned. Crustacean Compassion believes that such research, conducted on a very limited number of animals, may be essential to saving many other billions from suffering. In the case of the Elwood experiments, Elwood reports that he tried to use the minimum stimulus for a meaningful experiment and that in some experiments the animal decided when to leave the situation and thus terminate the harmful stimuli. Some creatures were able to be returned to the seashore.
We would urge researchers to consider whether animal research is fully ethically justifiable on the grounds of benefits to decapods themselves; to consider whether research into positive experiences could yield the same insights as research into pain and suffering; and to apply the 3Rs (reduce, refine, replace) in their fullest capacity regardless of whether this is mandated by law in their country.
Decapods React to Harmful Situations
There are two parts to pain. The first is a basic, unconscious reflex response to avoid something that will cause harm. The second is a negative felt experience which often triggers a long-term change in behaviour and helps protect from injury in the future (1). This can be compared to accidently putting your hand on something hot. Before you’ve had a chance to acknowledge it, your body pulls your hand away. That’s part one of a pain response. A moment later you realise, and suddenly feel, the hot sensation and because it was a negative experience, you learn to be more careful around hot things next time. That’s part two of pain.
When assessing evidence of pain in animals, we're examining the difference between a basic reflex response and a full painful experience.
In Elwood and Adam’s 2015 paper, they found that a painful situation triggers a stress response in crabs. They reported that this, combined with other findings, such as decapods changing their behaviour long-term after a painful incident, demonstrates that decapods are capable of experiencing pain (2). In another paper, Braithwaite (2010) concluded that the nervous system of decapods responds to pain in a more complex way than simple reflexive responses, supporting that they are capable of not only part one (the reflex response), but also part two, of the painful experience (3).
Decapods Tend to Their Injuries
Self-protective behaviour is when an animal tends to their injuries and pays attention to a painful part of their body. This is a common behaviour in vertebrates, including humans.
A selection of studies provide evidence of this behaviour in decapods.
In a similar way to vertebrates, researchers observed crustaceans rubbing and holding an injured area, as well as limping and reducing use of injured body parts (4). In another study, an inflammatory substance was injected into the claws of crabs. They found the crabs would hide in the corner of the aquarium, hold their injected claw off the ground, and become fidgety, shaking and rubbing the affected area (5). Similarly, Elwood (2019) noted that manually-declawed crabs would touch the wounded area and shudder (1). This behaviour has also been recorded in prawns who had their antennas damaged. The prawns would groom the injured antenna and rub it against the side of the tank (6).
Decapods Give Up Important Resources to Avoid Pain
A motivational trade-off is when an animal gives up a highly valuable or desired resource in order to avoid a harmful situation. If an animal is willing to give up something important to avoid a painful experience, it suggests that the animal can feel pain.
The following studies provided decapods with highly valued resources, but in situations that could cause them pain. They found that decapods do give up important items to avoid pain, therefore supporting the idea that they are capable of experiencing pain.
Crabs are nocturnal prey animals so a dark shelter is a very important resource. In 2016, Magee and Elwood provided crabs with a dark shelter, but when inside they would receive a mild electric shock. At first the crabs would enter, but when electric-shocked, they would leave and stand in the light, unsheltered area (7). Another highly valued item is a hermit crab’s shell. Researchers electric-shocked hermit crabs through their shells and then offered them a new, lower quality one. The negative experience of the shock led to crabs giving up their higher quality shell to avoid the shock. Increasing shocks resulted in crabs moving to new shells quicker with less contemplation regarding shell quality (8). These studies provide examples of decapods trading-off valued resources to avoid pain, a type of behaviour that is more than just reflexive (1).
Decapods Respond to Painkillers
Studying an animal’s response to painkillers can provide useful insight into whether that animal is capable of feeling pain. If the animal responds differently to a painful situation when given painkillers, this can be used as evidence that the animal can feel pain.
The example studies below compared the response of decapods to painful situations both with and without painkillers and found that their behaviour did change, supporting the idea that decapods are capable of experiencing pain.
In two scientific studies, researchers caused injury to the antennae of prawns. The behaviour of different groups was assessed based on whether they did or did not receive painkillers. In both studies, the group that received painkillers for the procedure showed less pain-associated behaviours like tail-flicking and rubbing or guarding the injured spot (6, 9). Another study observed how shrimp reacted to a mild electric shock with and without morphine (a strong painkiller). The group of shrimp not receiving morphine had an immediate strong negative reaction compared to a reduced reaction from the morphine-administered shrimp. These studies support that decapods are capable of experiencing pain by demonstrating that their behaviour during a painful procedure changes with painkillers (10).
Decapods Learn and Form Memories
Learning and memory formation is important because the second part of pain is the long-term change in behaviour which provides protection from injury. Elwood (2019) stated that memory formation can help differentiate between mere reflexes, and an actual painful experience (1).
The following studies are examples of when decapod crustaceans have been found to avoid pain by learning and remembering a previous negative experience.
A study back in 1992 found that decapods remembered to avoid a specific cause of pain for at least 24 hours (11). A few years on, researchers found decapods remembered other crabs for up to 4 days (12). In another test of memory, crabs were provided with a highly valued dark shelter. However, once inside the shelter, the crabs would get a mild electric shock. It only took two attempts to enter the shelter for the crabs to learn to avoid it next time (1). Other studies also made use of a mild shock-shelter and yielded similar results, finding that crabs quickly learnt to avoid it. It was inferred that this was the result of memories they had formed from previous shocks (13, 14). Furthermore, Appel and Elwood (2009a) gave hermit crabs a mild electric shock through their shell before offering them a new one (15). They slowly increased the length of time between the shock and the new shell offering, and found that crabs still opted for a new shell a whole day later.
Decapods Experience Stress and Anxiety
Stress and anxiety are closely linked to pain. Pain is a negative experience which is often associated with a stress response (16). Experiencing pain causes individuals to become more risk averse, which helps protect from injury and extend survival. Anxiety is the name given to this process of avoiding risk as a result of pain (1). Therefore, evidence of stress and anxiety can indicate that an animal is capable of, and has experienced, pain.
The studies discussed below provide evidence of stress and anxiety in decapods. Studying this is important as it provides insight into how painful experiences may impact decapods.
In 2014, Fossat et al observed the behaviour of crayfish exposed to a stressful situation. They then injected them with an anti-anxiety drug and recorded any changes in behaviour. They found that before the drug, crayfish actively avoided the stressor. Once the drug had been administered, behavioural signs of anxiety disappeared (17). Their results provide evidence that some invertebrates are capable of experiencing anxiety. In another study, researchers diluted antidepressants in the water tanks of crayfish and observed their behaviour. Crayfish exposed to antidepressants were twice as likely to come out of their shelters, and spent more time out looking for food than the antidepressant-free group. The antidepressants affected the crayfish because, like humans, their brains use the hormone serotonin (18). It has been established through research that measuring lactate is a useful indicator of stress (19), and there are numerous studies of increased lactate levels in decapods following a stressful experience. For example, increased levels of lactate were recorded in crabs after declawing (20) and in shrimp after extensive handling (21).