Crabs on the Move: The World’s Strangest Mass Migration

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In this final Swarms Minisode of the season, Katy Reiss and Laura Fawks Lapole lose their minds (in the best way) over the most chaotic, moon-synced crab love party on Earth: the migration of Christmas Island red crabs.

We’re talking:
 🦀 50 to 100 million land crabs
 🌧 Timed to what we're convinced is a witches curse....
🚧 Roads shut down
🌊 Pina colada breaks (probably)
💥 And babies launched off seaside cliffs like nature’s carpet bomb

This migration is so massive, locals build crab bridges and the entire island turns into one giant crustacean mosh pit.

But here’s the kicker—these crabs know how far they are from the ocean… AND what phase the moon is in. With brains the size of a Tic Tac.

🎧 Listen in for weird crab romance, the female egg drop of the century, and a surprising twist: these babies? They’re not just adorable—they’re lunch.

Transcript

Laura: [00:00:00] we have for the last couple of e episodes been talking about insects, which have six legs. My next one does have an extra legs than US people. But these are deca pods, which are crabs.

Katy: Yep.

Laura: So I wanted to talk about the Christmas Island red crabs, which you may have heard about before.

Katy: yeah, this is a good one. This is a

Laura: They're like pretty well known to us, like niche nature people. So you might not have heard of them before if you're just a normal bull person.

Katy: Yeah.

Laura: , I 'cause crabs. I was like, oh, there are so many species of crabs that actually do swarm, specifically soldier crabs are known for it. But I was like, no, no, no.

Let's do Christmas Island. So Christmas Island is a territory of Australia located in the Indian Ocean, like I think it's northwest of Australia, so like up by Indonesia.

Katy: Yeah.

Laura: It sounds very whimsical, but what I'm about to tell you makes it a bit more creepy than festive.

Katy: Yeah.

Laura: So the Christmas Island red crab is [00:01:00] endemic, which means it's a species that is only found on Christmas Island or the nearby Cocos Islands. They're. Far apart, but not in the 

Katy: Yeah. Yeah.

Laura: These crabs, just to give you a picture in your brain of what they are. They're red, hence the name, but they're about four and a half inches, 4.6 to be precise.

So decent sized 

Katy: That is precise. Yes.

Laura: So they live, mainly live most of the year. They spend most of the time in moist rainforests. So the island is kind of a plateau on the top. They most of the time spend their time up here in the moist rainforests. But they can even be found in people's backyards because people do live on this island.

They dig burrows that they almost never leave during the dry season because they're not good, with, without moisture. So they like dig these little burrows and leave and plug 'em up with leaves to stay damp. But they're like mostly underground

Katy: Yeah.

Laura: due to where they live. They mostly eat fruit, leaves, flowers and seedlings.

But they will also eat dead animals if they like are nearby their [00:02:00] whole. And African giant snails, which are invasive. So it's good that they found them as a food source. They're incredibly important to the ecosystem. Which for it right now I'll talk about. They're important because they aerate the soil with those burrows and they recycle vast amounts of nutrients back into the soil by eating all the dead stuff and then pooping it out and making the soil good.

But what they're infamous for is their insane migration across the island, each rainy season resulting in a giant swarm. So because we're talking Australia, they've, and this climate, there is not necessarily like seasons as much as there is dry season 

Katy: In what? Yeah. 

Laura: Not like what most of us are thinking like spring, summer, fall, winter.

So as soon as the first rainfall of the wet season occurs, typically in October or early November, these crabs emerge in the millions from their burrows, just millions. Their goal is to make it to the ocean where they can mate and spawn for their eggs. So at least this [00:03:00] swarm, this is like for good reasons, like we've talked about,

Katy: They're not devouring. yeah, 

Laura: yeah, yeah.

This, these, these are just 

Katy: This is more on the Starling. Yeah. This is more on the Starling side. They just wanna exist.

Laura: They're star-crossed crab lovers. So amazingly they also time when they emerge not just on the wet season, but also on the phases of the moon, because of course they do,

Katy: I was gonna just gonna say, I was like, if you're gonna say Moon, I knew you were gonna say Moon.

Laura: because horseshoe crabs do that. So why wouldn't other crab, the crabs in the moon, man, they're like all about, they're like, again, very 

Katy: All the moon. Yeah,

Laura: So they al always, this is a quote from the website, they always spawn before dawn on a receding high tide during the last quarter of the moon.

Katy: that is,

Laura: So specific, 

Katy: but I also, 

Laura: curse for this entire species.

Katy: is. It is. 

Laura: to only spa, not the receding high 

Katy: That would make an amazing T-shirt. That whole saying[00:04:00] 

on the front, on the front of a shirt. 

Laura: Yeah, it would, yeah. 'cause it's either that or a pro, an ancient prophecy, like one of the 

Katy: Yeah. It has to be either you're predicting when a biblical plague is coming of the swarms in the locust,

or it's just love craps. It's fine.

Laura: Yeah. But as weird as everyone's who's listening, what

Katy: Right.

Laura: tides are, ba tides are based on the moon. Okay. So if we want the highest tide possible, that's, I guess 

Katy: Yeah, they, yeah, they understand.

Laura: so they've like, and it's, it's not just one night, it's a couple nights, but it is a short window.

So 

Katy: They're like, we gotta get this on, and we gotta get it on right 

Laura: well that's the craziest thing. So depending on weather, if the weather cooperates, if the rain happens during the lunar, like depending on when, in the lunar phase, the rain hits, they either take their time to get to the ocean and they might stop for days at a time on the way snacking, taking drinks and , like just hanging out.

It, they like, like 

Katy: drinks. I'm like, I just imagine like the logos handing them [00:05:00] like tiny pina coladas and stuff. They're

like, thank you. 

Laura: cute. A little crab water break or, and so some they've been, I was reading an, a scientific journal that said they might stop for one to seven days at a time on their way to the ocean. 

Katy: Yeah. 

Laura: But if they're, if the rain doesn't happen until much closer to when they need to be doing stuff, they will hurry and not stop, which seems to indicate that not only do 

Katy: We gotta do this now. We gotta do this now. Yeah, 

Laura: but that, think about what that means. Not only do they know the phases of the moon, but they also know how far away they are from the 

Katy: Yeah. They're

Laura: if they need to hurry.

Katy: Yeah. Yeah. Right. they know if they need to hurt. 

Laura: or they know how many days they can spend to get 

Katy: I just Like how 

Laura: little crab brains.

Katy: know that they're just like, huh, we gotta hurry. We gotta be

Laura: Yeah. Well, they're 

Katy: do. 

Laura: okay, we're still like seven miles from the coast guys. I think we could probably spend another four hours, but then we gotta 

Katy: Yeah.

Laura: The boys go first on this journey and they're joined by the females because it's, they've gotta get there first to like stake out their territories, but the swarms [00:06:00] themselves are insane. Roads shut down and they've even built, you can look this up. There are little crab bridges. 

Katy: of course. 

Laura: climb up and over and the, you know, like sideways, just

Katy: Yeah. Right.

Laura: like total creeps going over over the roads.

Estimates, it depends on where you read Nat Geo with a, what's his face? What's the British guy 

Katy: David 

Laura: Attenborough. Attenborough says there could be a hundred million other places, say 50 million. So 50 million to a hundred million either way. Millions and 

Katy: I was gonna say, yeah, at that point, I know that there's a difference, but at the same time, I feel like there's not a difference. It's just

Laura: Yeah, it's an excessive amount of crabs. Yeah.

Katy: right.

Laura: And they've they're pretty well studied. Studies have shown that 

Katy: Well, yeah. If they're this predictable,

Laura: Yeah. Right. Like you just gotta wait around and track 'em. Studies have shown that they mostly walk in a straight line towards the northwest shore, not the, no, not the closest shore, the northwest shore.

So they're just 

Katy: specific. Yeah,

Laura: For their [00:07:00] couple special nights. One study that tracked the 

Katy: babe, we gotta get there now. Like.

Laura: yeah. Just all those, 

Katy: grabbing all their pina coladas. Yeah. 

Laura: or just like, she always takes forever, just,

Katy: this could make such a cute car,

A cute cartoon of

just 

Laura: it's like, have you, well, if you haven't seen the movie Migration People, you should 

Katy: yeah, that was pretty funny. 

Laura: But like, that could be, this could be the, something 

Katy: crab's version. Yeah. 

Laura: One study that tracked the crabs showed that they walked on average, 680 meters per day, which is almost half a mile.

Katy: Which is a lot. If you're a crab. I

mean, yeah, right. 

Laura: you know, like,

Katy: I mean just like,

Laura: or, or when you do that, it looks like you got cast a nets 

Katy: you know, just on my way to mate. On my way to

Laura: yeah. But. Um that, that was the average, but the record was some, some overachieving, some crab who [00:08:00] was in a rush. This was like the last crab out of his burrow. 

Katy: the first, so he is like,

I'm gonna get there first this year.

I'm gonna get the best spot. The best female. 

Laura: choosing. Yeah. The last, not this 

Katy: Yeah. 

Laura: that one walked, 1,460 meters in a day. So that's more, that's more than a mile 

Katy: my God. For a crab. That crab is hustling. Yeah.

Laura: like really fast.

Katy: stop. No time to

stop. I'm just

Laura: So for con and for context, the island itself is only 12 miles long and nine miles wide. 

Katy: yeah, It's a small island, guys like 

Laura: it takes them a couple days, but they are usually taking breaks if they if the rain cooperates, if typically the entire process lasts about three weeks. But that's a long time to be dealing with freaking millions of crabs.

You'd be like, my gosh, my commute like for three weeks. It's impacted by crabs. That's insane. You can't even keep using that excuse at work. Okay. That only works.

Katy: Only three [00:09:00] weeks a year. Does that work

Laura: yeah.

Katy: the

rest of the 

Laura: it was the crabs, I swear.

Katy: Nope. It's March. Can't

be the crabs guys

Laura: Yeah, yeah. And what dangers do they encounter? Well, besides traffic, which is definitely the main one at this point, there's dehydration if they don't get their drink breaks.

And, and then invasive yellow, crazy ants. I didn't have time to look that up, but I do the name yellow. Crazy Ants. They're crazy. I dunno.

Katy: Yeah.

Laura: But apparently they're responsible for the deaths of millions of crabs. 

Katy: Oh my God. Yeah. 

Laura: yeah, they just, I probably like the army ants 

Katy: Yeah. I just,

swarm and grab one.

Laura: So then what happens when they get to the beach?

Once, at the beach, we're talking millions of crabs and only so much square footage. So the boys are like wrestling over where they're gonna dig their burrows. Then they're all together, they've built all of these little burrows right at the shore. Then they wait for a female and then she chooses a burrow, I don't know how, and then goes down in there and they 

Katy: This, this is a hole.

I can just[00:10:00] 

like, 

Laura: the other way around.

Katy: I know for once

she needs to choose a good

Laura: crabs, crabs, breaking that glass ceiling.

So, and then once, once the male has made it, he's out. He just goes home. He walks the whole way back. So 

Katy: the opposite wack of shame. 

Laura: the three weeks. Yeah, yeah. He was like, did my crab duty and just going back for next year.

Katy: Did my, he's shuffling

back. 

Laura: females stick around for another two weeks while their eggs develop and they keep them in their little brood pouch. And they stay safe and moist in the burrows. So they're all just chilling, just chilling, nesting. But when the moon is right, or specifically, hold on, let me go back to my, 

Katy: more specifically when

Laura: the before dawn on a receding high tide during the last quarter of a moon, which is definitely a witch's secret.

Katy: It's gotta

Laura: it's so [00:11:00] specific.

Katy: we gotta figure out how to use that for

some merch somehow. 'cause that is so

Laura: Well it's like, it's like it's that he tells you like, Hey, when and where should we meet? You just throw out that line, you know, and people are like, what?

Katy: I feel like to the northwest. You gotta go to the

Laura: Before dawning on the receding. But you have sitting like really ominous anyway. The females come out and pack onto the rocks sometimes 100 per square meter. There's images of this. It's just chicks on chicks, on chicks, like everywhere. They're just stacked on top of each other, just waiting to let go of their eggs so that they can go home.

Katy: Yeah. They're like, we're done with this.

I came here what I did to do. Yeah, go

Laura: as soon as this might take a couple nights for all the females to do this, but I think it's just like a once and done thing for each individual. So they 

Katy: So they're not all waiting. They're holding.

Laura: I hope not for days. 

Katy: They're just,

holding. Because I imagine like a,[00:12:00] 

Laura: just dump them. Oh, I, the way you were standing there, I was picturing all of them holding each other's claws in support and then just, they have that little flappy thing that they just let their eggs out and they're like, okay sisters.

Katy: a, like a back of a conveyor. Like a

back of a plane just opens up.

Yeah.

Laura: in, in like the most sincere form of female solidarity.

Katy: Yeah. Right. together.

Laura: That's what they actually just are like, that's what the whole witch thing, this whole like prophecy, the sisterhood of the crabs, like all of this. They, they, once the water reaches them, they let go those eggs and they have about a hundred thousand of them each. So just whoosh. It probably feels amazing.

As soon as the larva, as soon as the eggs hit the water, the larva hatch, 'cause they're ready to go. 'Cause they've just been waiting inside these eggs. So they touch the water and they're like one of those little growing animals, like 

Katy: yeah.

yeah. 

Laura: and they stay in the ocean and the nearby pools for about [00:13:00] another month before they come on to land and head mom's head home.

Then babies come on land a month later and head 

Katy: Back to where there. 

Laura: The babies are five millimeters big 

Katy: Oh my gosh. 

Laura: five millimeters. The pictures are absurd, but there are so, so

Katy: So wait, is this an additional three weeks then that you can

use this as an excuse. Okay. So a month later you can use, so about

two months out of a year, 

Laura: crabs. Yeah. 

Katy: we had a little bit of a week a break here, and now we're back for baby crab traffic,

Laura: yeah. Yeah. Baby crab traffic. And they don't go as far in this first year. They like spend a couple of years lower down, But 

Katy: Okay. Okay. Okay. Okay. 

Laura: Most typically, most years mo most years most don't make it. Sometimes their years where none make it at all. 

Katy: Oh, whoa. 

Laura: But you know how like trees every once in a while have those mast years where they just release a ton.

Same thing with these crabs. Every once in a while something happens where they just release more eggs than normal and then a lot make it back up onto the beach to [00:14:00] sustain the population and so many that it looks like red algae like coming 

Katy: crazy. That's crazy. Yeah. 

Laura: And this is the last thing that makes them incredibly important for their ecosystem is baby food. In the opposite way that we normally think of baby food, babies that are food. So they're a really important food source. They're really important food source for all the ocean 

Katy: just, I said the babies that are food like.

Laura: Not to be confused with baby food, babies that are 

Katy: Our food. 

Laura: They're, I mean even things like whale sharks are eating them. They come to the shore and they're like, Christmas, Christmas Island feast, baby feast.

Katy: The whale shark that Luke and I track from one of those bracelets is over in this area. I gotta see if it's off this island 'cause it's in the same area. 

Laura: waiting three months. It's like, let's go crab fest. Yeah.

Katy: Yeah.

Laura: Whole new meeting. 

Katy: Let's go baby food. Right, 

Laura: that's, they want their crab babies.

Katy: right.

Laura: So that's [00:15:00] Christmas Island Red Crabs. It's crazy. 

Katy: Okay. I was just gonna 

Laura: one that I would like to 

Katy: for, for me, I feel like still top of everything we've talked about to see is the caves in Vietnam

Laura: Oh yeah, a 

Katy: and then we could just swing south. 

Laura: actually not that far away. Yeah. In the grand scheme 

Katy: south and hit this like that.

Those would 

Laura: have to go at the right time of year. My gosh. I would be dying laughing. I'd be like slightly creeped out by that many crabs, 

Katy: Oh, it'd be so 

Laura: laughing at the 

Katy: because surely this has to be like an expected tourist

Laura: Oh, a hundred 

Katy: surely the crab merchandise has to be

Laura: on point

Katy: right and so good. And if not, we need to research this. 'cause if not,

Laura: to be marketing that. 

Katy: we need to. Yeah. 

Laura: Christmas. Island already alone is marketable, and then a crab migration. Just little Santa hats on everything 

Katy: Well, but then again, it depends on if you like crabs or not. Because if you like it, then it's like a hoho ho crest, festive.

But if it's not, then it's like a ho, [00:16:00] ho horror,

you know what I mean? Like it's one way or the other. 

Laura: Yeah. It's like Krampus versus 

Katy: Yeah. It's one way or the other. I'd be so excited that, yeah, definitely add that to , my second spot of things that now I wanna go see. 'cause that would

be, that would be really cool. All righty guys.

That was the last of the swarms episode, so make sure you guys go check us out on Patreon, support us if you can, and then follow us on our social media channels. Just search Wildly Curious podcast. Be sure to follow us there and support us, and we will talk to you guys next week.

Laura: We should have one more episode, 

Katy: Yep. One more after this. 

Laura: All right. Talk to you guys one more time.

Katy: Bye.

Laura: Bye.