Crabs-Is this going to hurt?

by Matthew Norton

For any living creature the world around them is a dangerous place, full of things that can inflict damage on them, which makes it crucial for their survival that they can sense and respond to the danger and minimise the damage. Virtually all animals do so through a sensory mechanism called nociception, which allows them to sense and respond to damage through unconscious reflexes. When we touch something sharp, or hot and then flinch, that is our nociceptive response kicking in before the pain.

foot on nail
This will cause pain, but the initial response will be a reflex by nociception, not pain.

Pain, from an evolutionary perspective, is a step up from nociception with the damage being associated with a negative sensory and emotional experience which encourages use to avoid that specific danger in the future. In non-human animals it can be difficult to distinguish pain from nociception, since we cannot ‘measure’ their emotional state. This has prompted much debate into which animals can, or cannot feel pain, especially among invertebrates. Nonetheless a series of criteria have been proposed to determine if an animal can feel pain, which have been applied in various experiments on crabs.

Some of these criteria require crabs to deviate from their usual behaviour in response to the initial damage and then modify their behaviour in the long term, based on their experience (i.e. memories) to avoid future damage. Such behavioural changes have been seen in shore crabs (Carcinus maeans) and hermit crabs in response to an electric shock. In particular hermit crabs may temporarily abandon their adopted sea snail shells, which they rely on to protect their soft abdomen, if the shock comes from within this shell. Should a new snail shell be provided for up to a day after the initial shock they may move into this shell, demonstrating the longer term avoidance behaviour and the involvement of memory.

hermit crab
Hermit crabs rely on the empty sea snail shells to protect their soft bodies. There would have to be a real risk of harm from inside the shell for them to abandon it.

Other criteria require some involvement of the central nervous system, including the brain, to process the initial nociceptive information and coordinate a response, a crucial step for the previously mentioned behavioural changes. Again, there is some behavioural evidence with the glass prawn (Palaemon elegans), part of the same crustacean group as crabs, and the crab (Hemigrapsus sanguineus) where they intensively groom the part of their body where they have sustained damage. This awareness of where they have been hurt would require the brain to receive information on where they have been damage and then send out information to all the other body parts involved in this grooming behaviour.

When their long antennae is injured these prawns groom it intensively, suggesting they are aware of where the damage is.

These crabs fulfil these pain criteria, but for other criteria there is more limited evidence. For example another criterion requires the crab to be stressed as it suffers damage and one study on Carcinus maenas did find that electric shocks increased the concentrations of lactate in their body. This chemical can be linked to stress, but is also linked to rapid and/or prolonged activity (build ups of lactate is what causes us to ache when we exercise) and is not enough to prove stress on its own.

An even more problematic criterion for crabs to meet is having the required level of consciousness and sentience to be able to feel pain. This is mainly because of the difficulty in defining these terms, which involves thinking philosophically, which does not mix with attempting to find ‘hard’ evidence. Therefore we must rely on imperfect evidence and make an assessment on whether animal are likely to feel pain beyond reasonable doubt. If this approach can be applied to our legal system then it can also be applied here.

From a human perspective

The issue around whether animals can feel pain is especially important given that within the last year the UK government declared in a vote that no non-human animal can feel pain. This is despite the extensive body of scientific evidence which suggests, beyond reasonable doubt, that all vertebrate animals are able to feel pain. Normally I try to keep politics out of these articles, but the complete lack of consensus between government policy and scientific evidence, compiled by researchers who have spent years studying animal pain, cannot be ignored.

One piece of legislation that may be now at risk is the Animals (Scientific Procedures) Act 1986 which protects all vertebrate animals and, after later amendments, cephalopod molluscs from unnecessary harm during scientific research. Under this Act any researcher, or researchers, that involve these animal groups have to submit an ethical assessment and then be granted the appropriate license.
Other countries have brought in similar legislation, although the invertebrate animals groups they protect can vary, especially for decapod crustaceans (crabs, lobsters, hermit crabs). For example UK, EU and Canadian legislation only covers cephalopod molluscs, whereas New Zealand extends this protection to crabs, lobsters and crayfish and Norway and Switzerland protect all decapods.

animals in pain legislation
Fish, cephalopods and marine mammals (top row) are all protected from suffering under UK law. Crabs, lobsters and crayfish (bottom row) are protected under similar laws in other countries, but not in the UK.

For fish and cephalopods the debate around whether they can feel pain is very similar to that around decapod crustaceans. For example some have argued that because fish show learned avoidance behaviours towards a source of damage, which is suppressed with morphine, they can experience pain. Yet others argue that, like in crabs, they lack the required brain function and level of consciousness to feel true pain.

So why is protection more consistent in fish and cephalopods compared to decapod crustaceans? One possible explanation is that because fish and cephalopods have brains and nervous systems that more closely resemble ours we instinctively think that they feel pain like we do. This is part of a particular way of thinking called anthropomorphism, where we assume non-human animals think, act and generally experience the world in the same way we do. In some cases this can be a positive thing, inspiring empathy for animals, but it can become less influential with animals that lack similarities to us.

Still, the difficulty we have in recognising pain in animals makes the whole issue very difficult to resolve to everyone’s satisfaction and so we have to resort to debatable assessments based on the evidence available. Considering the human perspective raises another issue, which is that our idea of pain does not necessarily apply to other animals, although we often assume it does.

Thanks for reading photo
Thanks for reading


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Image sources

Ar rouz. 2016. [CC BY-SA 4.0 (]. 6414 z.jpg

All other images are public domain and do not require attribution

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