On sea urchins

2025-09-14

My girlfriend recently purchased some air plants and is thinking about buying some sea urchin shells to put them in, since sea urchin shells are a popular and good looking way of storing air plants.

She asked me what I thought about the ethics of buying sea urchin shells and I didn’t know, so I thought it’d be a good topic to write about.

The way I see it, there are two questions to consider:

  1. Are sea urchins sentient?
  2. If they are, does harvesting a sea urchin shell cause them any pain?

If the answers to both 1 and 2 are “yes”, then I would say it’s unethical to purchase sea urchin shells. If the answer to either question is “no”, then I would say it’s fine.

Disclaimer: The question of whether an organism is sentient or not is extremely complicated and controversial, and involves a lot of neuroscience, biology, and philosophy. I’m not an expert on any of these topics, so we’ll learn about this together!

Are Sea Urchins Sentient?

First, I define “sentience” as the ability to have positive and negative experiences. If it’s possible for sea urchins to have negative experiences, then we should do what we can to avoid causing sea urchins to have them. I’m going to use Animal Ethics’ guide for determining whether an organism is sentient:

There are three general criteria for deciding whether a being is sentient. These involve considerations that are (1) behavioral, (2) evolutionary, and (3) physiological. 1

The author also notes that the physiological criterion “should be the determining factor as to whether a being is sentient”.

Within the physiological category, there are two further criteria we can look at:

  1. Does the organism have a central nervous system?
  2. Does the organism have other physiological signs that may point to the ability to experience pain such as nociception or chemicals that act as analgesics?
    • Since the experience of pain is a negative one, I will assume that if an organism can experience pain, then they are sentient.

Nociception is a process by which pain is created in many organisms. Sensory neurons called nociceptors detect harmful or potentially harmful stimuli and alert the central nervous system, which creates pain to direct the organism to respond to the threat. If an organism has nociception, it’s likely (but not definitive) that it experiences pain.

Nociception is the detection of potentially injurious stimuli and gives rise to pain which usually has not only a sensory component but is also an emotional experience. [emphasis mine] 2

But not all brains are properly equipped to translate the alarms from nociceptors into pain:

In order for pain to be actually experienced, that information has to be received by a brain that is organized in such a fashion as to make it not only capable of processing it, but of processing it in a way that results in the experience coded by it. And what is unknown as of today is how brains need to be organized in order to give rise to this experience. 3

What I take this to mean is that nociception can direct an organism to respond to a threat, but possibly without translating that threat into an experience of pain. So nociception is not a guarantee of sentience. This makes sense, since the reflex triggered by putting your hand on a hot stove happens before the experience of pain; it makes sense that some organisms would stop at just developing the reflex, if the experience itself wasn’t advantageous.

But how would an experience of pain be advantageous? The experience of pain may help us avoid potentially damaging situations for longer than a pure reflex would:

What further advantage is gained by having an additional system that enables the experience of nociceptive inputs as an unpleasant emotion? It may be that the “emotional” component provides a long-lasting motivation that enables the animal to better maintain its tissue integrity (Bateson 1991). A nociceptive response may be organized as a reflex (Sneddon et al. 2003) but may not be associated with a lasting memory and motivational change. Pain, on the other hand, might induce a long-term memory and be coupled with learning to avoid situations that gave rise to the original pain experience (Bateson 1991). The greater the tissue damage in the original experience the greater may be the unpleasant emotional response and the greater the motivation to avoid it in the future. Thus pain experience has a longer-lasting effect and protects the animal from future damage in a more effective manner than does nociception alone. 4

The presence of chemicals that act as analgesics would also suggest that the organism can experience pain, because the purpose of analgesics is to alleviate pain. For example, snails were observed to take longer to react to a hot plate if given morphine:

The terrestrial snail Cepaea nemoralis, when placed on a 40°C hot plate, lifts the anterior portion of its foot. The latency of this response is influenced by morphine and by naloxone in a dose-dependent and time-dependent manner. Morphine increases the time taken to respond, whereas naloxone reduces it…These results indicate that an opiate system may have a role in this behavior, which resembles that reported in vertebrates. 5

This suggests that they experience pain, if morphine was able to seemingly reduce it.

So, to determine if sea urchins are sentient, our questions become:

  1. Behavioral
    1. Do they behave in ways that might make us, as sentient beings, think that they might be sentient?
  2. Physiological
    1. Do sea urchins have a central nervous system?
    2. Do they have nociception and/or chemicals that could act as analgesics?

I’ve ignored the evolutionary considerations in this blog post, since I know the least about them and I think it won’t be necessary.

Behavioral Considerations

In spite of their relatively simple nervous system, the echinoderms show complex behavioral patterns. They are able to turn over if placed on the back, and can orient themselves to currents in the water. In conclusion, these slow moving animals are able to survive even without a complicated nerve system, but their survival also depends on the protection offered by their stiff and spiny bodies. 6

Sea urchins were observed to have “straighter and faster” trajectories when subjected to predator-conditioned seawater compared to normal seawater:

The trajectories from sea urchins subjected to predator cues were, on average, straighter and faster than those coming from controls, which translated into differences in the diffusive properties and unpredictability of their movement patterns…Our results highlight that fear from predators can be an important driver of sea urchin movement patterns. 7

They don’t have eyes per se, but can detect and react to light:

Their behavioral repertoire, however, is rather complex. This is especially true for the urchin’s reaction to light. Not only can they detect looming visual stimuli from any direction and accurately point their spines towards them, but they are also able to resolve objects and move straight in their direction. What adds to the mystery of these behavioral capabilities is the fact that sea urchins are eye-less and instead feature dermal photoreceptors spread over their entire spherical body. 8

It’s important to note as well that Elwood, 2011, thinks that the behavioral component (as opposed to physiological) is the most important for determining if an organism can experience pain:

It is thus behavior that provides the greatest insights into the likely experience of pain. Rapid avoidance learning, coupled with a prolonged memory, indicates central processing and is consistent with pain… 9

The only evidence I could find of long-term memory in sea urchins was a project from three Japanese high-schoolers, but there was no accompanying paper. The findings are potentially interesting, however:

when the sea urchins were allowed to be attacked by starfish, they changed their behavior and they showed avoidance. Once the sea urchins showed avoidance behavior, they maintained this memory even after 12 days without a contact with the starfish. 10

I consider this fairly weak evidence given that it was a high school project and not peer-reviewed as far as I can tell.

Physiological Considerations

Nervous System

Sea urchins don’t have a centralized nervous system, but they do have a decentralized one.

Their nervous systems include an integrated network of nerves, but lack a cerebral ganglion. An important part of the system is a nerve ring that encircles the mouth, and five radial nerves that extend from the ring into different parts of the body. The nerves control the coordination of movement in the unique water vascular system. If the main radial nerve is cut, however, the row of suckers may still continue to move independently. 11

Nociception & Analgesics

I couldn’t find any evidence that sea urchins have nociception or chemicals that act as analgesics.

Neurons

As a bonus, I wanted to see how many neurons sea urchins have. I couldn’t find an exact answer, but according to this list of animals by number of neurons, starfish have about 500 neurons. Since starfish and sea urchins are both echinoderms, I will assume that sea urchins have on the order of 500 neurons as well. For reference, a fruit fly has 150,000 neurons. If graded sentience is true, then this would help place sea urchins and other echinoderms near the bottom of the spectrum of sentience.

So Are They Sentient?

Given that Animal Ethics rates the physiological criteria to be the most significant, and sea urchins fail both tests, if we were to judge based solely on this criteria we would have to say that they are not sentient. Indeed, Animal Ethics themselves conclude the same thing in a different post:

[I]n order to have experiences it is necessary to have a centralized nervous system. And some animals lack such a system. This implies that there are animals who cannot be sentient. First, we would include here those beings that do not have a nervous system, such as Porifera (the phylum that includes sponges), and those who do have a nervous system which is not centralized, such as echinoderms and cnidarians [emphasis mine] 12

But Elwood, 2011, disagrees that the absence of a central nervous system means an organism cannot possibly experience pain:

[T]he possibility of fish experiencing pain has been dismissed because human pain is experienced in parts of the cerebral cortex whereas fish lack this structure (Rose 2002). If one accepts this argument then the possibility of pain being experienced by any invertebrate must be dismissed because none has a central nervous system (CNS) built on the vertebrate plan. However, according to the same logic it could be suggested that because crustaceans or cephalopods lack any of the visual system found in humans they must be blind. This is not the case as both have a well-developed visual ability, each based on an entirely different CNS and receptors. Thus clearly the same function can arise in different animal taxa using different morphology, and it appears to be illogical to accept this reasoning for some experiences but to dismiss it for pain (Elwood et al. 2009) 13

I think we don’t have enough information to know definitively; I’m not comfortable making a decision either way. I would be interested to see studies on sea urchin nociception or rapid avoidance learning, which could help sway me one way or the other. From Elwood, 2011, again:

Substantial research on various taxa is necessary to assess which [invertebrates], if any, show (1) rapid avoidance learning of noxious stimuli, (2) prolonged responses directed to the specific site on their body where the noxious stimulus was applied, and/or (3) tradeoffs between avoidance and other activities that would indicate central decision making rather than reflex reaction 14

I just don’t have enough confidence to say, “no, sea urchins are not sentient; it’s ok to capture and kill 139 million 15 16 of them every year.”

How Are Sea Urchin Shells Harvested?

It’s suprisingly hard for me to find a definitive answer to this question. Some people definitely find them on the beach, and this seems like the most plausible way. When sea urchins are harvested for food, the shells are cracked open to get at their gonads, which looks like it renders the shell completely unusable. I think it’s unlikely that live sea urchins are harvested solely for their shells, since the gonads are too valuable as food.

I think it’s likely that most sea urchin shells you can buy online came from already dead sea urchins found on the beach or in the sea. All commercial sea urchin operations that I’ve watched videos of cut the shell in half and completely ruin the shell.

As a result of this, my guess is that it’s probably fine to purchase sea urchin shells.

Why does this matter?

Invertebrates vastly outnumber vertebrates: 97% of all animal species are invertebrates17. If we look at individuals, the number of vertebrates alive in any given moment is about 4.066 trillion, while there are around 10 quintillion insects 18, and I think this number isn’t even including invertebrates that aren’t insects, like echinoderms. This means invertebrates make up at least 99.999593% of the global population at any one time.

Most people just assume that either invertebrates can’t feel pain or that their interests don’t matter. But given how many of them there are, this is a huge mistake. If we turn out to be incorrect in our assumptions, we would be committing atrocities of a sort never before seen, even when taking into account factory farming.

Thankfully, purchasing sea urchin shells turns out to be probably fine. If you disagree with anything I wrote or I got something wrong, please let me know!

Citations

  1. https://www.animal-ethics.org/criteria-for-recognizing-sentience/↩︎

  2. Lynne U. Sneddon, Evolution of nociception in vertebrates: comparative analysis of lower vertebrates, Brain Research Reviews, Volume 46, Issue 2, 2004, Pages 123-130, ISSN 0165-0173, https://doi.org/10.1016/j.brainresrev.2004.07.007↩︎

  3. https://www.animal-ethics.org/criteria-for-recognizing-sentience/↩︎

  4. Robert W. Elwood, Pain and Suffering in Invertebrates?, ILAR Journal, Volume 52, Issue 2, 2011, Pages 175–184, https://doi.org/10.1093/ilar.52.2.175↩︎

  5. Martin Kavaliers et al., A Functional Role for an Opiate System in Snail Thermal Behavior. Science 220, 99-101(1983). DOI:10.1126/science.6298941↩︎

  6. Sømme, L. S. (2005). Sentience and pain in invertebrates (Final Report No. 317018M). Norwegian Scientific Committee for Food Safety. https://vkm.no/download/18.d44969415d027c43cf1f5b4/1499249763667/413af9502e.pdf↩︎

  7. Pagès, J.F., Bartumeus, F., Romero, J. et al. The scent of fear makes sea urchins go ballistic. Mov Ecol 9, 50 (2021). https://doi.org/10.1186/s40462-021-00287-1↩︎

  8. Arnone, et al., Uncovering the mechanisms of decentralized vision in sea urchins. https://www.hfsp.org/hfsp-news/uncovering-mechanisms-decentralized-vision-sea-urchins↩︎

  9. Robert W. Elwood, Pain and Suffering in Invertebrates?, ILAR Journal, Volume 52, Issue 2, 2011, Pages 175–184, https://doi.org/10.1093/ilar.52.2.175↩︎

  10. https://isef.net/project/anim040t-learning-and-long-term-memory-in-sea-urchin↩︎

  11. Sømme, L. S. (2005). Sentience and pain in invertebrates (Final Report No. 317018M). Norwegian Scientific Committee for Food Safety. https://vkm.no/download/18.d44969415d027c43cf1f5b4/1499249763667/413af9502e.pdf↩︎

  12. https://www.animal-ethics.org/what-beings-are-not-conscious/↩︎

  13. Robert W. Elwood, Pain and Suffering in Invertebrates?, ILAR Journal, Volume 52, Issue 2, 2011, Pages 175–184, https://doi.org/10.1093/ilar.52.2.175↩︎

  14. Robert W. Elwood, Pain and Suffering in Invertebrates?, ILAR Journal, Volume 52, Issue 2, 2011, Pages 175–184, https://doi.org/10.1093/ilar.52.2.175↩︎

  15. Aminur Rahman, M., Ismail, M., Shamim Parvez, M., Asadujjaman, M., Ashik, A.-A., & Habibur Rahman Molla, M. (2022). Echinoderm fisheries: their culture, conservation, bioactive compounds and therapeutic applications. Journal of Biological Studies, 5(3), 413–443. https://doi.org/10.62400/jbs.v5i3.7053 [Used for annual sea urchin harvest data: 63,000 tons in 2018]↩︎

  16. https://a-z-animals.com/animals/sea-urchin/ [Used for sea urchin average weight: 1 lb]↩︎

  17. https://www.nationalgeographic.com/animals/invertebrates↩︎

  18. https://www.livescience.com/animals/how-many-animals-have-ever-existed-on-earth↩︎