For years, I have taught what I was taught in college, the central maxim of invertebrate research: "No brain, no pain." You must have a brain in order to comprehend the notion, "Ouch- this hurts!" Yes, you can respond to stimulus without a brain, much as a bacteria would- it is too bright, so it moves away, it is too hot, so it moves towards the cold. But it requires a bit more than a collection of ganglia to process the concept of pain.
We all have pain receptors as one of our nine senses. (Smell, Taste, Sight, Hearing, Touch- and separate receptors in the skin for Heat, Cold, Pressure, and Pain. It's just that five of our senses are in the same organ- skin- so we lump them together.) Without those pain receptors, we wouldn't know pain. Those rare individuals, such as some lepers, who lose the ability to sense pain realize at their own peril how valuable a sense this is. Much as we might like to get rid of it, it helps us avoid problems like losing a limb because we didn't know how much it was being damaged by the acid.
But it is more than pain receptors. All of us (capable of reading this blog) also have brains, and rather impressive ones at that, capable of comprehending elementary thoughts like those here, or the sublime, like, "Ouch! This hurts!" You don't need to be self-aware to comprehend this, but you do need to be able to experience subjective reality, much as your dog or cat do. Your dog doesn't sit there contemplating his existential existence- he does know when something is happening to him.
For a long time, we have found this not to be true of the invertebrates. Excluding the amazing cerebral Cephalopods, invertebrates just don't really have brains. They have collection of ganglia- nerve cells- but nothing really complex to call a brain. And therefore nothing capable of understanding subjective reality, such as pain.
Thus, when scientists work with invertebrates, we have some greater amount of leeway. When pithing a frog for dissection, one must be very conscious of the pain inflicted. You do so only for a greater good, such as helping students understand how a frog works, and you do it quickly, to minimize the pain. This issue isn't present when vivisecting a crawdad, or an ant, or a snail. It is a great responsibility and burden to inflict pain on an animal, and you do so only when necessary. That necessity is removed when dealing with vast majority of invertebrates.
This is not to say that we can then willy-nilly kill any sort of invertebrate creature we want. For we are psychological beings, some would say with a soul, and when we kill another creature, or inflict pain, or appear to do so, it has an effect on our own souls. Much as a child playing non-stop Halo learns that it is okay to kill others without repercussions, stepping on ants who don't feel pain teaches the child that our actions that appear to harm life can be done with impunity. The lobster sounds like it is screaming when being boiled alive, but it is only the air escaping from between the animal's carapace. It has no brain, so therefore can't feel pain. Yet, that sound of screaming has an effect on the chef or any human, and can lead to a deadening of the emotive center which is the very definition of sentience- our own sentience. It is the effect of our actions on all creatures which matter even when dealing with those lower lifeforms, as evidenced by the clear trend of boys stepping on ant hills, often leading to boys harming dogs and cats, and then to seeing even other humans as mere fodder for their schadenfreude tendencies.
But we clearly distinguish those creatures that we care for for our own mental health, and those we care for because of the mental health of the creature itself. And lest an individual think they do not- that they care for all creatures because they are creatures, that they are vegetarian, or following Buddhist principles- I challenge them to fully apply that concept. Quit eating the yogurt, full of so many bacteria. Quit using antibiotics, killing off those nasty bacteria. Quit living, in fact, for your body is constantly fighting off those bacteria in the inner replay of evolutionary force. This is not merely rhetorical allegory. As pointed out in the introductory paragraph, bacteria sense- but not one suggests they have the ability to feel pain. There is a line, and everyone draws that line, albeit at different points. Even the Buddha did not care for germs in the same way (though of course he could not possibly be aware of their existence). Sensing does not entail sentience, although it is a precondition for it. Where, therefore, do we draw the line?
As I said, at the place where there is a brain. And it can be argued where there is enough of the animal to have a brain, or how much ganglia and brain is needed to comprehend pain. Clearly a virus does not have it, nor a sponge, nor an anemone- and certainly not a plant, all the pseudo-scientific theories out there not withstanding. And for a long time, we have been bolstered by the teaching that the invertebrates, excepting the Cephalopods, also fall into this non-sentient category.
Until now. Until today. For a study has just come out indicating that small hermit crabs also feel pain. They subjected the crabs to mild shocks, not so much that it would make them leave their shells, but enough that, when offered a better, shockless shell, they rejected their old home for the new one. This indicates a step towards not simply responding to stimuli, but actually remembering what is negative- or being aware of one's subjective environment and feeling pain.
More study is certainly needed before final conclusions can be drawn. Technically, if true, it would apply only to hermit crabs, but could be extended out reasonably to other Crustacea. It does not indicate that organisms with less of a brain, such as the adult Tunicate or earthworms, could also feel pain, and one would hope for similar studies on other groups of animals to determine if there is a like response. Similarly, one would like control studies conducted on organisms like jellies and sponges, who obviously have no way to comprehend pain, being completely brainless, to see if they respond in the same way, indicating that the study does not indicate surprising new sentience.
All those caveats withstanding, it brings one to pause for a moment. To consider. As the authors of the study rightly bring up, it has an effect on all those Crustacea we catch and eat. Is the best being done to minimize possible pain as they come up in the net. (No.) Certainly little is done for fish, but we also know that fish have very few pain receptors. If this study is born out, it may, and should, revolutionize the way we approach fisheries management.
But closer to home, it causes me to consider how I have treated all those organisms in the past. Like I intimated above, I have always been careful even with the organisms that don't seem to be sentient, if only for my own sentience. But there is care and there is care. And I can assuredly state that I have not cared for these worms and crawdads and snails with the thought that they were capable of experiencing pain. I have considered this for months now.
And it troubles me greatly.
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