Do fish and crustaceans feel pain?

Don’t eat me! (photo by Rafael Ortega Diaz)

It would be amazing if we could put ourselves inside the minds of other animals – do they think, feel and reason like we do? Or are they little more than glorified robots? Or something in between? At the moment, we don’t know much. It’s impossible to experience the consciousness of another human, let along another species – and extremely hard to figure out what other species ‘feel’.

This problem was not conveyed very well by last week’s ‘news’ that ‘fish don’t feel pain’. This was actually based on a review article – not on new evidence – which only concluded that ‘fishes are unlikely to feel pain’. Not quite the same as saying they definitively don’t (something that would be impossible to prove, anyway). There’s an interesting analysis of the review on the Practical Fishkeeping website that disputes the review’s conclusion, with help from an expert in the field.

What really didn’t come across from the media coverage is how difficult it is to draw any conclusions at all about what fish actually feel. The arguments can go either way – if fish recoil from painful stimuli, does this mean they consciously feel pain, or that they are unconsciously and ‘instinctively’ removing themselves from danger?

Luckily, another news story this week – about pain in crustaceans – shows how a cleverly designed experiment can shed a bit more light on a tricky subject like this. The study showed that crabs can learn to avoid electric shocks, going to the extreme of leaving shelter and venturing out into the open – something they’re not normally keen to do.

The researchers concluded that this response fit the criteria for pain – the crabs recoiled from the shock, and then avoided it, at a potential cost to themselves. As one of the scientists commented, ‘They leave what is a desired place – a dark shelter – to go out into this dangerous light environment – they are giving up something very valuable.’

Although this is far from being a window into the crabs’ minds, it gives us a bit more information to work with than simply observing animals’ immediate reaction to potential pain. The crabs were given the choice of two shelters in a brightly lit tank, one of which then delivered an electric shock. When the same crabs were put in the tank for a second test, those that had been shocked the first time around didn’t avoid the shelter where the shock had been given. But after being shocked once more, they learned – and avoided the same area in subsequent tests.

The scientists argue that the crabs’ ability to learn suggests this isn’t just a reflex reaction to pain, but a change to their behaviour that will affect the choices they make in the future.

This experiment focused on observing behaviour, but researchers also observe anatomy to figure out whether other animals feel pain in the way we do. For example, the review publicised last week points out that ‘C fiber nociceptors, the most prevalent type in mammals and responsible for excruciating pain in humans, are rare in teleosts [bony fish] and absent in elasmobranchs [sharks and their close relatives] studied to date.’

Nociceptors are nerve cells that respond to stimuli that could cause damage (for example heat) and send a signal to the spinal cord or brain that triggers a response (for example moving away). But just because fish don’t have the same nerve cells as us, does that mean they don’t feel pain? It certainly suggests they don’t feel the same pain that we do, but that is hardly proof that they don’t feel pain at all.

Statements like ‘fish don’t feel pain’ make me nervous because it’s a convenient conclusion for humans to make. That way, we don’t have to worry about fish welfare or feel bad about killing these animals. But research like the study on crabs can turn our assumptions upside down – at the moment, there is very little concern about crustaceans’ welfare, but maybe that needs to change.

Personally, I think our rules on animal welfare can be pretty arbitrary – the shock over burgers containing horsemeat being sold in the UK is a good example. Of course we should be able to trust the labels on our meat – but what’s so awful about eating a horse, compared to eating a cow?

Attitudes like this are completely irrational – and to me, assuming that fish (and other animals that show complex behaviour) don’t feel pain is similarly strange. Why wouldn’t they? I think we need a better reason to assume they don’t feel pain than the rather unscientific ‘they are very different from humans’. Perhaps we should be giving more animals the benefit of the doubt in the absence of hard evidence either way.


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