Are horseshoe crabs…crabs?

This is But Why: A Podcast for Curious Kids from Vermont Public. I'm Jane Lindholm. On this show, we take questions from curious kids just like you, and we find answers. If you've been listening for a while, you already know we love field trips at But Why, when we get out of the podcast studio I've created in my closet and go somewhere cool to make an episode. Earlier this month, we got the chance to go to the ocean to learn more about a very unique creature, one that's been

I'm Benicio. I live in Silver Spring, Maryland. I'm nine years old. Why are horseshoe crabs called horse shoe?

My name is Amir. I'm nine years old, and I live in Limerick, Ireland. My question is: why are horseshoe crabs called horseshoe crabs?

Hi, my name is Eva Rose, and I'm eight years old, and live in Vancouver, Canada. Why are horseshoe crabs called horseshoe crabs if they don't look like crabs?

You've sent us a lot of horseshoe crab questions, so it's a good thing we've met up with a scientist who studies these incredible arthropods, because she can answer them all.

I'm Sara Grady. I'm the senior coastal ecologist for Mass Audubon. We are at a little tucked away beach.

I see what looks like a little hump of something heading towards the shore. Is that a horseshoe crab?

That is a horseshoe crab. They're coming into the shore as the tide is coming in, so that they can lay their eggs.

And that's what we're here to see. So that's very exciting to me. Is it exciting to you as a horseshoe crab expert?

It's always very exciting. What's most exciting is when you see a pair of them, because they form these pairs of a female and a male, and they walk around together.

Oh, wait, there's three here.

Yeah, yeah. So they this is a pair of crabs. So there's a larger crab in the front, that's the female, and then there's a smaller crab in the back that is a male, and then there's a second male that has been attracted to the action that's going on and is trying to maybe get involved.

Just to be clinical about this, the horseshoe crabs are mating, and it's how they make more horseshoe crabs.

So what happens is the female will dig a nest, and actually, that female is doing that right now, so she's digging in the sand. She's going to lay her eggs, and then they're going to move forward a little bit, and then the male will fertilize those eggs, and in about two weeks, those eggs will hatch out.

I read that a female horseshoe crab can lay 80 to 100,000 eggs a year.

Yep, yep. They are pretty much a walking box of eggs, actually. And the eggs are a really beautiful, greenish blue color. They're very tiny, about two millimeters, so about the size of maybe the end of a pencil lead. And yeah, they lay them in little clusters in the sand.

So the female is moving forward right now, apparently dragging both of these males. So she she's already laid some eggs?

She may have laid some eggs already. Yeah, yep.

There looks like there's another male coming along for the chase now.

Yeah, yes. There are usually a lot more males than females. So if there's a female, she almost always has a male attached to her, and then there will be other males that are coming along for the ride.

I see another male over there. It's like they're all racing to get to this one female.

Yes, yeah. And when we see a pair that is not buried in the sand, we can pick them up, and I can show you.

There might be a pair over there. We waited in the shallows to where we saw two crabs scuttling towards the edge of the water. Sara picked one up so we could get a better look at this unique creature.

The crab has three parts to its body. So it has this front rounded part, it's called the prosoma. And then this part here, this sort of, I don't know, trapezoid shaped part, is called the opisthosoma, and it has these little spines on the side. And then this long, pointy part the tail is called the telson. So they have these three body parts.

One of the main questions that we've gotten from kids is, why are horseshoe crabs called horseshoe crabs?

My name is Ivy. I'm six years old. I live in Gilbert, Arizona. And why does horseshoe crab start with a horseshoe?

My name is Jack. I'm seven years old, and I live in Belfort, Pennsylvania. Where do horseshoe crabs get their name?

Well, if you look at the front shape of the horseshoe crab, it looks like a horseshoe. It's a U shape, and that's that's it, yeah.

And the horseshoe part that, that big armored part really helps protect a lot of their organs, right?

Yeah. So they're really well protected both on the back of their shell, the formal name being a carapace, for the crab. They're also really well protected underneath, for the most part. If you look on the underside, they have all of these different claws here, and they're different on a male in particular. So we know that this is a male horseshoe crab because his front claws look like boxing gloves. So those are called boxer claws, and then the rest

Yeah, they have like five little appendages that come out of it. These are wild looking!

Yeah. So this, you know, just like a hiking pole or a ski pole, helps the crab push themselves forward as they walk

How many claws does one crab need? across the mud, and then these two tiny claws here are called chelicerae, and that's how they push their food into their mouth. So this is the horseshoe crab's mouth, this spiky hole here. So they use these claws to kind of push stuff down in there so that they can eat it.

I know, right? So let's see. Let's count them. So we've got our two special front claws on the male. Then we've got another 1, 2, 3, on each side, so we're up to eight. And then we've got these pusher claws. So that brings it up to 10, right? 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and these little tiny ones don't really count. So, yeah, 10 claws. Their most sensitive part are these flaps here, which are their gills.

So the reason they're called horseshoe crabs is somewhat self explanatory. The main carapace looks like a horseshoe. But why are they called crabs? They're not really crabs, are they?

Right, well, most people, I think, when they see something that has claws and is walking around in the ocean, will think that it is a crab of some sort. Most crabs that you see are going to walk sideways. Horseshoe crabs are walking forward. They are more closely related to spiders and scorpions. And in fact, these little claws, if you were to look up close on a spider, you would also see those on a spider.

So they're sort of named crabs, not by mistake, but by humans just making assumptions.

Right, right, just like, you know, there are other marine creatures that are misnamed, like starfish and jellyfish, neither of which are fish, right?

Let me just pause here for a second. Did you know that horseshoe crabs aren't really crabs? I didn't. They're more closely related to spiders and scorpions, as she said. So we've been calling them the wrong thing all along. At least we got the horseshoe part right.

My name is Emilio. I'm six years old. I live in Columbus, Ohio. How did horseshoe crabs live before dinosaurs, and how are they still alive today?

How long have horseshoe crabs been alive?

They've been around for about 450 million years. Yeah. So they're one of the oldest living creatures walking around here. Yeah.

How did they survive that long? How did they survive when dinosaurs didn't and I mean, what's the secret of the horseshoe crab?

There have been a lot of extinctions between the time that the horseshoe crab evolved and now and they survived by being tough. And I think really finding the right body shape. Crabs tend to do really well in general, even the later crabs in evolution. So it's a good body to have. They really don't have a lot of natural predators when they're in the water, and so they can just keep going.

Does this one need to stay wet?

That's a great question. So part of coming into spawn is that they often will end up out of the water, and they have these special gills called book gills, and they're almost like the pages of a book. So there's these flaps on top, but then within each of the tougher flaps, there's pages almost and so those book gills will allow the crab to breathe as long as those can stay wet, so the they can pull oxygen out of the air through the water that's on their book gills.

So their book gills will stay wet for a little while, at least, even if they're out.

Yeah, about two days.

Oh, that's a long time.

Yeah. They they're okay for about two days or so. Of course, when they're on the beach, they are more vulnerable. A gull--

Sara, watch out. There's something coming for you, right behind you. Look out behind you.

Oh, look what we have right here. Yes, another, another crab. This one looks a little bigger, so I'm gonna see maybe it might be a female. Nope, another male.

One of the other things I notice about this one that you're holding is that there are some things attached to it, other shells, other creatures attach themselves to horseshoe crabs and get a ride.

Yeah, so they have these slipper shells that are on the back of the crab. Sometimes they'll have barnacles. This one has some algae on it. And horseshoe crabs actually have a flatworm that lives on mostly their gills. It's called a Limulus leech. Limulus is the genus, scientific name, for horseshoe crabs, and it only lives on horseshoe crabs, and it's actually a flatworm. It's not a leech.

My name is Luisa. I'm four years old. I want to know if horseshoe crabs pinch. I'm from Lynn, Pennsylvania.

Do horseshoe crabs pinch?

They don't. Nope. I'll show you right now that you can put your finger right in the claw.

I'm gonna do it too.

Yeah, you can pick a claw, any claw, and stick your finger in there. If you can get it.

It won't even grab me. It's trying to grab the microphone. It's like, "What are you doing?"

Yeah, they're, they're, they're very friendly, actually. You wouldn't think so, but, but yeah, one of my favorite things to do, actually, is if you kind of put your hand right in the middle of them, they kind of will give you a hug with their claws. Oh, I think it just poked your microphone with

Should we be picking up horseshoe crabs? Is it okay to No, they're just folding in half and you happened to have your hand in the wrong place. be picking them up and making them give you a hug?

That's a really good question. I think if you're

That's sort of upside down, then. interested in looking closer at a horseshoe crab, it's a good idea to pick it up and get to know it a little bit. Make sure you put it back in the water. Don't ever pick a horseshoe crab up by its tail. You want to pick it up by the front, by that rounded horseshoe part, and you can hold it just like a bowl of cereal in your hand. So that's gonna keep it

Upside down. And they like to, they like to fold up in half. So you can just hold the crab like that. And you don't want to disturb any crabs that are actively mating.

Hi, I'm Gray. I'm five years old. I live in Wexler, California. Why do horseshoe crabs have the pointy thing on the bottom?

Tell us a little bit about this very long tail on the horseshoe crab.

If you imagine that the crab has ended up on its back and it's stuck on the sand or maybe in a little bit of water, it needs to kind of pole vault itself back over. So that's what that tail is for it. It kind of will twist and plant the end of that in the sand or the mud and flip itself over.

But it's so long and you're not supposed to pick them up by it. Do they get ripped off or lost?

They do sometimes, yeah, yeah. Sometimes it will get caught and stuff, and then you'll just see they have a little stub or something like that.

We know that horseshoe crabs don't pinch but can they sting you with that tail?

Nope, nope. They are not gonna sting you. They're not gonna try to hurt you with that tail. They don't have any sort of venom or anything like that.

My name is Quinn. I'm five years old. I'm from Rocket Center, New York. What do horseshoe crabs eat?

What do horseshoe crabs eat?

They eat a variety of little creatures that live in the mud.

So they're carnivores.

They are, yes, yep. They eat little worms. They will eat small shrimp, like animals called amphipods, that live in the mud. And they also will eat small shellfish. And in fact, that's kind of what got horseshoe crabs in trouble for a while with people, because everybody thought that the horseshoe crabs, especially on Cape Cod where we are, were eating all of their clams, and they got really upset with the

Wow.

Yeah, yep. So they do eat some little clams and things like that, but it's not a huge part of their diet.

So now we know they're not actually eating all the clams that the humans would like to be eating?

Yes, that's right, yeah.

What eats horseshoe crabs?

Not a lot. But something that's really important that they do is feed migratory shorebirds. So when horseshoe crabs lay their eggs, not all of the eggs end up buried under the sand where they're going to turn into new horseshoe crabs. Instead, some of them end up closer to the surface, and there they can provide lots of food for shore birds like the red knot, which is a bird that migrates all the way from Argentina, from the Tierra del Fuego, way way south, all the way up to the Arctic.

Shall we look at some more horseshoe crabs and put this guy back and let him get on with the process of finding someone to mate with?

Sure, sounds good.

Coming up: we know what horseshoe crabs eat and what eats them. But do we eat horseshoe crabs? And if not, why are people catching them?

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This is But Why: A Podcast for Curious Kids. I'm Jane Lindholm. We're on a field trip to Cape Cod with scientist Sara Grady, a coastal ecologist who works with Mass Audubon. Sara is teaching us all about horseshoe crabs and answering the many questions you sent in for us to ask her. In the spring, you can sometimes find these arthropods--who, remember, aren't really crabs at all--close to shore. They usually live a little deeper out, spending most of their

I just want to show you, we'll put these right back, but I can pick them all up at once.

She's so much bigger than the male.

She's very big. Yes.

How big do they get?

Well, the biggest ones are down in Delaware Bay. They get bigger as you go south toward that area, and then they get smaller again as you go down toward the Gulf of Mexico. They can be over a foot across, plus. Yeah.

So this one's big, but she's not big by...

Shes pretty big. She's pretty big for horseshoe crabs in Massachusetts. I'm gonna put these three back. And as you notice, the tide has come in since we've been talking, yeah, so that's what they're following, the incoming tide.

So why do they want to lay their eggs up here, sometimes it looks like not totally out of the water, but and sometimes out of the water here up at high tide?

Well, it's a place where, once those eggs are buried in the sand, they won't get washed away as often, right? So if they were to make their little nest under the water, every time there was a wave, it would be kind of messing that nest up. This way, it's up at the edge of the water, and the water will get there, but not as often. And they also in places, especially like, again, down in the mid Atlantic, they really

How tiny are the babies when they hatch? What do they look like?

They are called trilobite larvae, and they look like a mini horseshoe crab. They're only, again, I mean, they're just a little bit bigger than the egg they were in, so they're only about two or three millimeters across, and they look exactly like a tiny horseshoe crab, except they don't have a tai. And the reason they don't have a tail is it's usually not a good idea to have something sharp and pointy inside your egg.

True. I hadn't thought about that.

Yeah, yeah. So grow that part once you're, you know, safe outside the egg.

Hi. My name is Jude, and I'm five years old, and I live in Vancouver, Canada. Do humans eat horseshoe crabs?

Do humans ever eat horseshoe crabs?

Not really. There's a very small market for eating the eggs. Very, very small, just like, you know, if you've ever ordered sushi with fish eggs on the outside or something like that, it's similar to that, but that's mostly the Asian horseshoe crabs. It's not like we're exporting American horseshoe crab eggs to to Asia.

However, humans do capture horseshoe crabs and kill them and use them. Why are horseshoe crabs important to humans?

So right now, there are two reasons that horseshoe crabs are harvested. One of them is for bait, and that's for the whelk fishery. So a whelk is a giant snail, and

Which people do eat.

Which they do eat, Yes, yep. And there's a market for that. And so they love the smell of horseshoe crabs, especially horseshoe crab eggs, so females will catch more whelks. And so there's an industry for collecting horseshoe crabs for bait. Fortunately, you're no longer allowed to harvest horseshoe crabs while they are spawning, which is going to help the population grow. The other reason that horseshoe crabs are harvested is for their blood. So

When you say synthetic, you mean a human made version that doesn't require killing any horseshoe crabs to get it.

Exactly.

To take the horseshoe crab blood, crabs are captured and some of their blood is drained out. The crabs are then put back in the water. But not all of them survive that whole procedure, so researchers are trying to figure out how to make sure the crabs are treated fairly, and, as Sara mentioned, they're trying to find ways to make a chemical that does what the horseshoe blood does, so they don't need to endanger the crabs at all.

My name is Hazel. I'm eight years old. I live in Gilbert, Arizona. My question is: what color is horseshoe crabs blood?

Their blood is blue. Yeah, it's a sort of a sky blue color, I suppose, a lighter blue, and that is because the molecule that's used to carry oxygen in horseshoe crab blood is based on copper. It's called hemocyanin. And it's kind of like the hemoglobin that's in our blood. So we have red blood cells, our blood is red, and that molecule in our blood has iron in the middle. So Jane, what color does iron turn when it's exposed to oxygen?

Rusty red.

Yeah. And what color does copper turn when it gets too much oxygen?

Kind of a blue, which is confusing, because if you see a penny, you think it's a copper color, which is like an orangy metallic. But if you've ever seen a copper roof, after a long time, it's kind of a sea blue.

Yeah or the Statue of Liberty.

Oh, yeah, I forgot about that.

Yeah, yep. So that's why their blood is blue.

So are horseshoe crab populations in trouble?

It depends on the place, and a lot of places they have been over harvested, and a lot of people have worked hard to try to protect them.

So what's the status now?

They are increasing in some places, stable in others and then there are other places where they have declined. So it's kind of all over the place. Horseshoe crabs take about 12 years between the time that they hatch out to the time that they come back to the beach to lay their eggs. And so that means that anything that's done to protect them when they're laying their eggs, we're not gonna see if the protections worked really well until those crabs grow up and come back.

My name is Sophie. I'm five years old. I live in Delmont, Pennsylvania. How long can horseshoe crabs live?

So 12 years until they are adults, essentially, until they're mature and can make new horseshoe crab babies. How long do they live in total?

It's hard to know, because unlike something like a fish or like even a clam or a tree, you can't cut it and count rings or count layers on various bones. They don't have bones like that. So we have to kind of go by size, and also, to some degree, by the things that are growing on the shell can also be used to kind of get somewhat of an age.

You mean, like, you'd be like, "Look, dude, you have that barnacle. That barnacle is at least seven years old, which means you must be even older."

That's right. "You've had that jewelry for a long time."

But I thought horseshoe crabs molted and got rid of their shells.

The molting slows down once they become mature, so they can live up to about 25, 30 years, we think, yeah.

And how often do they molt in a lifetime?

Oh, it's about 15 times.

So they get a new shell 15 or 16 times. But most of that is when they're really young.

Yeah, they molt six times in their first year, yep. And they grow from being, you know, about the size of a kind of a dull pencil lead, all the way up to being bigger than a quarter.

Wow, in one year. That's a lot. I mean, it still sounds pretty small, but if you're a crab, that's a lot of growth.

Yeah, yeah, yep.

There's like, a ton of them over there.

All right, let's keep moving. Oh, here's a pair that's actually buried in.

Oh, yeah, wow.

So she came in, she's dug this nest, she's laying the eggs, and then they'll probably leave in about an hour.

So they're out here on the very edge, where we can see them right now, and there are so many of them. So cool, because I never see horseshoe crabs, and I'm amazed. But where are they normally?

Well, when it's not time for spawning, they stay in the deeper water, and they're just out there eating. Because they're cold blooded, they do slow down in the wintertime, so they'll kind of dig into the mud and stay there, and that's actually when they get a lot of the barnacles and things on them, because they pretty much turn into a rock for a while.

We figured we'd better let the crabs get on with their mating and spawning. But I had to get Sara to do one more thing while I had a microphone in her face. Sara has written a whole song about horseshoe crabs. It's a specific type of song called a sea shanty, and I sprung it on her as a surprise that I knew about the song, and wanted to see if she remembered enough to still be able to sing a verse.

[SINGING] A little bird called the red knot can fly so far, yes, quite a lot. From South America to the Arctic on their migration. While they fly, they stop to rest. They think that horseshoe crab eggs are best. For getting fat and bulking up and reaching satiation. Feeding this little bird is quite important, I'm sure you've heard. How these two animals are strongly linked together. Their blood is blue. That's because of copper. It clots up as an infection stopper...it...[laughs]

I was liking that verse,

oh yeah, I can't remember.

That was great. Yay yay yay. Clap. I've had that song in my head ever since we got Sara to sing it. Thanks to Dr. Sara Grady, senior coastal ecologist with Mass Audubon, for sharing her deep knowledge of horseshoe crabs with us. We're going to put some videos on our YouTube channel and social media channels if you want to see what these crabs look like. If you're lucky enough to see a horseshoe crab, observe them very carefully. They are wild