Do Crabs, Crayfish or Shrimp Feel Pain?

Today I am going to be exploring all things relating to crustacean’s pain and neuroscience! So, do crabs, hermit crabs, crayfish or shrimp feel pain? This is a very tricky problem, especially for those of us who are working with these model organisms or even keep them as pets in home aquariums.

The short answer to this question is that crabs, crayfish, or shrimp do response to harmful stimuli. It means that they can experience pain to some degree. The main problem though is that definition of pain cannot be fully applied to crustaceans and other invertebrates.

The point is that the conclusions are often based on argument by analogy, which suggests that if behavioral responses to particular harmful situations are similar in crustaceans and humans then they are probably mediated by similar affective states.

This is far from the truth!

Whether invertebrates are able or not to experience pain (as humans do) is a highly controversial issue. So, keep reading if you want to know more about crustaceans, their neuro systems, reaction to negative stimuli, and pain.

What is Pain?

So first of all, let’s start with the main question… what is pain?

In common terms, pain is usually defined as an unpleasant sensory and emotional experience associated with actual or potential illness or injury.

Here comes the first problem with crustaceans; crabs, crayfish, shrimp, etc. cannot describe their emotional experiences. Therefore, scientists had to modify the term of pain as ‘an aversive sensory experience caused by an actual or potential injury that elicits protective and vegetative reactions, results in learned behavior, and may modify species-specific behavior’.

Basically, crustaceans in pain should learn how to avoid the harmful stimulus and demonstrate sustained changes in behavior that have a protective function to reduce further injury and pain.

From a neuroscience perspective, pain is a sensation generated by the brain, usually when it receives stimuli that it interprets to be a sign that something is potentially causing harm or danger.

The type of nerve that transmits these signals are known as nociceptors, which comes from the Latin Nocere, “to harm”, meaning they are indicators of harm.

However, it is not that simple. Some scientists believe that simply noting a response to noxious stimuli is not sufficient, because nociception does not equal to pain.

Nociception and Pain

The ability to detect dangerous, damaging stimuli is adaptive in terms of survival, and thus the evolution of an early warning system in animals seems intuitive. Otherwise, animals would continue to damage themselves repeatedly, resulting in disease, loss of limbs, and even mortality.

Note: Think back to any time you accidentally touched something too hot, cold, etc. The painful feeling you felt was originally transmitted by a nociceptor. You may not even need to see the source of the pain for that.

However, in crustaceans, there is no suggestion that nociception involves an unpleasant feeling and there is no implication that central processing and decision-making are involved in responses. On the contrary, nociception often involves a reflex response that just helps minimize tissue damage.

It means that nociceptors do not always mean that nociception leads to pain.

In addition, there is an opinion that the pain does not imply only unconscious reflex responses that assist in withdrawing from tissue-damaging stimuli. It also includes awareness of such stimuli with the intervention of the conscious part of the “brain”, which is most difficult to prove in invertebrates.

In simple words, while a behavior might seem to be the most obvious indicator of pain we have to be aware that mechanical reflexes can exist without the experience of pain.

There are multiple reports of the occasionally noted absence of behavioral responses in such conditions that we would expect to elicit great responsiveness from vertebrates. For example, some insects will continue to feed whilst being eaten alive by predators or, as shown in the case of the male praying mantis, by their mating partners.

Nociception, Pain, and Crustaceans

So, now we know that the main function of pain is to aid future avoidance of the painful stimulus, whereas nociception enables a reflex response that provides immediate protection but no awareness or long-term behavior change.

But do crabs, shrimp or crayfish learn to avoid painful stimulus?

In one of the experiments, scientists wanted to distinguish the difference between pain and a phenomenon known as nociception in hermit crabs. They used mild electric shocks when hermit crabs used one of the shells.

The tests showed that hermit crabs started exploring alternative options (shells) after receiving several mild electric shocks.

So, the crab’s swift shock avoidance and discrimination learning (between the two shelters) clearly showed that shock affects their choice of shelter and was consistent with the definition of pain used for other species.

In other experiments, crayfish (Procamarus clarki) had to choose between dark shelter (natural preference) and lit shelter.  Every time it chose the dark shelter, it received a mild electric shock.

As a result, crayfish quickly learned to respond to these associations by walking to a safe area in which the shock was not delivered. In addition, the shocked crayfish had relatively higher brain serotonin concentrations coupled with elevated blood glucose, which suggests a stress response.

Note: The association from a single trial between the shock and the light chamber persisted for up to 3 hours. Whereas in another experiment, crayfish could ‘gradually developed avoidance behavior’ following 20 trials per day for 32 days.

The evidence collected from those studies thus strongly suggests that crustaceans do indeed feel pain.

But… it is a bit more complicated than that.

Another test was to alter the cues available to the crabs. So, if the crabs had associated a particular pattern of stripes with the shock.

The data show that crabs learned to walk either to their left or to their right to get to the non-shock shelter and avoid the shock. However, there was no evidence that crabs used cognitive maps.

These findings indicate swift avoidance learning, which is a key criterion/expectation for pain experience, but this alone does not prove that crabs can experience pain which is all consistent with the concept of pain.

Pain Indicators in Crustaceans

To decide where or not crabs, crayfish, or shrimp feel pain, some scientists suggested adding more criteria that might be used to indicate pain. Specifically:

1. the presence of nociceptors,
2. avoidance learning,
3. a suitable central nervous system and receptors,
4. protective motor reactions that might include reduced use of the affected area, limping, rubbing, holding or autotomy,
5. physiological changes,
6. trade-offs between stimulus avoidance and other motivational requirements,
7. reduced evidence of pain experience if treated with painkillers.

I have already described the presence of nociceptors and avoidance learning. According to the studies, they fit at least the minimum criterion for pain experience. So, let’s start from here.

3) Central Nervous System and Suitable Receptors in Crustaceans

The central nervous system in crustaceans consists of:

• brain (cerebral ganglion or ganglia);
• ventral nerve cord linking a series of small nerve cell clusters.

The crustacean brain is quite unlike that described in vertebrates. It is very simple and consists of only 3 nerve cell clusters (cerebral ganglion or ganglia).

The brain can control some things, but lots of things like moving, and mating, are controlled by the other ganglia. It means that even if the cerebral ganglia are disabled, some parts of the crustacean would still move and react in a purposeful way while responding to external stimuli.

Note: Some people may say that their brain is too small and simple to feel the pain. Well, this argument is not valid because the brain size does not necessarily equate to complexity of function. For example, the brain of large crustaceans, such as a lobster, is likely to be considerably larger than that of many vertebrates.

You can also read my articles:

• Crayfish Internal Anatomy
• Dwarf Shrimp Internal Anatomy

4) Protective Motor Reactions

The most common sign of pain in animals:

• reduced use of the affected area,
• limping,
• rubbing,
• scratching,
• autotomy,
• as well as other changes in normal

For example, crabs will autotomize limbs (cast off) in situations that do not involve haemolymph (blood) loss, for example, if placed on a hot plate or if the limb is injected with acetic acid.

Note: It is worth noting that while we observe these behaviors in response to painful stimuli in vertebrates, most invertebrates don’t show the same behavior. However, not displaying certain external behaviors doesn’t necessarily mean that an organism isn’t experiencing pain. The evolution is too complex, maybe other animals learned to hide their pain to avoid being seen as vulnerable and getting targeted by predators.

5) Physiological Changes

In vertebrates, pain also causes physiological changes such as respiratory patterns, tachycardia, pupil dilation, changes in blood flow, etc.

Even though, there has been limited examination of similar responses in crustaceans. These physiological changes were still registered in some cases.

I have already mentioned that shocked crayfish had relatively higher brain serotonin concentrations coupled with elevated blood glucose, which suggests a stress response. Another scientist recorded a reduction in heart rate following claw autotomy in an agonistic encounter and suggested this could be an attempt to avoid excessive blood loss. These effects of claw removal lasted more than 24 hours.

According to another study that was published in the journal Biology Letters, European shore crabs physically react to a pain stimulus.

When the researchers tested their hemolymph (or “blood”), it had three times the amount of lactic acid. Basically, this is a clear physiological sign of stress.

6) Trade-offs Between Stimulus Avoidance and Other Motivational Requirements

Trade-offs clearly involve some form of processing in which the different needs of the animal are weighed. It means that the response should be the same regardless of other motivational priorities. It should not be just a reflex (caused by nociception).

For example, whether hungry or satiated the reaction must be the same.

This is important for pain research as it is a strong indicator that the response to the noxious stimulus is not purely reflexive.

7) Reduced Evidence of Pain Experience If Treated with Painkillers

The effect of painkillers should reduce their sensitivity to the shock in a dose-dependent manner.

For example, in the crab, Chasmagnathus granulatus, electric shock delivered via two small holes drilled into the carapace causes a defensive threat display. Injection of painkillers reduced the crabs’ sensitivity to the shock, with the analgesic effect declining with increasing duration between injection and shock.

Another experiment showed that harmful stimuli applied to a specific antenna resulted in a reflex response and long term behavioral attention to the impacted area which disappeared when benzocaine a local anesthetic was given.

This is particularly interesting that painkillers apparently decreased response to injury in crustaceans.

The point is that if the reflex responses (to injury) were only as a basic means of self-preservation and had nothing to do with pain, then the local anesthetic should not have had any effect. Because it does nothing to remedy the injury and has everything to do with reducing the sensation of pain.

The very fact that such things as anesthetics seem to mitigate response to injury definitely suggests that the harmful stimuli go beyond just a simple notification and they need to avoid the cause of that stimuli merely for survival purposes.

Tests, Ethics, Legislation and Pain in Crustaceans

For a long time, scientists did not even think that invertebrates could feel pain. Everybody thought that they just simply reacted mechanically. Well, compared to our complex nervous systems it made sense. But their nervous systems do have their own complexity.

As far as I know, currently, scientists still do not have to clear any ethics forms in order to experiment with crabs, crayfish, shrimp, etc. However, the situation is changing.

For example, in Switzerland, they elected to legislate for lobsters to be stunned or knocked out prior to boiling them.

In Conclusion: Do Crabs, Crayfish, or Shrimp Feel Pain?

What is pain? It may seem like a question that everyone already knows the answer, but it is actually quite a subjective idea and even humans experience it very differently.

It is way more complicated with animals. Even more, nobody really understands what pain actually is in animals and crustaceans in particular. That is why there has been inconclusive research indicating that lobsters, crabs, crayfish, shrimp, and other crustaceans, may experience pain – just not in the same way humans do.

Perhaps the most compelling argument that crustaceans feel pain is not their behavior but rather the presence of nociceptor resembling structures in their nervous system and their reaction to painkillers.

Nonetheless, it is currently impossible for us to know exactly what their experience is in response to stimuli. Due to the very nature of pain itself, we might never know for sure! Therefore, approaching them on the assumption that crabs, crayfish, shrimp do feel pain is more humane.

It is an important question in bioethics and in the field of animal care because we do not want to cause them distress. We need to always advocate for treating living organisms around us with respect. After all, there is a tremendous amount of evidence that suggests that they do feel pain.

References:

1. Shock avoidance by discrimination learning in the shore crab (Carcinus maenas) is consistent with a key criterion for pain. The Journal of Experimental Biology 216, 353-358. 2013. doi:10.1242/jeb.072041.
2. Pain in aquatic animals. The Journal of Experimental Biology (2015) 218, 967-976 doi:10.1242/jeb.088823.
3. An invertebrate perspective on pain. Animal Sentience 2016.018: Mather Commentary on Key on Fish Pain.
4. Painful feelings in crabs. J. Exp. Biol. 216, 353-358.
5. Defining and Assessing Animal Pain. The Humane Society Institute for Science and Policy. 11-2014.
6. Behavioural indicators of pain in crustacean decapods. Ann Ist Super Sanità 2009 | Vol. 45, No. 4: 432-438.
7. Evidence for pain in decapod crustaceans. Animal Welfare 2012, 21(S2): 23-27. doi: 10.7120/096272812X13353700593365. ISSN 0962-7286.
8. Pain and stress in crustaceans? Applied Animal Behaviour Science 118 (2009) 128–136