Sea urchins: Spiky balls of ouch


Sea urchins are essentially folded-up sea stars. Very poky folded-up sea stars with spikes poky enough that even I might consider putting some shoes on my feet. Maybe.

Today, we’re finishing our exploration of the hedgehog-skinned echinoderms. Climbing up the Tree of Life one group at a time.

Most sea urchins look exactly like you envision them: balls with spikes, but some look a little different: Sand dollars are flat sea urchins without spikes, for example or the pencil urchins with their blunted thick spikes. But even the more typical round balls of spiky doom come in all kinds of colors and sizes.

Chapter 1: Sea urchin basics

We know of more than 900 species of sea urchins, so we won’t be able to cover them all, or even get to all the different groups.

Funnily enough, while the echinoderms are called hedgehog-skinned, the urchins are also named after hedgehogs. Urchin is apparently an old word for hedgehog. But, I admit they deserve the double comparison, as most of them actually have spikes.

Most sea urchins look like someone added nails to a baseball from the inside. Spikes surround a spherical body. These balls of ouch are usually a couple centimeters or inches in diameter, but can range from a few millimeters to the size of dinner plates. The smallest one is Echincyamus scaber at 5.5 millimeters in diameter..) That’s tiny!

The largest is the red sea urchin. That’s the one that reaches dinner plate size. They are typically about 18 cm, so about 8 inches, and their spikes can get up to 8 cm, so about 3 inches long. Ouch!

Chapter 2: Sea urchin anatomy

Like our other echinoderms, these things have ossicles, so hard plates of internal armor. In the case of urchins, these ossicles are fused to form a rigid shell.

You’ve probably seen these shells in tourist shops before where they are often sold as decoration. If you look at an empty shell, you’ll see not only the round bumps that the spikes attach to but also why I call them folded-up sea stars.

Dead sea urchin shell with visible “arms” (Source: Unsplash)
Living sea urchins (Source: Unsplash)

Sea urchins have the same five-fold symmetry as sea stars (well, we’ve talked about this being a bit more complicated but let’s not get into that again…). Depending on the species, there are more obvious “arms” visible inside the shell. They obviously can’t move those, though.

So, if I tell you sea urchins move around, you’d probably assume they walk on those spikes. But, sand dollars don’t have those, so that can’t be it. The spines can move, kind of like your shoulder in the socket, but they aren’t the best leg-replacements, as they only have that single joint at the base. Most species have to lengths of these spines, the primary and secondary ones, but that still doesn’t help with moving themselves anywhere.

Instead, urchins have tiny holes in their shells that let tiny adhesive tube feet through that allow them to move. The tube feet stick out of the hard shell at the bottom. So, while the spikes are mobile, they don’t really use them to walk. They might use them to push things along a bit or to turn themselves back on their right side, though. And, honestly, they don’t really walk fast anyway. Somewhere between a few centimeters per hour and a couple centimeters per day..)

Fun fact: some sea urchins can use their tube feet to see light. It’s how some urchins know when to spawn. Yeah, I know, it’s weird. It’s even weirder that they are now thought to use their entire body as an eye.

There are modified tube feet that kinda look like mini tentacles around their main mouth, too. Wait, what? Main mouth? Yes. In between the spikes are tiny mouths called pedicellaria. There’s movable jaws in each, too. But, they also have a real mouth, usually at the center. In some species, it’s a structure called Aristotle’s Lantern, a combination of teeth or teeth bands, plates, and a soft tongue-like thing.

Yes, that lantern thing was first described by that Greek philosopher dude (well, kinda, but let’s not get into that). The mouths can “scrape, cut, chew and bore holes into the toughest rocks,” as the UC San Diego put it. The urchin can do these things so well, in fact, that those very same researchers copied it for a space exploration device.

It’s also what makes urchins so detrimental to kelp forests. But we’ll get to that.

Behind that creepy lantern-mouth thing sits a pharynx, an esophagus, and finally the intestine that reaches all the way to—you guess it—the anus on the top of the urchin. Like in sea stars, there’s also the madreporite for the water system that makes, among other things, the tube feet work like water-powered suction cups.

It’s impressive to me that they can control all this stuff without a true brain, but we’ve established that a lot is apparently possible without one. Instead, they have a simple nervous system with a large ring around the mouth (what’s with the brain donut again?) and nerves running from there to the water systems and tube feet, spines, and such.

Chapter 3: Sea urchin sex

Most sea urchins are broadcast spawners, like the cucumbers, but some hold onto their eggs with their spines. I find it amazing how many examples there are of animals that are technically egg-bearers but then turn out to be ovoviviparous, so kinda life-bearing.

Broadcast spawning always sounds so easy, because they just throw their eggs and sperms into the water column. But for that to be successful, some coordination is necessary. The moon is often used for these synchronizations.

So, these eye-less, brain-less things can still use the moon to predict when to release their stuff. Impressive, right?

It’s probably why it’s so detrimental to many species when humans turn the night into day with their endless light pollution, especially in larger cities. How is an urchin supposed to tell the difference between the moon’s lunar cycle and the bright lights of a coastal town?

But as we’ll get to light pollution another time, let’s move on to some examples that are well, cool as a cucumber, I mean, urchin.

Chapter 4: The red pencil urchin

While the typical urchins have spikes that scare even my barefoot-living feet, the pencil urchins are much less scary and look really ducking cool.

Red pencil urchin by Scott Roy Atwood – Own work, CC BY-SA 3.0,

With fewer, thicker, and dulled spikes, they look like someone stuck a bunch of pencils into a baseball. In the red pencil urchin, the spines are usually, you guessed it, vividly red. The body, however is much darker which makes it look extra cool that they have white rings around their spikes.

While it would already be a lot harder to ram those blunted pencils into your foot, they also are not poisonous, so even if you do, the hole in your foot is your biggest worry.

And at up to 20 centimeters (eight inches), and with most of them a vivid red, it would be quite the feat to step into them to begin with.

What I don’t get is why people think the spines are cool souvenirs. You can buy a kilo of them for a couple bucks and it only gets cheaper the more you buy. The thins people will display in their living rooms or collect in a drawer… always amazes me.

I’d much prefer looking at the vivid red spikes in the coastal areas where they are common like around Hawaii where they are known as Ha‘uku‘uku‘ula‘ula.

Chapter 5: The eccentric sand dollar

Another example that’s not as poky as the default urchin are the sand dollars. While they are technically urchins, they don’t much look like it. Instead of balls with spikes, they are flat and spineless. Okay, technically they have tiny bristles, so not entirely spineless. Though those spines feel more like a tough fabric than anything dangerous. Nothing compared to the actual poky spikes of the more typical urchins.

Sand dollars in Southern California. Photo credit: Kate Hildenbrand

We once dived above a site where hundreds of these things, probably thousands, had come together until they covered the bottom. Countless brown and gray coin-shaped animals sticking out of the sandy bottom everywhere.

Remember how I told you urchins don’t use their spines to walk? Well, that’s actually not true for the sand dollars which use their tiny spines. As they move on and into sand a lot, they get more traction with their fuzzy spines than with their tube feetsies.

They have even tinier hair-like cilia on their bodies that move food toward their mouths. Like the typical urchin, they have a lantern structure that allows them to chomp down.

They might not look much like urchins but if you take a close look at a living specimen or even a quick look at a dead specimen, you’ll see that they have the same sea-star pattern on their shell as the urchins. Speaking of dead ones: the tests, so shells, of the sand dollar, no matter the species, are popular souvenirs, too. Displaying dead things is popular apparently, especially when it comes to the ocean where people don’t often see the living things.

Chapter 6: Sea potatoes?!

Okay, one more weird one before I get to talk about kelp and sea otters. Because, how could I not talk about something called a sea potato.

Heart urchins or sea potatoes have an oval or heart-shaped test covered in short spines that look almost like fur. They live in burrows which they keep stable with mucus from where they stick out their tentacles. They aren’t regular tentacles but rather modified tube feet, but they still work like tentacles. Like the sand dollars, they use their shorter spines to move, probably because of their shared preference for soft sandy bottoms.

Instead, they use their tube feet to breathe and perceive their environment.

Oh, and they usually don’t have those complex lantern-mouths but simpler mouths that don’t need to chew on thing, because they eat much smaller particles.

Chapter 5: Urchins, otters, and kelp

And with that, we’ll move on to the urchin I’ve seen way too much of—though I’m still fascinated by them: the Pacific Purple Urchin. When we dived around the Channel Islands near Los Angeles, we saw kelp forest munched up by purple urchins.

As I said earlier, those lanterns are very effective at destruction. This aggressive feeding behavior can wreak havoc on kelp forests and turn an ecosystem wonderland into an urchin barren. There are now sea urchin barrens all over the globe.

Sea urchin barren map by Filbee-dexter and Scheibling (

Sea urchins, mostly thanks to their spikes, don’t have many non-human predators. Just not worth the hassle. Sure, some animals eat them, and so do humans, but thanks to the added stressors of the climate crisis like warming oceans and habitat loss of their predators, sea urchins are taking over some of the most beautiful ecosystems.

The heroes in the fight against the urchins are the otters. If you’ve ever watched a video on keystone species, you’ve probably heard about the interactions of otters, urchins, and kelp before. I’ve got a video about that very interaction on my channel. But the short version is: sea otters know how to crack the shells of urchins without poking their fingers on the spines. A dead urchin can’t eat kelp.

Another predator, the sunflower star, was the star of our sea star episode. Yes, yes, pun intended. But thanks to the detrimental sea star wasting syndrome, most of them didn’t survive 2014 and it took until recently for them to even remotely recover. They, like the otters, have figured out how to eat urchins. Without them, there are more and more urchins out there with fewer and fewer predators to eat them.

When more and more urchins appeared across California’s coastline, researchers tried to find out what happened. They found that sea otters did indeed eat a lot more urchins—and their numbers increased rapidly, but they seemed to prefer the urchins from intact reefs, so the barrens remained bare.

My first thought was that the otters wanted to keep the balance in the ecosystems they relied on and thus chose the intact ecosystems instead of the barrens, but the truth is more simple: the urchins from kelp forests had much larger nutrient-rich gonads while the barren urchins were slowly starving and not worth the effort, so-called zombie urchins. But no matter why they do it, by choosing the urchins from intact ecosystems, these wonderful protectors of the kelp are keeping the remaining patches as healthy as possible. And, in a way, this helps regrow the kelp forests in the barren areas, as spores from the happy kelp float over to the barrens.

Now, we just need to protect the otters, mitigate the effects of the climate crisis, and stop actively destroying the planet to give nature a chance to recover their balance.



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