Snails and bivalves: a discussion of indicators of sentience

There are few studies on bivalve and gastropod sentience. This is unfortunate because it leaves us with little evidence that can guide our actions towards these animals, although if any of this research is invasive then it may be good that it has not been done. In this article we will address the limited scientific literature about bivalve and gastropod sentience. We will argue that there are no conclusive reasons to deny that they are sentient.

Bivalves are a grouping of aquatic animals in the mollusk phylum with a soft body enclosed in a hard, hinged shell.¹ The most commonly known types of bivalves (each of them including several families of animals) are clams, scallops, mussels, and oysters.²

Gastropods are a different grouping of mollusks, which are also soft-bodied animals. Snails are gastropods (all gastropods are either snails or slugs), which can include many different types of both land and aquatic gastropods partially or fully enclosed in a hard shell. They range in size from less than an inch to up to 15 inches.³ The word snail is a colloquial term that refers to gastropods with shells and slug is a colloquial term to refer to gastropods without shells. While for the purposes of this article we will only discuss snails, everything said about them applies to slugs as well.

Bivalves and snails are used by humans in large numbers, mainly for food.⁴ Methods vary, but often mussels and other bivalves are steamed alive “until they open,” clams are eaten raw, scallops are seared in pans while they are still alive,⁵ and snails are starved and then boiled alive.⁶ In addition, snails are sometimes used in toxicology testing⁷ and for the production of cosmetics and personal care products. Their mucus is extracted through the process of “milking,” which consists of putting salt or acid on a snail so the snail secretes slime as a defense mechanism. This leads to extreme dehydration followed by death for the snail. If these animals are able to feel pain, milking could cause them great pain, comparable to experiencing salt being poured on a full-body open wound.⁸ This is in addition to the agony they might feel from dying of dehydration.⁹

Moreover, bivalves and snails in the wild are harmed in many ways due to factors such as lack of food and water, disease, weather conditions, injuries, and other threats to their physical safety. In many situations it would be possible to help them. This means that if they are sentient, by not helping them we’re allowing many forms of suffering that we could prevent.

In many cases it is thought that it is not morally problematic to hurt them, because these animals are not commonly regarded as morally considerable. Many of those holding this view have a similar attitude towards most other animals. Some people, however, show greater disregard for animals like bivalves and snails. There is a tendency, even among many animal advocates, to give consideration only to animals that are most similar to humans, even though other animals are also sentient, which means they can also be harmed.¹⁰

If bivalves and snails are sentient, then there are great wrongs being done to them on a large scale. We will now examine the evidence of bivalve sentience.

Evolutionary considerations

Sentience is expensive neurologically and metabolically, meaning that it requires a great deal of brain power and space for supporting nervous system structures. So, without an evolutionary reason to retain this trait, it is unlikely that it would be retained in species over time. The brain is subject to the same selective pressures as other traits. Therefore, when energy and resources are limiting and demands on neural processing are reduced, brains would be expected to get smaller.¹¹ Therefore, if sentience (including the ability to feel pain) is functionally unnecessary for animals such as bivalves and snails, we should expect them to evolve to become non-sentient.

It is theorized that the evolutionary function of pain is to cue animals to react to harm, usually through moving away from the source of harm.¹² For example, if an object is very hot to the touch, pain will guide you how to carefully handle it without burning yourself. Additionally, if you do burn yourself while handling it, you will remember that experience to avoid repeating it. While reflexes are basic reactions that happen prior to conscious reflection, pain allows for more nuanced reactions and long-term learning in order to avoid harm. Therefore, if an animal doesn’t have the ability to move out of harm’s way, there might be no evolutionary reason for the ability to feel pain.¹³

Many argue that because bivalves are not able to move independently, it is likely that they do not experience pain. However, scallops, file shell clams, and the larvae of many bivalves do in fact swim.¹⁴ In addition, as we will discuss, bivalves have the ability to close their shells to avoid sources of harm, and feeling pain (which requires sentience) could allow them to do so in appropriate circumstances.¹⁵ Gastropods are able to crawl slowly and in some cases to swim. So the argument that they can’t feel pain because they can’t move is based on an inaccurate claim.

A related argument against the sentience of bivalves and gastropods is the lack of complex senses, like eyes that can perceive detailed objects. Gastropods are able to sense light through their eyes, and some bivalves have simple eyes that are able to detect changing patterns of light and motion, but they lack well developed eyes like those of other mollusks such as octopuses and squids.¹⁶ Snails navigate primarily through their sense of touch and their sense of smell.¹⁷ Therefore the sensory abilities of bivalves and gastropods are probably not very complex.

There’s another general response to arguments against sentience based on evolutionary considerations. Bivalves and gastropods are both mollusks and are therefore evolutionarily related to clearly sentient mollusks like octopuses and squids. This means they have some traits in common, though we can’t know for sure which traits. Traits are conserved as long as they are evolutionarily valuable, so it is possible that sentience is a conserved trait present in all mollusks. However, this evolutionary argument is not strong evidence, because even though these animals belong to the same phylum (Mollusca), the evolutionary distance between them could be large enough for sentience to be present only in some of them.

Nociception and response to noxious stimuli

Evolutionary considerations are one type of indicator that animals may be sentient. Another one is nociception. Nociception refers to the nervous system’s detection of aversive stimuli that an organism encounters. Nociceptors are sensory neurons that enable nociception by detecting noxious stimuli where they are located in the body.¹⁸ Many anesthetics rely on blocking nociception to mitigate the noxious stimuli signals that reach the brain.¹⁹ Whereas pain is a conscious experience in the brain, nociception is the lower level detection of noxious stimuli, which can be translated into pain in an animal capable of experiencing it. Nociception may be required in order for pain perception to take place by allowing the organism to detect noxious stimuli. However, nociception is not itself pain, so the presence of a nociceptive system does not by itself demonstrate pain. For example, an animal might have nociception in order to enable a purely reflexive response, but not be capable of experiencing anything.

There is substantial evidence that gastropods have nociceptors.²⁰ Most notably, the nociceptive system of Aplysia, a giant sea slug, is very well-documented.²¹ There is also evidence that land snails (Cepaea nemoralis) and sea slugs (Tritonia diomedia) have nociception.

Gastropods and mollusks show evidence of reacting to noxious stimuli. It has been suggested that snails might have opioid responses to relieve pain. Only sentient animals can feel pain, so a response resembling pain relief suggests sentience. It has also been shown that mussels have aversive responses to cold water,²² and produce morphine (possibly to regulate the amount of pain that they experience, though morphine might have other functions) in response to muscle damage.²³ Fear is another type of negative response that only sentient animals can experience (the term “experience” indicates conscious awareness, or sentience). Scallops have been found to show increased heart rates in response to cues from predators.²⁴ Increased heart rate is a possible symptom of fear. While this is not proof of these animals’ abilities to feel pain and fear, it indicates that they might.

The nervous systems of bivalves and snails

There are two main indicators of sentience related to the structure of animals’ nervous systems: the extent that they are centralized and the neuron count. The degree of centralization of the nervous system is perhaps the most important evidence concerning the sentience of an animal. The most highly centralized nervous systems include brains. Gastropods and snails do not have brains, but their nervous systems are centralized to a certain extent, and they have analogous structures, which are several pairs of ganglia (clusters of neurons) connected by a nerve cord. The centralized organization of these structures may be sufficiently complex to perform at least some of the functions that brains do. At this point, the available knowledge about this cannot confirm or deny whether they can support sentience.

The neuron count of an animal can suggest whether or not that animal has enough complexity to support consciousness. We know sentience comes about from the interaction of neurons but we do not know how large a number might be required. Smaller animals do not require as many neurons as larger animals to perform the same functions. This is because smaller nervous systems are more compact. There are fewer neurons involved in each neuron circuit, so smaller animals can perform operations more quickly. In addition, many of the neurons of larger animals are needed to facilitate their much larger bodies and their much larger sensory fields.²⁵ Because of this increased efficiency, the minds of small animals may be able to support sentience with a much smaller than expected number of neurons.²⁶ Small clams have around 6,000 neurons.²⁷ Garden snails have around 60,000 neurons in their whole nervous system.²⁸ Aplysia have around 18,000 neurons throughout their nervous system.²⁹ For reference, common octopuses have half a billion neurons throughout their nervous system,³⁰ and dogs have 2.25 billion.³¹

Gastropods typically have five pairs of ganglia, with the two cerebral ganglia constituting a brain. Aplysia are also known for having extremely large neurons, the largest in the animal kingdom in absolute terms. The size of these neurons allows them to do more with fewer neurons, because huge neurons can innervate large areas, control multiple behaviors, and increase the speed of behavioral responses.

Other possible indicators of consciousness in bivalves and snails

There are some studies that explore the mental abilities of bivalves and snails such as learning, memory, and altering behavior in differing circumstances. These three abilities are also found in the minds of clearly sentient animals, suggesting that the underlying mechanism by which this happens in sentient animals (sentience) is also present in bivalves and snails. Where there is outward similarity, there may be similarity in the underlying explanation. But just as with the studies on physical pain, these findings constitute only weak evidence for sentience.

One study showed that terrestrial snails were able to learn by association. Researchers found that the snails grew less interested in a formerly attractive food after encountering it together with a repulsive substance.³² It has been shown that Aplysia sea snails exhibit long term sensitization. This means that the snails can become sensitized to noxious stimuli after they are exposed to it and they react more strongly to subsequent exposures.³³ Snails are also capable of this form of learning.³⁴ These types of learning (sensitization and learning by association) might be some of the simplest forms of reaction to noxious stimuli, with even single celled organisms arguably showing some level of this.³⁵ These forms of learning constitute only weak evidence of sentience at best because they can be explained by simple mechanisms.

Mussels are able to alter their responses according to differing danger levels. When they face a perceived danger, such as the smell of a predator or some sudden variation in their environment, they close their shells, even if this makes it impossible for them to eat. Solitary mussels have been observed protecting themselves, and consequently refraining from eating, for longer than those who are in a communal tank. Thus, it seems as though grouped mussels sense a lower risk of harm. This indicates an ability to balance and trade off different needs and risks (such as threat of predators, significance of group size, and demand for food) against one another and adjust their behavior based on context.³⁶ Reflexive responses, such as an automatic kick from a hammer blow at the knee in humans, can happen unconsciously but more nuanced responses to noxious stimuli may require consciousness. It is unclear if this behavior in mussels more closely resembles reflexive behavior or behavior that requires consciousness.

Garden snails balance different biological needs for eating, mating, and avoiding predators. They maintain home territories and will return to the same place to sleep or hibernate.³⁷ Though balancing needs and navigating do not necessarily indicate sentience, it might suggest they have an inner world that has some complexity and this could indicate an adaptive rationale for them to be sentient.

Conclusion

There is still uncertainty about bivalve and snail sentience. It is likely that we will be unable to definitively answer the question until we solve what has been called the hard problem of consciousness, which is basically the question of how consciousness arises. What we do know is that there is an enormous amount of possible suffering at stake. Given the possible sentience of these animals, the most prudent course of action appears to be to err on the side of caution and not harm them.³⁸

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Notes