August 03, 2010

Do slugs feel pain?

September 27th, 2009

Anyone who has, in the last few weeks, set foot in the kitchen of my house in Leamington will testify to the fact that I am suffering from a serious slug problem. Not all too long ago, I stepped on one of the little bleeders in my bare feet as I went downstairs to make a cup of coffee first thing in the morning – a most unwelcome wake-up call! Only after a fusillade of obscenities (and lengthy, OCD-style feet washing) did I realise, to my horror, that there were no fewer than eight, yes EIGHT, further slugs sliming around my kitchen. Traumatised and utterly disgusted, I went on a revenge-driven, murderous killing-spree with the sodium chloride.

I've never done that before; normally if anything like this were to happen while I was living with my parents, I would have just angelically requested of my dad that he remove the unwelcome guests for me, but alas, I was all alone in Leamington and had no choice but to do the deed all by myself. My kitchen is now littered with shrivelled little corpses sitting in lovely puddles of rusty-brown fluid. Revolting, I agree.

I must admit, even though they had been causing me considerable distress, I did feel rather barbaric as I watched the slugs writhe around for several minutes following administration of the salt treatment. It looked painful; death by dehydration seems like a pretty unpleasant way to die! But then, in an attempt to alleviate my contrition I tried to tell myself that it killed them reasonably quickly and they didn't suffer for long. Then I began to wonder... do they actually suffer at all? Do slugs and other such gastropod molluscs actually have a nervous system that is sufficiently developed to generate the sensation of pain as we know it? I was so curious that I felt compelled to do a little bit of research.

The nervous system of the giant garden slug, Limax maximus, is not, I confess, something I know a great deal about, and nor it seems does the rest of the general scientific community – it's not exactly a hot-potato. I thought that slugs and other similar creatures would be really evolutionarily primitive, having little or no capacity for generating synchronised responses upon stimulation, but I was completely wrong.

Higher invertebrates – some worms, flies and our friend Limax – have quite highly developed nervous systems, believe it or not – only a few notches down the evolutionary ladder from ourselves. They have highly developed sensory organs which send nerve impulses along sensory neurones to clusters of neurones in the head. These are called central ganglia, and are essentially a very primitive brain. Information is then relayed to muscles in different parts of the body through a nerve cord (not dissimilar to the vertebrate spinal cord) that runs from head to tail of the animal, and allows changes in behaviour. So, actually, the nervous system organisation in these invertebrates is actually rather similar to our own. Not great for the ego, eh?

But, back to the point, this fundamental ability to respond to outside environmental conditions is absolutely essential for the survival of the organism. It's a basic, cardinal function of virtually all organisms on earth – certainly all those I can think of. Even bacteria, arguably the most primitive living organisms on earth, have mechanisms for moving away from harmful chemicals. The reasons for these systems are obvious: to prevent damage and/or death.

Slug experiments of this type, well, actually ANY experiments of this type, that investigate slug behaviour and neurology are somewhat hard to come by, but I did find one in which a species of snail, Cepaea nemoralis was exposed to high temperatures (38ºC – not actually that hot, but consider; snails and slugs live in cool places such as under rocks and in crevices (and in my kitchen) and so 38ºC is pretty damn hot for them).

The first thing to happen when you roast a snail is that it retracts into its shell to minimise immediate damage. Secondly, if the snail remains on the hot-plate for more than 30 seconds or so, it will protract from its shell, secrete a thick, insulating, yellow mucous, and display searching movements – very sensible – in an attempt to get to somewhere that is not as hot. These searching movements involve contraction of the foot, (the part in contact with the hot-plate), and repeated turning of the body from side to side.

Think about it: this is a logical escape response. Humans would do exactly the same thing; Ever stepped barefoot onto hot sand on the beach on holiday and rapidly jerked your foot away with a sharp gasp? That's the acute escape response. If the sand suddenly became boiling hot while you were standing on it, I'm sure you would hop around in all directions to try and find the closest cool surface whilst hopping from foot to foot to try and minimise the area of your feet in contact with the sand. (Sadly, we aren't yet able to secrete a thick yellow mucous – one-nil to the snail).

So anyway, how does all this relate to the salty demises of my own invertebrate visitors? Well, when I cover my slugs in salt, I find the exact same behaviours as did the snail researchers. Ok, so they don't have shells to retract into, but the first thing that the slugs do is contract their bodies to about half of their normal length, and curl up at the edges. Then, they begin their characteristic writhing around that I described; moving their gait rapidly from one side to the other in an attempt to find somewhere less salty. Death comes too swiftly to allow the secretion of a mucous, but I bet that if you were to put the slugs on a non-lethal salty surface, there would be a mucous secretion, just like in snails.

So, to summarise so far:

Salt is detected by receptors in the slug's skin, causing sensory neurones to fire impulses to the clusters of neurones that make up the central nervous system. The CNS then sends impulses along different neurones, along the nerve cord to muscles in different parts of the body. The muscles then contract to bring about the escape response and writhing movements. Then they die and I have to clean them up.

Annnyway, back to my original question: do slugs feel pain? Well, I can't actually find any information directly on this topic besides the “Yahoo Answers” replies – call me skeptical, but I can't imagine any of these 'experts' actually know any better than the next fat person. My own feeling is that slugs DON'T feel pain in the sense that we know it, and my reasons for thinking this are thus.

The sensation of 'pain' is not generated directly at the area of damage. In vertebrates such as ourselves, damage stimulates pain receptors in tissues, and electrical impulses are sent to the brain. The brain then integrates and interprets the information, and makes you feel pain in the area that you've damaged. But there's the key point – pain is a feeling that is generated by the brain: specifically, if you're interested, in two regions known as the periaqueductal grey matter and the nucleus raphe magnus.

Remember; slug brains are nothing more than a collection of nerve cell bodies – they're certainly not developed enough to resemble anything like the periaqueductal grey matter or nucleus raphe magnus. This is one reason why I don't believe that slugs feel pain. Their brains just simply don't have the necessary apparatus to generate pain as we know it.

Of note in this discussion is a 2007 review on the degree of pain felt by lobsters when boiled. The review was published by the animal rights group Advocate for Animals, and it reported that: "scientific evidence ... strongly suggests that there is a potential for lobsters to experience pain and suffering,". As far as I was able to see, all of their evidence for such a claim was based on the fact that lobsters “have opioid receptors and respond to opioids (analgesics such as morphine) in a similar way to vertebrates...indicating that lobsters' reaction to injury changes when painkillers are applied”. The latter claim is fair enough, I won't argue with it – I found another, more impartial, study from a Brazilian laboratory that elegantly investigated the effects of morphine on snail escape responses, and did indeed find that morphine subtly changed the snails' reactions to injury. (Whoever thought of the idea and why somebody would bother wasting good morphine on a snail is beyond me, but it did show up some quite interesting results) In case you were wondering, lobster and snail nervous systems are highly comparable – I checked.

Anyway, while this study confirmed that morphine has some kind of effect on the snails, my objection to the AforA review is this: It is not scientifically sound to make such a definitive conclusion based on only one piece of flimsy evidence. Yes, the morphine clearly had an effect, but who's to say that the morphine had a painkilling effect? I see no solid evidence that it did. The snail certainly didn't describe it's feelings. Just because morphine has a painkilling effect in humans does not mean that it does in invertebrates, even if the same receptors are involved.

Amusingly, I am reminded of a comment in Private Eye magazine from several years ago which played on a quote by Tony Blair with regards to, I think, the Iraq War. “If I had lied to the public then I would have resigned. Therefore, because I have not resigned, I couldn't have lied!”. The Eye has here perfectly highlighted the same type of laughable illogic that fuels these ridiculous claims by biased researchers who only ever find – surprise, surprise – exactly what they want to find, even when faced with overwhelming evidence to the contrary. But don't get me started.

Anyway, my opinions still stand. Personally, I don't believe that slugs do feel pain; certainly not in the way that we know it, but maybe in the future some evidence will come to light that will prove me wrong. Certainly at the moment no solid evidence exists that proves that vertebrates do not feel any sensation at all – maybe they do! But certainly, not in the way that we know it. At least, I can sleep safe in the knowledge that I did not cause the slug the most unbearable agony that I initially thought I had.

You might also be pleased to know that the slug problem has now resolved itself, almost as abruptly as it arose!

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