The zoologist Jordi Casamitjana explains why vegans do not eat molluscs such as oysters, mussels, or snails, despite some people claiming they can.
It probably was my first act of animal liberation.
When I was a child, I used to free snails who had been collected after a rainy day and kept starving for a couple of days before being cooked alive. I grew up in Catalonia, you see, and there, like in France, snails are a traditional dish. Even if I was a meat eater — like everyone else in my family — I never ate any snail, as the idea of it always repulsed me. Instead, when my parents were not looking, I would open the bag where the snails were kept, and let them go free. I never thought much about it. It was quite instinctive for me to do so with snails — I wonder if I had done the same with chickens had I been brought up on a farm instead of a city.
I did eat squid, octopuses, and mussels (also a traditional food of Mediterranean countries), but I never tasted oysters because I also found the idea repugnant (they looked like snot to me). Many decades later, when I became an ethical vegan in 2002, I stopped eating any animal, including all molluscs (the scientific term for all the animals I just mentioned).
I say naturally because it would not make sense to avoid eating animals such as pigs, cows, chickens, fishes, lobsters, or turkeys, but still eat molluscs, right? And yet, some people who claim they are vegans do that. They still eat molluscs such as oysters or mussels (who belong to the sub-group bivalves inside molluscs) and call themselves Ostrovegans or Bivalvegans — I met a couple of them. For me, nobody who has access to nutritious plant, fungi or algae-based food but willingly eats any animal can be called a vegetarian, let alone a vegan, but it seems that some do, and try to rationalise it with all sorts of arguments.
As a zoologist specialising in animal behaviour (therefore, an ethologist) I can see how many of such arguments do not hold water, despite they sound scientific and biological. Unfortunately, far too many vegans with little knowledge of zoology are falling for these and are persuaded that perhaps they should reconsider their rejection of animal products, and start consuming some (becoming what some call post-vegans).
I think it would be worth debunking such arguments and explaining why vegans should not be eating any mollusc, not only octopuses and squid, but also snails, mussels, and oysters.
This is not a trivial issue only discussed in vegan echo chambers. It has been debated all over mainstream media, with headlines such as “Are Oysters Vegan?”— probably prompted by plant-based chefs who want to increase their revenue by selling some luxury food. Therefore, I feel I need to use my expertise and address it.
What Are Molluscs?
Molluscs are a group of animals who do not have a spine, so are part of what we call invertebrate animals. The term “mollusc” is the British English version (in American English it is spelt “mollusk”) of the scientific term Mollusca, which is the second-largest Phylum of invertebrate animals. It has around 85,000 species, most of them aquatic. Modern molluscs have a soft body, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body, and a big muscle called “foot”. The most well-known molluscs are snails, slugs, mussels, clams, oysters, scallops, cockles, octopuses, squid, and cuttlefish.
They are sub-classified into seven Classes: Gastropoda (single-shelled, like snails, or without shell, like slugs), Bivalvia (two-shelled, like clams or mussels), Aplacophora (solenogasters), Monoplacophora (segmented limpets), Polyplacophora (chitons), Scaphopoda (tusk shells), and Cephalopoda (nautilus, squids, cuttlefish, and octopuses).
Perhaps the Class of molluscs that has shown the most human-like intelligence is the Cephalopoda (octopuses and squid), a name that means “head-footed”. These are exclusively marine molluscs who have prominent and large heads, sophisticated eyes, and tentacles. The foot has become a set of arms around the mouth. The shell is either internal or absent, except in the species of the sub-class Nautiloidea (Nautilus).
Gastropoda (snails and slugs) is one of the most well-known Classes, and the name means “stomach-footed” molluscs. These have a developed muscular foot that is used for crawling. They have a head with eyes and tentacles, and except for the slugs, all gastropods produce an outer shell which is spirally wound. They live in marine environments, freshwater, and on land.
The class Bivalvia (mussels, clams and oysters) is named because they possess two shells (“Bi” means “two”), but they are also known as Pelecypoda (meaning “hatchet-footed”). They are bilaterally symmetrical, the head is greatly reduced, have no tentacles but a foot that sometimes can be seen sticking out of the shelves (called “valves”), which are joined by hinges and other structures to open or close them. All species are aquatic.
So, all molluscs are animals, and this is why vegans do not consume them. A vegan is someone who follows the definition of veganism as defined by the Vegan Society that created the term in 1944, which states “veganism is a philosophy and way of living which seeks to exclude — as far as is possible and practicable — all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals.” If you ask the Vegan Society today whether it is OK to consume oysters, for instance, they will be unequivocal about it: “Veganism is about rejecting the notion that animals are food or products, therefore we don’t view oysters as something to eat,” said a Vegan Society’s spokesperson to the Guardian in 2019.
Molluscs are animals, so vegans also seek to exclude all forms of exploitation of molluscs, including taking them from their natural habitat, breeding them, selling them, trading them, killing them, cooking them, or consuming them, because unless one is trapped in a desert island without food or edible vegetation, it will always be possible and practicable to eat something else.
That is the most basic reason why vegans don’t eat molluscs of any type (including mussels and oysters). There are more reasons, though.
All Molluscs Are Sentient Beings
What is sentience? In a nutshell, sentience is the ability to have positive and negative experiences, which requires at least two things: firstly, senses to perceive sensations from stimuli coming from the environment, and, secondly, a nervous system to process such sensations and translate them into experiences which allow the animals to react accordingly, depending on whether they are negative or positive (i.e. fleeing from an adverse environment or moving towards a source of food). They do not need to be deep or complex multi-layered experiences, and they do not need to be reached via high-level cognitive processes (from what some call “consciousness”). Simple experiences such as the equivalent of the thought “I don’t like this, whatever this is” will do.
If sentient beings are capable to move, they can use this capability to react appropriately to their experiences, and in consequence, live longer and reproduce more. So, the cocktail you need to produce evolutionary valuable sentience is having senses, a nervous system, and muscles (capable to contract on command). If you have these three things, with time, you are likely to evolve into a lineage of sentient beings, because once you have attained sentience, you will become better at dealing with the environment, and therefore you will have an advantage over non-sentient beings.
On Earth, early members of the Animal Kingdom evolved the three essential components of sentience, and once they did that, most of their descendants maintained their sentience. This is why all Eumetazoa (true animals) we see today (that include all vertebrates and all invertebrates except the sea sponges) are sentient beings.
Even bivalves? Yes, even them…but this is not what ostrovegans would make you believe. They will tell you that mussels and oysters are no longer sentient, so this is why they consume them. Well, let’s see if that is true. Let’s check if bivalves have senses, have a nervous system, and can move with muscles.
First of all, all bivalves have senses. In particular, all have mechanoreceptors (touch) and chemoreceptors (taste). They also have statocysts which help them to sense and correct their orientation, and light-sensitive cells that can detect a shadow. Some have actual eyes, such as scallops, who have more complex eyes with a lens, a two-layered retina, and a concave mirror.
All bivalves also have a nervous system. They don’t have a central nervous system, but we know this doesn’t matter if they still have enough interconnected neurons spread in different parts of the body. Their nervous system consists of a nerve network and three paired ganglia (groups of neurones packed together), named cerebral, pedal, and visceral. The cerebral ganglion innervates the labial palps, anterior adductor muscle, anterior part of the mantle, and the sensory organs. The pedal ganglion, which innervates the foot, is absent in oysters, who have reduced feet, and the pair of visceral ganglia innervate the gills, heart, posterior adductor muscle, and posterior part of the mantle.
There is evidence that these nervous systems allow for the feeling of pain. Research has found endogenous morphine (a natural analgesic) in specific bivalve’s tissues which appears to be involved in the response to physical trauma. They show altered morphine levels in response to trauma and adjust to changing environmental conditions. Endogenous opioids and opioid receptors have been found in the common mussel Mytilus edilus, particularly in immunocytes. Cadet et al. (2002) studied the edible blue mussel (Mytilus edulis) and found that rapid exposure to cold temperatures from room temperature altered ganglionic opiate processes, suggesting that opiate signalling is involved in this organism’s response to thermal stress. In other words, if it gets too cold too quickly it hurts them.
Finally, we have the issue of mobility. Even if many bivalves live a sedentary lifestyle, they do move. Sometimes very slowly (as adult marine mussels can attach and detach byssal threads to attain a better position in the subtract, and can move around to find better locations with what scientists call a Lévy walk ), sometimes at mid-speed (as freshwater mussels have a muscular foot that helps them burrow and move small distances), sometimes quite fast (as sea scallops swimming away from immediate danger), sometimes in their larva stages (as in marine mussels where the round microscopic planktonic larva drifts for three weeks to six months, before settling on a hard surface), sometimes they move parts of their bodies only, which has the same effect of fleeing from a bad situation (such as closing their “valves” to no longer be “out there”), as in oysters and others. If we had shells, we could just close them instead of running away from danger too — it is a genuine reaction of a sentient being who has experienced danger approaching, as its senses perceive it and its nervous system correctly processes the information received.
From an ethological point of view, we can see how bivalves are sentient because they react to negative experiences appropriately. For instance, Leonard et al. found in 1999 that blue mussels in an estuary along the coast of Maine can somehow sense high-predation areas and develop thicker shells, while attaching to their substrate more strongly. Clams and scallops have simple eyes and chemosensory organs located along the periphery of the mantle and if they detect a threat, they escape by swimming in the opposite direction. Also, all bivalves close their shells when they detect danger and open them when they detect food, which is the “sentient” thing to do.
A 2012 study by Wilson et al. showed very interesting results for freshwater pearl mussels, Margaritifera margaritifera. These bivalves often aggregate, presumably to reduce predation risk to each individual to be predated, and the researchers wanted to study if they aggregate more if there is higher predatory pressure. They used dimming light, vibration, and touch as novel stimuli to examine the trade-off between motivation to feed and motivation to avoid predation (closing their valves and ceasing feeding). They found that mussels within a group showed shorter closure times than solitary mussels, consistent with decreased vulnerability to predation in group-living individuals. Also, mussels exposed to the odour of a predatory crayfish showed longer closures than control mussels, highlighting the predator assessment abilities of these mussels (performing better than humans might have done in similar circumstances).
In conclusion, there is no evidence that bivalves are not sentient and there is some convincing evidence that they are. This is why vegans do not consume them as vegans do not consume sentient beings.
Molluscs Legally Recognised as Sentient Beings
Some molluscs are now officially recognised as sentient beings in several jurisdictions. These are, unsurprisingly, the cephalopods. The European Directive 2010/63/EU on the protection of animals used in scientific procedures includes “any live cephalopod”, meaning that over 700 species of cuttlefish, squid, octopuses, and nautiloids are protected from some scientific experiments due to their sentience. Canada, New Zealand, Norway, Switzerland, and some states of Australia also protected them. In 2022, octopuses and Decapod Crustaceans became the first invertebrates to be officially considered sentient beings under UK law.
If you observe the behaviour of an octopus it is very difficult not to conclude they are sentient. The nervous system of cephalopods is the most complex of all invertebrates, as they have a well-developed brain, part of it in the head and the rest spread among their tentacles (so, it could be said they have nine interconnected brains). All cephalopods have a brain-to-body weight ratio comparable to some vertebrates but organised very differently, as 40% of an octopus’s neurons are located in a central brain while 60% are in the arms. Octopus brains have forms of short- and long-term memory, versions of sleep and the ability to recognise people. In experiments, they managed to solve complex mazes and completed tricky tasks to get food rewards. Octopuses are also considered to have consciousness according to the 2012 Cambridge Declaration of Consciousness – a document written by computational neuroscientist and neurophysiologist Dr Philip Low and signed by an international group of prominent scientists gathered at the University of Cambridge, UK.
Octopuses are so obviously sentient to everyone that they have beaten the highly social bees, ants, termites, and wasps in the race to be officially recognised as sentient beings. For me, all insects, arachnids, and crustaceans are also sentient, and their legal recognition as such is one of the goals of the animal rights movement I am part of. Interestingly, in a 2009 study by Chittka & Niven, they showed that moose exhibit 22 distinct behaviours compared to bees exhibiting 59, despite the obviously larger moose brain — and yet, moose are officially considered sentient in all countries, but bees in none.
Octopuses are sentient because they evolved from sentient beings to live very dynamic lives where intelligence pays off. But octopuses, mussels, snails, and oysters all evolved from a common mollusc ancestor who was also sentient, so for bivalves not to be considered sentient, they should have lost this precious quality through evolution. Is this likely?
Evolutionary Reasons not to Consume Molluscs
You may think that any of the molluscs we have been talking about are primitive forms of animals, but they are not. They are as evolved as you and I are. They are not like the Porifera (Sea Sponges), which are very primitive animals (often called proto-animals) that have not developed senses, organs, or nervous systems yet. On the contrary, molluscs have all sorts of senses, fully-formed organs, and they have a nervous system.
All mollusc classes evolved from a common marine ancestor (sometimes called arch-mollusc), who had a single mineralised dorsal dome-like shell, a head with light-sensitive ocelli and s single pair of tentacles, a ventral flat muscular creeping foot, and under the mantle, they have an oesophagus, a stomach, an intestine, digestive glands, a heart, arteries, sexual organs, gills, and a nervous system composed by several ganglia in three different locations (cerebral ganglion, pedal ganglion, and pleural ganglion). So, these ancestral molluscs were sentient beings as they had senses to perceive the environment, a nervous system to process the information from the senses (including cerebral ganglia having a function of a brain) and could move with their large foot closer or away from the stimuli perceived depending on whether the experience was positive or negative.
Through millions of years of evolution, these archaic molluscs evolved into many other molluscs. Some became predators and moved a lot and fast, so their nervous systems become bigger to handle it, their senses became more sophisticated, and their muscles became better adapted to high-speed movement and hunting. These became the cephalopods. Others went in the opposite direction by becoming plankton feeders mostly fixed on a substrate, so their nervous system became smaller, their senses simpler, and their muscles became adapted to opening and closing shells, or slowly digging with the foot. These became the bivalves. A third group became better adapted to breathing air, so their shells and senses changed for a life on land, they moved slowly with their foot, and their nervous system probably did not change that much in size as they had to deal with similar situations than the archaic molluscs had. These became the gastropods.
However, the most important point I like to make is that I believe these three groups retained their sentience, because on Earth, once you evolved into a sentient being, you become better equipped to solve problems, so you will have an advantage over those who have not evolved sentience yet — or somehow lost it. I believe that all the bivalves, although their nervous system became smaller as they did not need to process that much information due to their diet and sedentary lifestyle, did not lose their sentience. I believe this is the case because if some did and others did not, those that did would not be able to outcompete those that did not, so sentient bivalves would have an evolutionary advantage, and these would be the ones we would see today.
You could argue that having sentience has also a cost, as it requires more energy and more demanding physiology, so in environments with very few resources perhaps non-sentient beings would have an evolutionary advantage. After all, we often see animals who have evolved in caves to have lost eyes as they are biologically “expensive” to produce. Dr Diane Fleischmann, an ostrovegan, says that pain is biologically expensive, and feeling pain requires more calories to feed tissue capable of sensing and distinguishing helpful stimuli from harmful stimuli. However, dying for not closing one’s valves because of not feeling pain anymore is even more biologically expensive, as would lead to death. I would argue that it would be biologically “cheaper” for molluscs who already feel pain to retain it once they become sedentary, rather than to evolve a different mechanism for closing their valves or retreating their feet in the presence of danger without the use of any pain (they would have to evolve a completely different system to do that, which makes no sense to me).
Once you are sedentary, reducing the size and number of senses may save resources, as these are arguably more anatomically and physiologically complex than the ganglia. But if the senses can still send some information, the nervous system will be able to process it correctly if its sentient quality has remained intact. Therefore, for me, it makes more economic sense to lose senses and muscle first, before losing sentience. A bivalve with an intact sentience functionality but with smaller ganglia, as there is less information to process, would have an advantage over one that lost “computational” power first before losing senses and muscles.
But if there is a simplification of anatomy, the question is why. It is possible that plankton-eating molluscs would not have evolved to be sedentary because their environment was poor in resources. On the contrary, the abundance of plankton in one location may have been the reason to stop moving around looking for food, so there may not have been that important to simplify anatomy for “economic” reasons because there would be enough food to afford not to lose anything. Animals that, through evolution, lose senses are often parasites, cave animals, or animals living at the very bottom of the oceans, but most bivalves live on shores or shallow waters where biodiversity is high (and therefore food is abundant). The evolutionary pressure to reduce organ size might have been driven by the need to fit the entire body inside the shell so it can be protected from many predators, not as a response to fewer resources. It would make sense, then, that the part of the nervous system that is reduced is the one needed to move many muscles, not the one needed to have experiences that would allow avoiding predation. This hypothesis is consistent with what we find in oysters, perhaps the most sedentary of all bivalves, who have lost the ganglion that deals with the movement of the foot, but not the cerebral ganglion.
The Precautionary Principle
We do not know whether the evolutionary reduction of neurological activity in bivalves has led to the loss of sentience. However, we cannot assume that sentience was lost just because the nervous system became smaller, because we do not know the minimum number of neurons, and the minimum number of connections among neurons, needed to generate sentience. The sensible thing to do, especially for a vegan who is not an expert on bivalve neurophysiology, is to apply the precautionary principle, and assume they still have sentience. Someone who assumes the opposite, not only would be in breach of fundamental principles of veganism (such as the belief in the sentience of all animals and the avoidance of speciesism) but would be either an irrational supremacist with severe cognitive dissonances or a carnist propagandist (perhaps with a vested interest in the seafood industry or any other animal exploitation industry that feels threatened with veganism).
It would be handy if there was anything in the bivalve’s anatomy that could point us toward the conclusion they have not lost sentience. Well, I think there is. If sentience would disappear once becoming sedentary, you would see the nervous systems disappear until they would not be any ganglia left, just scattered nerves, with very few neurones. And yet, we still see the nervous ganglia in all bivalves today, and even more, we still see the cerebral ganglion (cerebrum means brain). And it is not that small. It has been estimated that a lobster (another officially recognised sentient being) has about 100,000 neurones, a sea slug has 18,000 neurones, a pond snail has about 11,000 neurones, and a clam has around 10,000 neurons. So, not much difference between a snail and a clam, right? After all, some nematode worms, who clearly move around and go hunting for other creatures, only have about 400 neurons. All this should be sufficient to, at least, give the benefit of the doubt about whether bivalves have lost all sentience (one of the most evolutionary valuable characteristics an animal can have).
In Molluscs, the nervous system may not be divided into a central nervous system and a peripheral one as we see in vertebrates, but would be more scattered through the body, and although you see a clear brain in cephalopods compared with the other molluscs (as they need it due to their lifestyle) you still see a nervous system in all the others, including bivalves, that still have ganglia (including the cerebral ganglia) that have a similar brain function, but for a simpler life. Scientists do not define sentience based on having a single brain or a central nervous system because, as octopuses have proven, you can achieve sentience with a nervous system that is spread in different parts of the body.
Those who claim it is OK to eat mussels and oysters because they do not feel pain or do not have a central nervous system are often not scientists with any knowledge of bivalve biology or sentience. They like to believe some scientists and disbelieve others to confirm their biases. I am a scientist and say it is more likely bivalves are sentient than the opposite, and yet, I bet that some ostrovegans with no scientific background at all will say I am wrong and criticise this article based on what they heard other scientists might have said. Why believe them and not me — or those other scientists I mentioned in this article? Many scientists still claim fishes do not feel pain, so should these ostrovegans become pescatarians then? These scientists question the research that found opioid reactions in fishes suggesting they can feel pain, but these are also the reactions found in bivalves also suggesting they feel pain, and yet the same type of studies are judged differently depending on whether they confirm or deny the ostrovegans’ biases.
Although I am a zoologist, I am not an expert on bivalves or their neurophysiology, but I am an ethologist, so my expertise is in animal behaviour. In the same way I gave expert testimony as an ethologist before the Catalan parliament to debunk the claims of some Spanish veterinarians working for the bullfighting industry who said that bullfighting bulls do not feel pain when tortured in bullfights, I can give testimony here that, as an ethologist, looking at the behaviour of bivalves, I can say they behave like sentient beings.
Even if we were not able to prove molluscs are sentient beings by making convincing experiments with them studying how they react or whether they feel pain (some of such experiments would be unethical, by the way), we should conclude that, due to their natural behaviour in the wild, their anatomy, and their evolutionary history, there is no evidence that any of them are no longer sentient beings.
On the other side, the sea sponges are not molluscs, but proto-animals from the group Porifera, which do not move at all or have any nervous system, so they are unlikely to be sentient as they never evolved from any sentient being. Plants, fungi, and algae don’t have senses or a nervous system either, so are also unlikely to be sentient as they never evolved this complex trait.
This is why vegans consume plants, algae, and fungi, but they do not consume molluscs. They do not consume them because they are animals and are sentient beings who evolved from other sentient beings, as we did.
Why Vegans Don’t Eat Bivalves
Even ostrovegans know they are not really vegans. Some are honest enough to admit it. In 2020, Kelsey Piper, a journalist from Vox, dared to ask the question “Are you personally vegan?” to one of the most influential animal rights philosophers who most people assumed must be a vegan, the utilitarian Peter Singer. He replied the following: “Strictly speaking, no. For example, I don’t think that bivalves — mussels and clams — I don’t think they can suffer, so I eat them. I would certainly eat cellular-based meat, once it was available. And I’m not really strict about avoiding free-range eggs.” So, he is not even a vegetarian, and his egg consumption would not even allow him to claim the ostrovegan title.
Some would be less honest, though, and even if the definition of veganism is clear in terms of avoiding all animals — not all sentient beings — they insist on calling themselves vegan even if they eat some animals. It seems, though, that there are several types of eating habits among the ostrovegan community. Some only eat oysters because they are the most sedentary of all bivalves, some only eat oysters and mussels (because for being sessile they think they are not sentient), whereas others also include clams and scallops (because they don’t care if they move around). Some may also eat snails because they may not consider them sentient either. Eating a mussel but rejecting a clam is like eating a pig but rejecting a dog — it cannot get more speciesist than that.
Why stop there? If they would only reject animals whose sentience has been officially recognised, they would then be happy to consume the majority of animals on Earth. All invertebrates except cephalopods and decapod crustaceans, which means, all the insects, spiders, centipedes, scorpions, jellyfish, sea urchins, worms, etc. Many human cultures eat insects (in the Congo, Cameroon, Uganda, etc.), sea urchins (in Japan, Corsica, etc.), and snails (in France, Catalonia, etc.), and yet, I bet that many self-defined ostrovegans would not touch any of these. They would not eat foxes, badgers, possums or rats either, even if they are dead and therefore no longer sentient beings, and they died in the wild by accident (the so-called road kill). They would not eat the bodies of their best friends either (canine, feline, or human) who died by natural causes outside any exploitative industries, despite their bodies not being sentient anymore and they were not killed to be eaten.
If we are honest, ostrovegans’ culinary choices have little to do with sentience or the knowledge about who is sentient or not, but with an aversion to the vegan philosophy and its uncompromising ethical stand. It has nothing to do with the necessities of a healthy diet, but with the laziness of an unhealthy habit (seafood contains cholesterol and water contaminants, by the way). It has nothing to do with being rational and coherent, but with being irrational and inconsistent. For ostrovegans, sentience is just an excuse for the unjustifiable.
For proper vegans, on the other side, their response could not be more cogent: Vegans do not eat squid because they are animals; they do not eat snails because they are animals; they do not eat clams because they are animals; and they do not eat oysters because they are animals. And vegans believe all of these are sentient beings, not only because the precautionary principle advises doing so, but because there is no evidence that any of these animals are not sentient, and there is some convincing evidence that they are.
Vegans do not eat bivalves because bivalves are molluscs, and molluscs are animals, part of the animal kingdom vegans so much care about. Vegans exclude the exploitation and cruelty of any member of this kingdom, no matter who they are, and no matter if they have proven how sentient they are.
The first lesson for a hypothetical vegan citizenship exam should probably be this one:
“Vegans don’t eat animals.”