Why planarians may hold the key to sentience: about primitive sentient organisms and immortality

By Guillaume Reho @ 2025-10-23T17:55 (+28)

Context

This post is based on one of my PhD dissertation chapters with some twists: it covers the subject with a more casual tone, adds discussions on other very primitive animals, and I emphasize on my personal opinion on this matter rather than using an impersonal scientific voice. I also talked about it during the event Les estivales de la question animale, the replay being available on youtube. The talk was in French, which was another motivation for me to share it properly in English.

The cover image is from here, with credit to Rattiya Thongdumhyu/Shutterstock.

Key takeaways

 

Who are planarians? 

Planarians represent many species from the Tricladida order, which is found inside the Platyhelminthes phylum, so they are flatworms. Do not get thrown off however, because planarians represent the only order of flatworms that are not parasitic, unlike their ugly counterparts (e.g. tapeworms or liver flukes). Trust me, they are small crossed-eye cuties. Most studied species live in freshwater ecosystems like ponds, lakes, creeks, etc. 

Planarians are usually known for their remarkable regenerative capacities[1]. Cut them in half, and each half will grow back its missing parts. A tail ? No problem. A head ? No problem. Their body is filled with pluripotent stem cells, allowing them to regrow entirely from almost any part of their body, even with very little amount of cells. Thus, they are often classified as “immortal” animals (more on that later).

Figure 1: Planarian regeneration. Photos from Ivankovic et al. 2019[2].

Another remarkable trait that planarians have is their nervous system. Unlike the nervous system of annelids (roundworms), other flatworms or most other invertebrates, planarians’ nervous systems occupy a massive amount of space inside their body. Its physiology also shares much more similarities with those of “higher-level” nervous systems (such as ours) than other invertebrates usually do. Here are a few examples: they mostly have chemical synapses rather than electrical synapses, multipolar neurons rather than unipolar or pseudounipolar neurons, extensive dendritic ramifications, highly developed dendritic spines, most of the same neurotransmitters, etc. 

Interestingly, in his book The First Brain[3], biologist professor Oné R. Pagán not only recalls that planarians’ nervous systems show surprisingly many similarities with “higher” animals nervous systems (like those of vertebrates), but also that planarians display one of the earliest form of cephalisation and of specialization into a “central” nervous system versus a “peripheral” one. Planarians (and other flatworms) also show predatory behaviors, which gave them the nickname of the “first hunters”. Hence, planarians not only display similar nervous physiology with much "higher” animals, but they also display relevant evolutionary arguments for the presence of very primitive forms of consciousness and/or cognition that could help them survive and hunt for prey.

Figure 2: Planarian nervous system. Left image: View of the complete nervous system using in situ hybridization and immunostaining, from Nishimura et al. 2008[4]. Right image: Views of the brain using the same techniques focused on specific neurotransmitters, from Ross et al. 2017[5].

About animal immortality

There are a few species of animals that we sometimes consider as “immortals”. The most famous ones are usually jellyfish, tardigrades and starfish; some sharks, turtles, lobsters or axolotls may be mentioned as well. To my knowledge, there is no clear definition of what an “immortal animal” is. If we talk about aging and the senescence of cells, the Animal Aging and Longevity Database identified 8 animal species of turtles, sharks, mollusks, fish and urchin that show negligible senescence. However, these species still display a maximum longevity, one of the most famous one being the Greenland Shark at 507 years, because of predation, diseases, food scarcity, etc. These are really old individual animals that we could see the birth of and the death of. These are not, arguably, immortal, as all individuals die off at some point. They may theoretically live way longer in appropriate conditions, but not indefinitely: their senescence is really slow, but not absent. To this list can be added lobsters, axolotls or any reptile that show some sort of limb regeneration: they may get old and regrow limbs, but they are arguably not immortal individuals. Tardigrades are also in their own category: they are highly resilient to extreme conditions, but not immortal as individuals. 

Where it gets funky is when animals cheat death: when immortality is not about lack of senescence, but about rejuvenating and regenerating whole new individuals from a few stem cells. This is where “true immortality” might be reached. Animals capable of multiplying themselves are much more rare, and most of them are amongst the most “primitive” animals, as the common ancestor we share with them are basically the ancestors that separate animals and mushrooms. These animals are sponges and cnidarians (jellyfish and polyps)[6]. However, there are some slightly “higher” species that are also able to regenerate a whole new body from only a part of it: some starfish species and planarians. 

All those species are able, when cut in half (for example), to regrow two identical individuals. In the case of cnidarians, only the polyps are able to do that as they represent the early asexual phase of the reproductive cycle of cnidarians, the late sexual stage being the jellyfish. Some jellyfish species, however, are able to rejuvenate into their early polyp phase, hence their reputation for immortality. Interestingly, one planarian species is also mentioned in the Animal Aging and Longevity Database as having negligible senescence, but they are the only species without a maximum longevity. Here is a thought experiment to understand why:

As portrayed in this 3 year old post on the forum that I dug up, imagine one planarian called Wormy. Wormy has a head, a funny cross-eye look, is gliding around searching for food, and enjoys relaxing under his small rock. Now cut Wormy in three equal parts: one is the head, one is the trunk, one is the tail. As two weeks pass by, you now see three complete planarians gliding around (look back at Figure 1). Is Wormy the former head, trunk, tail, all three parts, or none of them ?

Wormy is a ship of Theseus and immortality is a trick 

The difficulty to answer this question recalls the Theseus paradox: if Theseus replaces parts of his ship, piece by piece, until every piece has been replaced, is it still Theseus' ship ? Then, if Theseus reconstructs another ship using every piece that was removed from its original ship, thus recreating his original ship, is it Theseus’ ship again ? 

Similarly, is Wormy still Wormy if, when cut multiple times, all his cells have been regenerated ? Are all his other body parts reconstructing Wormy as well ? 

You may now notice how these species trick us into thinking they are immortals: they actually create clones of themselves by reproducing asexually (they do this on their own by budding or fissioning[7]). By creating clones, they blur the line of what we consider as separate individuals: we mostly see them as some sort of colony, like bacterias that divide to multiply, and not as individuals. Are the initial individuals still there, only creating copies, or can they still die off ?

Figure 3: Budding hydra, from Wikimedia Commons by SadiesBurrow.

To me, immortality in its common sense does not exist in animals, or at least does not have a relevant biological meaning. As bacterias are not immortal just because they reproduce asexually, animals that reproduce asexually are not immortal either. I guess this may not surprise you very much, but it is commonly misleading. 

Still, the important underlying question is: which characteristics would you use to identify what an individual (still) is ?

What makes Wormy different

When looking at polyps (such as the hydra) or sponges budding, the easy way to define an individual is through morphology: we can clearly see one complete organism that has some sort of arm growing on its side that will create another, new individual. In cases of regeneration however, such as the Wormy case, we can sense an itch on the definition of what an individual is. When talking about polyps and sponges, we may define an individual as a unit, as one organism (i.e. a bunch of cells glued together). But Wormy is not just an individual unit, he is an individual someone, with a name and a personality that perceives life on its own. Wormy has a personality because I gave him one in this example, but the reality is strikingly similar: unlike polyps, sponges or starfish, planarians have a bilateral symmetry, a head with two ocellis that look like eyes, a pharynx that gently pat food before eating it, a curiosity for their environment, a sort of nose that constantly sniffs around, are able to learn, have preferences in their environment, etc[1][8]. Planarians are way more similar to us, and to most other animals that we encounter, than any of the other “immortal” animals. Because Wormy was given a name and a personality, he stopped being an individual unit, and became an individual someone. It may simply be a semantic issue, but it displays how we perceive these animals.

In this sense, I would argue that what represents Wormy is his sentience. As Jonathan Birch defines it[9], a sentient being is a system with the capacity to have valenced experiences, which implies phenomenal consciousness, commonly defined as the “what it’s like” state, and thus a first person experience.

Consequently, to answer the “where is Wormy” question, we would need to know which part of Wormy contains his sentience. Arguably, if we had to search somewhere, it would be in the brain. If planarians are sentient, Wormy would thus probably be the head part, which contains the brain, while the trunk and the tail parts would regenerate brains and produce new sentient organisms, although they would still be genetic clones. But are they sentient, though ?

Why sentient planarians would matter 

Before answering this question, you may wonder why such a question might be worth considering. Planarians are not particularly used for any industry, apart from some research on stem cells or ecotoxicology. They are often considered pests in aquariums, so we might reflect on the way we kill them, but they do not seem to outnumber other worms or insects in the wild, so they might not be a priority. To me, their importance lies in research about sentience itself thanks to their remarkable regenerative capabilities. If such an animal is sentient, then we can observe very specific lesions and regeneration of brain parts that would influence sentience markers. Imagine observing sentience markers appear day after day, while observing precise regeneration of specific brain parts. Such models could allow the study of the fundamental elements of what makes a brain become sentient. Of course, ethical considerations for such studies should be considered as well, but they may give us valuable insights into very primitive forms of consciousness, cognition and decision-making. Plus, research on planarians is usually cheaper than for other animal models, such as insects, as their maintenance is pretty easy (very small size, high reproduction, high resilience, etc). Such an idea is not new: in the 1960s, a new field called “protopsychology” was popularized, which even made it to a Scientific American issue cover, and developed the study of very primitive forms of psychology using planarians. These studies mainly focused on conditional learning and decision-making back then, but could benefit from nowadays standards to study sentience markers. That is, even more so today, as planarians' behavior and cognitive abilities are seriously neglected in favor of regeneration studies that solely focus on morphology, physiology and genetics.

Are planarians sentient? 

Considering the idea that even primitive animals could be sentient, let’s first take a quick look at sentience markers in the other “immortal” animals that we have discussed:

Sponges

Sponges do not have a “proper” nervous system. They display early cell lines that probably developed nervous cells in “higher” animals (which is called the neuroid system), and they do possess some nociceptive receptors from the TRP family that have major roles in nociceptive systems of “higher” animals[10], but the idea of sentience in sponges is as improbable as for plants.

Jellyfish and polyps 

Cnidarians may be considered the most primitive animals with a nervous system. However, their nerve cells are very loosely distributed around their bodies, which forms what is called a nerve net. Without any form of centralization of nerve cells into a brain, or at least ganglions, it would be difficult to consider cnidarians as potential sentient candidates.

Starfish 

Starfish sentience is a tougher question. Starfish are actually closer to us, as Deuterostomes, than most other invertebrates, which are usually Protostomes. This means that they are genetically closer to us than all insects, crustaceans or molluscs, which include animals that we consider sentient. This time, starfish do have a nervous system with a centralization around their center of symmetry. However, this centralization does not resemble a cephalisation, only forming a ring nerve, and does not seem developed enough for us to expect sentience. I found some literature discussing starfish welfare and sentience[11], but it did not seem very convincing to me and I did not dig very deep. Reach out if you know more about echinoderms sentience! 

Planarians

Finally, as mentioned, planarians have a nervous system that seems extremely developed for their small size. Their nervous system occupies a huge volume of their body and is very much centralized in their head: it's a clear cephalisation. They show two huge linked cerebral ganglions that are usually considered as a brain[3]. I will spare us the difficult question of defining what a brain is and if planarians ganglions should fall under that category; just consider that most researchers tend to use the term brain because planarians’ ganglions are very much organized with specialized cell lines and functions on specific nerve layers. Because I would consider planarians legitimate candidates for sentience, let’s look at the evidence.

For this exercise, I will use Birch’s sentience criteria framework[12]: 

The presence of nociceptors, i.e. sensory nerve cells that specialize in the detection and transmission of noxious stimuli, has not been properly demonstrated yet. However, we already know that planarians possess numerous nociceptive receptors (the membrane proteins, such as from the TRP family) that are specifically expressed in sensory nerve cells and that can be specifically tuned[13]. This was a central subject of my PhD and I would assume with high confidence that planarians probably do have nociceptors, even if not properly demonstrated yet[14].

Apart from nociceptors, other sensory neurons have been properly traced. We know that sensory information is integrated in structurally and functionally distinct parts of the brain[15]. We also have some insights into which neurotransmitters are mostly involved for each type of stimuli (mechanical, thermal, etc).

As for criteria 1, the precise paths of nociceptive sensory neurons have not been traced into the brain, so we do not know exactly where nociceptive inputs might be integrated, even if we have some evidence that seems to lead to the brain[16][17]. However, as far as I’m aware, there is not one single animal whose nociceptive system is independent from other sensory inputs, or not integrated at all. It would not make sense to me, evolutionary speaking, to isolate one specific sensory input from the other ones. Thus, while no evidence yet properly proves this point, I would also assume pretty high confidence that nociception is integrated in planarians’ brains as well. 

Analgesia and anti-nociceptive modulation were also a huge part of my PhD work. The literature shows dozens of research articles modulating planarians behavior by applying either analgesics, anaesthetics, narcotics or anti-inflammatory drugs[14]. Such molecules are clearly able to reduce behavioral and physiological markers of stress and nociception in much the same way they would in vertebrates or other invertebrates[18]. 

To the best of my knowledge, motivational trade-offs have barely been researched in planarians. We do have one study that explored decision-making by applying gradients of different stimuli to planarians’ environment[16]. For example, one environment was brightly lit on one side (which planarians hate) and dark on the other side (which they love); but the bright side was also at a pretty cool temperature (which they love), and the dark side at a high temperature (which they hate). After their position in time is measured, we can see on which side they spent the most time. This way, researchers were able to assess the level of importance that planarians gave to each stimuli against one another. Their conclusion was, by most important to less important: chemical stimuli, light, thermal stimuli, and thigmotaxis (texture). Thus, in our example, planarians would prefer the dark hotter side than the bright cool side. I have not encountered studies that explored trade-offs between threats and rewards in planarians. The level of evidence is still pretty low, but not absent. 

Self-protection in planarians seems absent. They show physiological markers of stress, such as an increased mucus production, and they contract most of their muscles in noxious environments to crawl, which help them protect themselves[19], but I don’t see how this would count for flexible self-protection. 

As mentioned earlier, the 1960s planarian researchers were interested in “protopsychology”, so exploring associative learning was actually a relatively big sub-field of research. The most famous (and infamous) researcher in this field was James V. McConnell. He is a fascinating character and I would recommend you to read about him: he worked on memory transfer using cannibalistic planarians and received a non-lethal mail-bomb from the Unabomber for it at the time. In his Manual of Psychological Experimentation on Planarians[20], McConnell reviewed most of the evidence we had at the time (and still have) relative to associative learning in planarians. In most studies, planarians were presented with a two-choice maze and were conditioned to choose a side with rewards (food) or punishments (electric shocks) using other neutral stimuli (such as the color on the sides of each maze branch). Even though planarians needed hundreds of iterations to reach a satisfactory level of success (e.g. 90%), these experiments showed that planarians were able to associate neutral stimuli with valenced stimuli (either noxious or positive) beyond simple habituation or sensitization.

To my knowledge, there has not been any analgesia preference test done on planarians. It could be done using similar mazes than from the criteria 7 where noxious stimuli would be induced (such as a low dose of irritant chemicals), and one branch of the maze would contain analgesics while the other branch would not. It would be tricky, since they are aquatic animals, but it's definitely doable. However, some evidence still shows dependence and withdrawal effects in planarians from opioids (such as morphine); plus, an endogenous opioid, met-enkephalin, was measured in planarian neurons, although we have not yet identified a potential opioid receptor[18]. This evidence may give us some insight into the effect of opioids in planarians, but further research needs to be done to conclude anything on this criteria. 

Conclusion from Birch’s framework

To conclude using this framework, we would need to assess levels of confidence to each criteria and conclude using the number of criteria that reached high levels of confidence (at least 2: some evidence, at least 3: substantial evidence, at least 5: strong evidence, at least 7: very strong evidence). My personal experience on this topic would let me have pretty high confidence on at least the 4 first criteria. They are relevant, but definitely not the most convincing, and may lack definitive proof. However, given the study on criteria 5 about decision making and the plethora of old studies on conditional learning on criteria 7, my estimate of planarian sentience definitely shifted up a bit, and I would consider that they show at least substantial evidence. 

Bonus framework 

As a means of comparison, I also tried to complete Sneddon’s pain perception criteria framework[21] using planarian studies. Without going into details for each one, here are the criteria that planarians seem to validate: 

Nociceptors - Pathway to central nervous system - Central processing in the brain - Receptors for analgesic drugs - Physiological responses - Movement away from noxious stimuli - Behavioural changes from norm - Responses reduced by analgesic drugs - Altered behavioural choices. (9/17)

Conclusion

Planarians may not be a top priority animal to study or to care for, nor a particularly sexy animal to work with, but given the particular place they have as very “primitive” animals that have an exceptionally developed nervous system with surprisingly high similarities with those of vertebrates, they may just be the perfect candidates to study what primitive forms of sentience might or might not be. Beyond research, considering such “primitive” animals as sentient could also shift our perceptions about invertebrates and expand our moral circle far beyond current considerations.

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