Do Sea Urchins Feel Pain? (Explained)

Pain is an awkward thing to test, and it’s not even easy to define. What actually is pain? We know it as humans, but how can we tell what animals feel? Especially invertebrates (animals without a backbone) like sea urchins that don’t even make any sound, and their bodies are very different from ours. Scientists have been looking for answers for years, but it’s still a lot of guesswork.

Since we can’t get inside animals’ bodies and fully understand their feelings, we can draw conclusions based on their behavior and anatomy. In this article, I’ll answer whether sea urchins feel pain or not based on scientists’ research, but let’s begin with a quick answer:

Sea urchins like other invertebrates can possibly feel pain, however, we haven’t exactly proven that yet. They have numerous sensory cells located in the spines, pedicellariae, tube feet, and round their mouth. These receptors are sensitive to touch, chemicals, water currents, and light.

However, this certainly doesn’t tell the whole story. Below I’ll explain how we can identify pain in animals based on scientific research. Furthermore, I’ll describe sea urchins’ nervous system and senses, behavioral responses of invertebrates, and explain long-term motivational changes and learning. Next, I’ll draw a conclusion based on other research. Read on!

How can we identify pain in animals?

As I mentioned, some animals can’t make any sound, and obviously, we can’t ask them if they’re in pain. We can’t measure pain directly, but there are ways to identify it in animals, and in our case, in invertebrates. 

We can define a few categories that can evaluate the potential for pain in sea urchins and all invertebrates. The first is their anatomy. Based on science, animals should have a suitable nervous system and sensory receptors to be able to feel. They detect sensory information and communicate it with the body.

The second is behavioral responses, such as limping, rubbing, or nursing wounds. Just like humans, when animals feel pain, they automatically rub part of their body that is irritated.

The third is ​​long-term behavior changes. Animals can show a difference in their behavior after experiencing a painful event exactly as humans do. For example, after touching fire, we know we should not do that anymore because it causes us pain.

Knowing these categories, let’s now take a look at sea urchins’ bodies and what we know about their responses to a noxious stimulus.

Nervous system and senses in sea urchins 

Sea urchins are marine invertebrates that belong to echinoderms together with sea stars, brittle stars, sea cucumber, and crinoid. It’s interesting to note that this group of animals is closer to humans and other vertebrates than any other invertebrates on the phylogenetic trees. However, their nervous system is pretty simple. 

Central nervous system

Sea urchins have five-parted radial symmetry, and their nervous system includes an integrated network of nerves but doesn’t have cerebral ganglion, which we can compare to a brain. A nerve ring is an essential part of the sea urchin’s body system. It surrounds the mouth and five radial nerves that extend from the ring into different body parts like tube feet, spines, and pedicellariae.

The nerves later control movement coordination in the water vascular system which is a complex set of canals characteristic of all Echinoderms. Their original function as simple respiratory–suspension feeding structures evolved into a broad range of functions like feeding, respiration, locomotion, excretion, and sensory reception.

Senses

Sea urchins have numerous sensory cells located in the epithelium (type of body tissue), particularly in the spines, pedicellariae, tube feet, and round their mouth. These receptors are sensitive to touch, chemicals, water currents, and light. 

Even though sea urchins don’t have eyes, their entire body functions as a compound eye. These animals can use their light sensitivity to protect themselves from predators. When shadows pass over, they react and point their sharp spines directly at the potential danger. 

Other examples of reacting to light are that some species seek shade and cover themselves with rocks or shells. For instance, Tripneustes cover themselves more in the summer than in the winter.

Despite their relatively simple nervous system, the sea urchins show complex behavioral patterns. For example, irregular sea urchins like sand dollars can turn over if placed on the back. Next, they bury themselves in the sand to hide from predators. 

In conclusion, these spiny animals have a relatively simple central nervous system but can register their surroundings through numerous sense organs. Their body is built differently than we know as humans; for example, instead of a typical “brain”, they have a nerve ring. Thanks to their spines and sensory cells, they can survive and protect themselves from predators. 

Behavioral responses to pain

Even though there has been no research on sea urchins specifically, it’s worth mentioning other research on invertebrates whose bodies are very similar to sea urchins. Professor Robert Elwood has been looking for ways to answer the questions if invertebrates feel pain and he conducted a lot of research, mostly on crustaceans. His results are fascinating and we can definitely draw some conclusions from them. 

Reflex vs. pain

It’s important to mention that reflex is often not considered a sign of pain. Reflex is an unconditioned response that we experience – for example, when we touch a hot iron, we immediately pull it away. Pain is the conscious experience that happens after the signals have reached the brain. 

Therefore, when an animal responds to something, we’d think it’s painful, it doesn’t necessarily mean the animal is in pain, but it’s a simple reflex. That’s why scientists often look for responses beyond reflex, for example, animals rubbing or nursing a wound.

Pain responses

Professor R. Elwood conducted one of his research on prawns. He brushed their antennae with acetic acid, and they began grooming their treated antennae with complex movements of both front legs. Furthermore, their rubbing movement decreased when a local anesthetic was applied before the experiment. 

Another animal the professor examined was a crab. He applied a brief electric shock to a part of the hermit crab body. He noticed the crab rubbing that sport with its claws for a long time. 

Further, he placed crabs in a bright tank with two shelters. Crabs prefer hiding during a day under rocks, so in this case, they would probably pick one of the shelters to hide. He gave them a brief shock inside one of the shelters, which forced crabs to go outside. After only two attempts, the crabs were more likely to switch shelters. He says, “So there is rapid learning, just what you would expect to see from an animal that experienced pain.”

In our case, however, sea urchins don’t have apparent organs to rub their wounds, as crabs do. However, what’s interesting is that Robyn Crook, an evolutionary neurobiologist, examined squids, and noticed that they might experience pain entirely differently. 

Right after a squid’s fin was crushed, nociceptors became active, but not only where the wound was. They became active across a large part of the squid’s body, extending to the opposite fin. It could suggest that if this animal feels pain, it may hurt all over, rather than feeling it in one spot. 

We’re not sure yet why this would happen, but it makes sense from the animal’s perceptive. The squid’s tentacles can’t reach many parts of its body, so it couldn’t even tend its wound. What’s more, squids are forced to move all the time because of their fast metabolism and keep hunting. An injured squid with all-over heightened sensitivity may stay more alert and sensitive to touch and visual stimuli. “Its long-term behavior changes,” she says. “This fulfills one important criterion for pain.”

Long-term motivational change and learning

The experiment on hermit crabs also showed the long-term changes in their behavior after noxious stimuli. Crabs received a shock in their shell, whereas other control crabs weren’t shocked. Next, the crabs were offered a new empty shell, and a change in motivation was observed. Shocked crabs more quickly approached and moved to the new shell.

This behavior change towards shells clearly shows that crabs are motivated to switch shelters after shock and suggest that they learned that the current shell is low-quality. This shift in motivation was noted 24 hours after the noxious stimulus and proves that the change in the behavior is not just a reflex. Such learning has been demonstrated not only on crabs but other crustaceans and fish as well.

Conclusion

People for a long time rejected the idea of pain in invertebrates like crustaceans because they thought these animals respond to noxious stimuli only by reflex. The examples above clearly show that this is not the case, but it also doesn’t mean that pain is proven. 

We simply don’t know, and probably never will know, what exactly invertebrates feel when exposed to noxious stimuli. What’s more, pain, in this case, can neither be proven nor disproven. However, we must accept the possibility of pain. 

We also don’t have a clear answer about if sea urchins feel pain, but knowing the research results on other invertebrates, we can at least draw similar conclusions. We should always keep in mind that sea urchins, just like other animals, are livings beings, and they deserve ethical consideration.

Sources

• Ruppert, Edward E.; Fox, Richard, S.; Barnes, Robert D. (2004). Invertebrate Zoology, 7th edition.
  
• Elwood Robert W. (2019) Discrimination between nociceptive reflexes and more complex responses consistent with pain in crustaceansPhil. Trans. R. Soc. B3742019036820190368 http://doi.org/10.1098/rstb.2019.0368

• Elwood R. W. (2021). Potential Pain in Fish and Decapods: Similar Experimental Approaches and Similar Results. Frontiers in veterinary science, 8, 631151. https://doi.org/10.3389/fvets.2021.631151