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If you grew up in a temperate climate, you probably spent some time bothering brown garden snails (Cornu aspersum, formerly known as Helix aspersa). I certainly did. Now, as a grown-up (supposedly) expert (supposedly) on the science and philosophy of consciousness, I've decided to seriously consider a question that didn't trouble me very much when I was seven: Are garden snails conscious?

Being an "experimental philosopher", I naturally started with a Facebook poll of my friends, who obligingly fulfilled my expectations by answering, variously, "yes" (here's why), "no" (here's why not), and "OMG that is the stupidest question". I'll call this last response "*gong*" after The Gong Show, an amateur talent contest in which performers whose acts were sufficiently horrid would be interrupted by a gong and ushered off the stage.

It turns out that garden snails are even cooler than I thought, now that I'm studying them more closely. Let me fill you in.

Garden Snail Cognition and Behavior

(Most of this material is drawn from Ronald Chase's 2002 book Behavior & Its Neural Control in Gastropod Molluscs.)

The central nervous system of the brown garden snail contains about 40,000 neurons. That's quite a few more neurons than the famously mapped 302 neurons of the Caenorhabditis elegans roundworm, but it's modest compared to the quarter million neurons of an ant or fruitfly. The snail's brain is organized into several clumps of ganglia, mostly in a ring around its esophagus. Gastropod neurons generally resemble vertebrate neurons, with a few notable exceptions. One exception is that gastropod neurons usually don't have a bipolar structure with axons on one side of the cell body and dendrites on the other side. Instead, input and output typically occurs on both sides without a clear differentiation between axon and dendrite. Another difference is that although gastropods' small-molecule neural transmitters are the same as in vertebrates (e.g., acetylcholine, serotonin), their larger-molecule neuropeptides are mostly different.

Snails navigate primarily by chemoreception, or the sense of smell, and mechanoreception, or the sense of touch. They will move toward attractive odors, such as food or mates, and they will withdraw from noxious odors and tactile disturbance. Although garden snails have eyes on the tips of their posterior tentacles, their eyes seem to be sensitive only to light versus dark and the direction of light sources, rather than to the shapes of objects. The internal structure of snail tentacles shows much more specialization for chemoreception, with the higher-up posterior tentacles perhaps better for catching odors on the wind and the lower anterior tentacles better for odors closer to the ground. Garden snails can also sense the direction of gravity, righting themselves and moving toward higher ground to avoid puddles.

Snails can learn. Gastropods fed on a single type of plant will prefer to move toward that same plant type when offered the choice in a Y-shaped maze. They can also learn to avoid foods associated with noxious stimuli, in some cases even after only a single trial. Some species of gastropod will modify their degree of attraction to sunlight if sunlight is associated with tumbling inversion. In warm ocean Aplysia californica gastropods, the complex role of the central nervous system in governing reflex withdrawals has been extensively studied. Aplysia californica reflex withdrawals are centrally mediated, and can be inhibited, amplified, and coordinated, maintaining a singleness of action across the body and regulating withdrawal according to circumstances, with both habituation and sensitization possible. Garden snail nervous systems appear to be similarly complex, generating unified action that varies with circumstance.

Garden snails can coordinate their behavior in response to information from more than one modality at once. For example, as mentioned, when they detect that they are surrounded by water, they can seek higher ground. They will cease eating when satiated, withhold from mating while eating despite sexual arousal, and exhibit less withdrawal reflex while mating. Before egg laying, garden snails use their feet to excavate a shallow cavity in soft soil, then insert their head into the cavity for several hours while they ovulate.

Garden snail mating is famously complex. Cornu aspersum is a simultaneous hermaphrodite, playing both the male and female role simultaneously. Courtship and copulation requires several hours. Courtship begins with the snails touching heads and posterior antennae for tens of seconds, then withdrawing and circling to find each other again, often consuming each other's slime trails, or alternatively breaking courtship. They repeat this process several times. During mating, snails will sometimes bite each other, then withdraw and reconnect. Later in courtship, one snail will shoot a "love dart" consisting of calcium and mucus at the other, succeeding in penetrating the skin about one third of the time; tens of minutes later, the other snail will reciprocate. Courtship continues regardless of whether the darts successfully land. Sex culminates when the partners manage to simultaneously insert their penises into each other, which may require dozens of attempts.

Impressive accomplishments for creatures with brains of only 40,000 neurons! Of course, snail behavior is limited compared to the larger and more flexible behavioral repertoire of mammals and birds.

Garden Snail Consciousness: Three Possibilities

So, knowing all this... are garden snails conscious? Is there something it's like to be a garden snail? Do snails have, for example, sensory experiences?

Suppose you touch the tip of your finger to the tip of a snail's posterior tentacle, and the tentacle retracts. Does the snail have tactile experience of something touching its tentacle, a visual experience of a darkening as your finger approaches and occludes the eye, an olfactory or chematosensory experience of the smell or taste or chemical properties of your finger, a proprioceptive experience of the position of its now-withdrawn tentacle?

(1.) Yes. It seems like we can imagine that the answer is yes, the snail does have sensory experiences. Any specific experience we try to imagine from the snail's point of view, we will probably imagine too humanocentrically. Withdrawing a tentacle might not feel much like withdrawing an arm; and with 40,000 neurons total, presumably there won't be a wealth of detail in any sensory modality. Optical experience in particular might be so informationally poor that calling it "visual" is already misleading, inviting too much analogy with human vision. Nonetheless, I think we can conceive in a general way how it might be the case that garden snails have sensory experiences of some sort or other.

(2.) No. We can also imagine, I think, that the answer is no, snails entirely lack sensory experiences of any sort -- and thus, presumably, any consciousness at all, on the assumption that if snails are conscious they have at least sensory consciousness. If you have trouble conceiving of this possibility, consider dreamless sleep, toy robots, and the enteric nervous system. (The enteric nervous system is a collection of about half a billion neurons lining your gut, governing motor function and enzyme secretion.) In all three of these cases, most people think, there is no genuine stream of conscious experience, despite some organized behavior and environmental reactivity. It seems that we can coherently imagine snail behavior to be like that: no more conscious than turning over unconsciously in sleep, or than a toy robot, or than the neurons lining your intestines.

We can make sense of both of these possibilities, I think. Neither seems obviously false or obviously refuted by the empirical evidence. One possibility might strike you as intuitively much more likely than the other, but as I've learned from chatting with friends and acquaintances (and from my Facebook poll), people's intuitions vary -- and it's not clear, anyway, how much we ought to trust our intuitions in such matters. You might have a favorite scientific or philosophical theory from which it follows that garden snails are or are not conscious; but there is little consensus on general theories of consciousness and leading candidate theories yield divergent answers. (More on this, I hope, in a follow-up post.)

(3.) *Gong*. To these two possibilities, we can add a third, the one I am calling *gong*. Not all questions deserve a yes or a no. There might be a false presupposition in the question (maybe "consciousness" is an incoherent concept?), or the case might be vague or indeterminate such that neither "yes" nor "no" quite serves as an adequate answer. (Compare vague or indeterminate cases between "green" and "not green" or between "extraverted" and "not extraverted".)

Indeterminacy is perhaps especially tempting. Not everything in the world fits neatly into determinate, dichotomous yes-or-no categories. Consciousness might be one of those things that doesn't dichotomize well. And snails might be right there at the fuzzy border.

Although an indeterminate view has some merits, it is more difficult to sustain than you might think at first pass. To see why, it helps to clearly distinguish between being a little conscious and being in an indeterminate state between conscious and not-conscious. If one is a little conscious, one is conscious. Maybe snails just have the tiniest smear of consciousness -- that would still be consciousness! You might have only a little money. Your entire net worth is a nickel. Still, it is discretely and determinately the case that if you have a nickel, you have some money. If snail consciousness is a nickel to human millionaire consciousness, then snails are conscious.

To say that the dichotomous yes-or-no does not apply to snail consciousness is to say something very different than that snails have just a little smidgen of consciousness. It's to say... well, what exactly? As far as I'm aware (correct me if I'm wrong!), there's no well developed theory of kind-of-yes-kind-of-no consciousness. We can make sense of a vague kind-of-yes-kind-of-no for "green" and "extravert"; we know more or less what's involved in being a gray-area case of a color or personality trait. We can imagine gray-area cases with money too: Your last nickel is on the table over there, and here comes the creditor to collect it. Maybe that's a gray-area case of having money. But it's much more difficult to know how to think about gray-area cases of being somewhere between a little bit conscious and not at all conscious. So while in the abstract I feel the attraction of the idea that consciousness is not a dichotomous property and garden snails might occupy the blurry in-between region, the view requires entering a theoretical space that has not yet been well explored.

The Possibilities Remain Open

There is, I think, some antecedent plausibility to all three possibilities, yes, no, and *gong*. To really decide among them, to really figure out the answer to our question about snail consciousness, we need an epistemically well-grounded general theory of consciousness, which we can apply to the case.

Unfortunately, we have no such theory. The live possibilities appear to cover the entire spectrum from the panpsychism or near-panpsychism of Galen Strawson and of Integrated Information Theory to very restrictive views, like those of Daniel Dennett and Peter Carruthers, on which consciousness requires some fairly sophisticated self-representational capacities of the sort that well beyond the capacity of snails.

Actually, I think there's something wonderful about not knowing. There's something marvelous about the fact that I can go into my backyard, lift a snail, and gaze at it, unsure. Snail, you are a puzzle of the universe, right here in my garden, eating the daisies!

Eric Schwitzgebel

http://schwitzsplinters.blogspot.com/2018/09/are-garden-snails-conscious-yes-no-or.html