Hannah Fry and Dara Ó Briain introduce Curious Cases

Summary

You're Dead to Me – Curious Cases Takeover: “Why Do So Many Animals Turn Into Crabs?”

Podcast: You’re Dead to Me (BBC Radio 4)
Episode: Hannah Fry and Dara Ó Briain introduce Curious Cases
Date: October 8, 2025

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Episode Overview

In this special crossover episode, Greg Jenner hands over the “You’re Dead to Me” feed to the hosts of BBC Radio 4’s Curious Cases, Dr. Hannah Fry and comedian Dara Ó Briain. The pair, joined by evolutionary experts and the director of a crab museum, tackle an internet-fuelled biological question: Why does evolution keep producing crabs (a phenomenon known as carcinization)? Humorous, fact-filled, and full of sideways banter, this episode explores evolutionary convergence, the true—and false—crabs of the crustacean world, and why pop culture can’t get enough of these quirky creatures.

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Key Discussion Points & Insights

1. Setting the Stage: “Everything Becomes a Crab” Meme

[01:08–04:03]

• Hannah and Dara discuss the cultural fascination with crabs and how the meme that “everything evolves into a crab” (carcinization) has seized the internet—and even science museums.
• Dara recounts a recent Natural History Museum exhibit speculating about possible alien body plans, noting, “The only really recognizable thing was a crab. They said, yeah, yeah, there'll definitely be a crab.” (Dara Ó Briain, 05:01)

2. The Central Question: Is Crab Evolution a Trend?

[05:34–07:19]

• Listener question: Why have so many different species independently evolved into crab-like forms? Is the “crab” the ideal shape, and could humans one day become crabs?
• Hosts introduce three guests:
  • Prof. Matthew Wills (Evolutionary Palaeobiology, Bath)
  • Dr. Joanna Wolfe (Evolutionary Biologist, Harvard & Santa Barbara)
  • Ned Sysat Williams (Director, Crab Museum Margate)

3. What Is a Crab, Anyway? True vs. False Crabs

[08:16–13:11]

• Dr. Wolfe clarifies the difference between “true” crabs (Brachyura, ~8,000 species) and “false” crabs (Anomura, e.g., hermit crabs, king crabs, squat lobsters).
• Ned and Joanna note the confusion in taxonomy and why we sometimes label things with “crab” in the name that really aren’t crabs.
  • “Hermit crabs are like my favorite crab, and they're not a crab.” (Dara Ó Briain, 11:00)
  • “A crab is a made-up word… horseshoe crabs are not crabs, neither are they horses or shoes, but we still recognize them for the wonderful beings they are.” (Ned Sysat Williams, 23:17)

4. Carcinization & Convergent Evolution Explained

[13:03–22:48]

• Carcinization: multiple, independent lineages (at least five times) have evolved the “crab” body plan—broad, flat shells and tucked-under abdomens.
• Prof. Wills uses the lobster as an example, showing how evolving into a crab results in sacrificing mobility (the ability to flick away backwards) in exchange for better armor and compactness.
• Not every animal can or will become a crab; convergent evolution requires pre-existing body plans and environmental pressures.
  • “You have to be in an environment where [crab shape is] an advantage… and have all the building blocks.” (Matthew Wills, 15:42)
• Carcinization is remarkable, but not unique—there are memes for crabs, fewer for e.g. dolphin/shark convergence.

5. Are Humans Next?

[19:13–19:34]

• Fry presses Wills: if enough time passes, will everything become a crab?
  • “No.” (Matthew Wills, 19:32)
  • The meme is fun, but oversimplifies the science.

6. The Flexibility—and Limits—of Evolutionary Trees

[19:40–22:48]

• Genomics—versus older comparative anatomy—has overturned much mammalian taxonomy: “Horses are more closely related to bats… Elephants and Sirenians (manatees) are cousins.” (Matthew Wills, 21:14)
• Convergence means wildly different animals can look and act similarly without being closely related.

7. Crabs: Why So Cool?

[23:38–25:12]

• Ned extols crabs as underestimated, ecosystem engineers whose burrowing supports mangroves and coastal life.
  • “Without crabs, mangroves choke and mangroves die. So crabs are ecosystem engineers.” (Ned Sysat Williams, 24:20)
• Crabs exhibit fascinating, sometimes bizarre behaviors—wearing hats (jellyfish, sea anemones), living inside sea urchins, and more.

8. Crab Diversity: From Fiddler to Coconut

[25:41–27:13]

• Crabs range from fiddler crabs (famous for their asymmetrical, display-ready claws) to giants:
  • Largest by leg span: Japanese Giant Spider Crab (can exceed an adult human’s arm span).
  • Heaviest: Coconut Crab (a land-dwelling, terrestrial “hermit crab” the size of a small dog).
  • “The heaviest arthropod ever known is alive with us now. And it's the coconut crab, which is not truly a crab, it's a hermit crab.” (Joanna Wolfe, 26:12)

9. What Makes a Crab, Biologically and Philosophically?

[13:11–14:29, 23:17]

• The crab “plan” is effective because it subdivides walking and manipulating limbs; most crabs have differently specialized claws.
• Left- or right-handedness in crabs develops depending on usage!
• Crabs, like language, are hard to define by hard boundaries—more philosophy than taxonomy, sometimes.

10. Memorable Moments & Banter

• Dara jokes about feeling betrayed that hermit crabs aren’t “real” crabs, launching into comedic mock outrage:
  • “You are not a crab! You're a crab of lies. You are False. I abjure thee, false crab.” (Dara Ó Briain, 30:27)
• Hannah explores the meme of “everything becomes crabs,” referencing Monterey Bay Aquarium’s social media:
  • “This is, you know, one of the most respected institutions in the entire world just posting a crab content meme with zero context.” (Hannah Fry, 06:36)

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Notable Quotes

• On Evolution and Popularity:
  • “Crabs are weird and funny looking… so why wouldn't you hinge a whole discourse about evolution on a crab?” (Ned Sysat Williams, 08:37)
• On Convergent Evolution:
  • “A lot of them looked like paramecium… The only really recognizable thing was a crab.” (Dara Ó Briain, 05:01)
• On True vs. False Crabs:
  • “There's two groups… they're the true crabs and the false crabs. That's not a value judgment, it's just based on taxonomy.” (Joanna Wolfe, 09:40)
• On Crab Functions:
  • “Lots of different appendages doing different jobs… even the pincers—they have a crushing claw and a cutting claw.” (Matthew Wills, 27:16)
• On Symbiosis:
  • “Some crabs will live inside the bums of sea urchins. Other crabs will wear jellyfish on their heads upside down, like, as kind of hats.” (Ned Sysat Williams, 23:45)
• On Crabs in the Food Chain:
  • “People will eat a lot of things.” (Dara Ó Briain, 18:24)

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Timestamps for Important Segments

• Introduction & Crossover Context: 01:08–05:01
• “Everything Becomes a Crab” Meme: 05:01–07:19
• Intro to Guests & Carcinization: 07:19–09:06
• True vs. False Crabs: 09:06–13:11
• Evolutionary Explanation (Crab Biology & Fossils): 13:11–19:13
• Crab Body Plan & Adaptations: 19:13–21:29
• Convergence & Evolutionary Trees: 21:29–22:48
• Crab Coolness & Ecological Importance: 23:38–25:12
• Giant Crabs & Diversity: 25:41–27:13
• Wrap-Up & Final Thoughts: 29:20–30:51
• Notable Banter: 30:27–30:51

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Tone & Style

The episode is highly conversational, witty, and full of good-natured teasing between Hannah and Dara, as well as their expert guests. Despite the humor, the discussion remains rigorous, delving into the nuances—and the mysteries—of evolutionary biology. Running jokes about fake crabs, internet memes, and crab “PR” combine with lively explanations to make a scientific topic genuinely engaging for all listeners.

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Conclusion

This episode both uncovers the science behind one of nature’s quirkiest evolutionary trends and celebrates the oddball appeal of crabs. With guest experts weighing in on taxonomy, evolution, and just why crabs capture the human imagination, listeners come away appreciating that the answer to “Why do things keep becoming crabs?” is as much about our perceptions—and internet memes—as it is about the power of convergent evolution. As always, the hosts make learning hilarious and accessible.

“I think this program has been absolutely excellent PR for crabs.” (Hannah Fry, 29:33)

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For more odd natural questions and answers with wit and science, subscribe to Curious Cases on BBC Sounds.

Transcript

[Ned Sysat Williams]

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[Dara O'Brien]

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[Dara O'Brien]

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[Ned Sysat Williams]

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[Dara O'Brien]

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[Ned Sysat Williams]

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[Matthew Wills]

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[Hannah Fry]

Hello, You're Dead to Me fans. It is Hannah and Dara here from Curious Cases.

[Dara O'Brien]

You might remember me from a past episode of youf're Dead to Me where I joined Greg Jenner.

[Hannah Fry]

Have you been cheating on me on different podcasts?

[Dara O'Brien]

Yeah, I do other stuff. I mean, but the implication of this is that these people only ever listen to youo Dead to Me in no other media in the last 20 years. And then we'll go, oh, who's that guy? Oh, he was once on an episode of youf're Dead to Me.

[Hannah Fry]

Yeah, and that is the reason why I recognized his extremely distinctive Irish voice.

[Dara O'Brien]

It was like, it was a really good episode about Leonardo da Vinci. I would strongly make ren. It was one of the live ones with a crowd in it, and it was very funny. What can you remember that I trashed Leonardo da Vinci. And then Greg Jenner went, he's one of my favorite people in history. And I said, he never built a weapon. He painted seven paintings, and he just basically bluffed his way around the Renaissance.

[Hannah Fry]

The man invented helicopters.

[Dara O'Brien]

He invented a humpba Humph Hump machine. I mean, no one's built it and made it work.

[Joanna Wolfe]

Right.

[Hannah Fry]

Okay, so if you're still angry with Dara o' Brien from his appearance on you're Dead to Me, the podcast where he trashed one of your heroes, Greg Jenner's heroes, Leonardo da Vinci, then please forget about that. Please forgive him.

[Dara O'Brien]

Okay, fine. Okay, look, I'VE reformed the show, doing this show. Yeah, I don't trash anything. And we wanted to share a little bit of what's coming up on Curious Cases. In fact, a little bit more than that. We're actually gonna dropped the entire first episode here for a while so you can have a listen to it.

[Hannah Fry]

Oh, are we? Yeah, we are. That's what we're doing on their feed.

[Matthew Wills]

Yeah, I know.

[Hannah Fry]

I'm sorry about that.

[Dara O'Brien]

That's a bit much. You just finished the thing going, I'm ready. I've walked the dark now. That's done.

[Hannah Fry]

If you thought you hated Dar o'.

[Dara O'Brien]

Brien before, wait till you get a full half hour about crabs. Disrespectful to Leonardo da Vinci. I am no more positive about crabs.

[Hannah Fry]

You know how you sort of keep track of your fans? You also keep track of your anti fans.

[Dara O'Brien]

Oh, they're more. Even more voc. My fans are such, like, I just. People go, oh, I think I saw you once. Yeah, that's all right. Whereas disliking me is for life. Look at the proper passion that people have.

[Hannah Fry]

Well, anyway, to both Dara's fans and non fans and people who are ambivalent about either of us, enjoy the episode on Crabs.

[Dara O'Brien]

And don't forget to search for Curious Case on BBC Sounds to stay up to date with the latest episodes and check out on our extensive back catalog.

[Hannah Fry]

You're about to listen to a brand new episode of Curious Cases. Shows are going to be released weekly wherever you get your podcast. But if you're in the uk, you can listen to the latest episodes first on BBC Sounds. I'm Hannah Fry.

[Dara O'Brien]

And I'm Dara Obree.

[Hannah Fry]

And this is Curious Cases, the show.

[Dara O'Brien]

Where we take your quirkiest questions, your crunchiest conundrums, and then we solve them.

[Hannah Fry]

With the power of science.

[Dara O'Brien]

I mean, do we always have them?

[Hannah Fry]

I mean, the hit rate's pretty low.

[Dara O'Brien]

But it is with science.

[Hannah Fry]

It is with science.

[Dara O'Brien]

Here's an interesting thing I did recently. I did something with the Natural History Museum just in a kind of a standing in the room while they're doing proper work, like whatever, for a new exhibit that's running at the moment about space. Right. Is there life in outer space? But as part of it, I was nudging them and they went for this, that they'd have a speculative bit at the end. They were thinking of doing this where they would basically say, well, what might life be like under different gravitational conditions and different planets. Exactly. On different planets. What it might look like. So they Kind of went, okay, what could be fairly safe to make a guess that an alien life might look like? Because it's not going to be Chewbacca, right?

[Hannah Fry]

You can't say that for sure.

[Dara O'Brien]

I'm fairly sure I can say it's not all going to be what looks like a person in a suit. And so they have this kind of like, you know, nicely presented, kind of a vague sense of what these things might look like. And a lot of them look like paramecium. A lot of them looked like, you know, very, very evolutionary, very kind of simple cell of life. Like you kind of go, okay, Gran, that makes sense. But the only really recognizable thing was a crab.

[Hannah Fry]

A crab.

[Dara O'Brien]

A crab. They said, yeah, yeah, there'll be a crab. No, there'll definitely be a crab.

[Hannah Fry]

The most evolutionary stable possible.

[Dara O'Brien]

It seems to be like a very evolutionary friendly thing, a crab. You're low, you're low, you're hard.

[Hannah Fry]

You can walk sideways.

[Dara O'Brien]

Oh, look, no one's expecting that. That's the weird thing about it. No one is expecting. Oi, where'd he go? I'm looking at you. I'm looking at you. I'm looking at with me crab eyes. I'm looking, staring your eyes. No, I'm now going left. Yeah, you know, where's that?

[Hannah Fry]

Crab eyes can move around. I mean, independent of the crab legs.

[Dara O'Brien]

I mean, we don't acclaim them enough. They're amazing.

[Hannah Fry]

I think you're absolutely right, which is.

[Dara O'Brien]

Why there's an excellent question about crab.

[Hannah Fry]

Certainly is. Hello, we are Kirsten and Emily from.

[Joanna Wolfe]

Cornwall and we're interested in the weird.

[Hannah Fry]

And wacky nature of evolution.

[Joanna Wolfe]

We read about carcinization, which is when.

[Matthew Wills]

Non crab organisms evolved to have crab like bodies.

[Hannah Fry]

Why have so many different species evolved into crabs?

[Joanna Wolfe]

Does this mean that crabs are the ideal life form?

[Hannah Fry]

And if so, could humans eventually become crabs?

[Dara O'Brien]

Okay, how cool do they sound? And also I really like the fact that there is implicit in this a sense of these two young women going, will this happen in our life? I mean, what year are we talking? We're young and we're staring at our old future ahead of us. But is part of it going to be a click, click, click, click, click? Yeah. Are we going to be casting anything with our claws?

[Hannah Fry]

You know, 2060. Yeah, everything's crabs. There's a meme about this. I don't know if you've seen this.

[Dara O'Brien]

No, I haven't.

[Hannah Fry]

All the people on the Internet have been making jokes about how eventually everything's going to evolve into crabs. There was Monterey Bay Aquarium, for example, you know, one of the most prestigious and respected marine institutions on the planet.

[Dara O'Brien]

Yeah.

[Hannah Fry]

They just posted a picture of some humans dancing and a crab in a corner with a party hat looking on at them, saying I was like them once. This is, you know, one of the most respected institutions in the entire world just posting a crab content meme with zero context.

[Dara O'Brien]

So we're basically saying this is a thing.

[Hannah Fry]

I mean, look, we've got questions of plenty.

[Dara O'Brien]

Yes.

[Hannah Fry]

That we, that we need to answer. And that is why we have got three brilliant guests who are going to help us get to the bottom of things. We have Matthew Wills, who is a professor of evolutionary paleobiology at B University, Joanna Wolf, who is an evolutionary biologist affiliated with Harvard and Santa Barbara, and Ned Sysat Williams, who is director of the Crab Museum in Margate.

[Dara O'Brien]

Ned, everything evolving into crabs, is this something people are aware of, do you think? Do they ask you about in the museum?

[Ned Sysat Williams]

Yes, yeah, people ask us about it probably every day. I've never quite been able to give a fully satisfactory answer to either myself or them. I think the thing is carcinization, which is the thing that we're talking about, which is about when animals evolve into a crab like form. That was memed quite heavily. It's not necessarily more of a thing than other convergent evolutions. It's just sort of won the PR game.

[Hannah Fry]

It's a pretty good one to be fair.

[Ned Sysat Williams]

Look, crabs rule. Why wouldn't you want to evolve into a crab like form?

[Dara O'Brien]

I mean, yes, there are various forms of convergence evolution, which is basically where evolutionary forces on different species will come with the same results. So they can claim it's part of a shared heritage, but just the environment has created our eyes. And octopus eyes for example, are very good example of that, like whatever.

[Hannah Fry]

But the idea that you get eyes even though you don't have shared ancestors.

[Dara O'Brien]

Exactly. Yeah, yeah, yeah. I mean dolphins and sharks. One is a mammal with fish and yet they have share fins and flippers and, and a similar kind of skin, like whatever. So it happens a lot. But as you say, there aren't memes about that.

[Ned Sysat Williams]

Let's, let's be real and I'm gonna go on record and just say that crabs are weird and funny looking and.

[Hannah Fry]

That is hot take from the crab.

[Ned Sysat Williams]

I know, I mean it's the fact. We got it on one of our boards in fact. But crabs are weird and funny looking and yet they are part of the world in which we inhabit. And they are intriguing because of their weirdness, their Spikiness and their funny lookingness. So why wouldn't you hinge a whole discourse about evolution on a crab?

[Dara O'Brien]

Joe, this is something you study extensively. You have a brilliantly titled paper, how to Become a Crab, for example. Did you ever think your study would become. Would get this much attention?

[Joanna Wolfe]

I didn't. And I probably wouldn't have written it the way that I did had I known. I don't know if I would have called it how to Become a Crab, because people can't become a crab.

[Dara O'Brien]

What is it that's so special about the evolution of crabs that has spawned all this?

[Joanna Wolfe]

Well, I think, like Ned said, it's actually just one example of many, many examples of convergent evolution across the tree of life. We should probably define what crabs are, right?

[Dara O'Brien]

Oh, yeah. Okay, Grant. I mean, I have a strong visual image, but I put it in a proper context.

[Joanna Wolfe]

There's two groups that are each other's closest relatives. They're the true crabs and the false crabs. That's not a value judgment. It's just based on.

[Hannah Fry]

It sounds a bit judgy. I'll be honest, it does sound a bit.

[Joanna Wolfe]

It's based on the taxonomy which was named first. So true crabs are also called Brachyura, and they represent about 8,000 living species. And those include groups like fiddler crabs, spider crabs, shore crabs. Most of the things that you think look like a crab are within the true crabs. Then there's the false crabs, which. The Latin name of that is Anomura. The false crabs are the closest relative of the true crab. So they're not like human level distant. But the common ancestor of those two is still almost 300 million years ago, which is older than the dinosaurs. Right. So it is a long time ago that these two groups split. And yet within the false crabs, these groups include things. And I'm sorry that the English names are so misleading. They include things like squat lobsters, which aren't a lobster. They have hermit crabs, which aren't really a crab. They have porcelain crabs, king crabs. None of this is a true crab.

[Dara O'Brien]

Hermit crabs are like my favorite crab, and they're not a crab.

[Joanna Wolfe]

I know, right?

[Hannah Fry]

False crab. They're a false crab, but still a crab.

[Joanna Wolfe]

No, Exactly.

[Hannah Fry]

We're subject to debate. What is a crab?

[Ned Sysat Williams]

Yes.

[Hannah Fry]

Really?

[Ned Sysat Williams]

Hugely. I don't think we can neatly put things in boxes necessarily. I mean, I do agree with you that there are definitely two very distinct families, but I feel that there is some overlap.

[Dara O'Brien]

I'm just taking it personally that this is these hermit crabs who are very Much the collectible crabs on beautiful beaches that you go, look, kids, look at this crab. And to have to know that I have lied all those times I said, look at this. I mean, if they're not crabs, what are they? Sorry, they're still crustaceans, but they're not.

[Joanna Wolfe]

They're like a false crab, if you will. So they're definitely still crustaceans. They're decapod crustaceans. So that also includes a little further away, lobsters and shrimps, the things that are officially lobsters and shrimp. Again, this is taxonomy. So this isn't necessarily describing what their morphology is looks like. What they look like, as Ned said, can vary quite a lot. And that, I think is kind of the crux of what is interesting in carcinization, because oftentimes there are things that you think look like a crab, but they're not closely related to other things. They don't have a shared ancestor with other things that also look like a crab. So the hermit crabs, though, I at least think of them as not looking like a crab, because what we think a crab morphotype is is basically this sort of rounded, flattened carapace and also having the abdomen folded under the body. Now, in a hermit crab, usually you don't see the abdomen because it's in the shell. Right. But they have a soft abdomen and it curls inside of the shell. If you took one out, it wouldn't just like be held completely under the body. I guess the equivalent of what it would be like is if you were permanently doing a crunch forever. That's basically like what a crab's. Oh, okay, okay.

[Dara O'Brien]

That's certainly vivid in my head because I just think claws. Claws and shell.

[Joanna Wolfe]

Yeah, but a lobster has claws and shells. Shrimp has.

[Hannah Fry]

Well, actually, Matt, you've got, I mean, a plastic lobster here in the studio with you.

[Matthew Wills]

Here's a lobster. It's also got a claws and a shell, but it's got its abdomen stuck out behind the. The body. And this is great because animals with this kind of body plan can move around in three different ways. They can walk around on their walking legs, they can swim. They've got little oar, like limbs on the abdomen, and they can sort of beat these in a rhythm and they glide along quite elegantly. But the killer thing they have is the ability to contract all of the muscles in the abdomen. And the back end of a lobster and the back end of a shrimp is splayed out into this lovely tail fan. And when they do this, they shoot off backwards at high speed. Now, if you become a crab, if you, if you bend the abdomen back down underneath this front part of the body, the cephalothorax, as it's called, then it makes you more compact and you can armor yourself. But what you sacrifice is this. It's called the carydoid escape reaction, if you want. It's this ability to snap yourself and to shoot off backwards a vast rate of life.

[Dara O'Brien]

So you lose the. I can only think of the yummy lobster tail.

[Matthew Wills]

It is yummy. It's fabulous.

[Dara O'Brien]

But you lose that. That is what is being folded underneath.

[Matthew Wills]

Yes. Yeah, that's okay, fine.

[Dara O'Brien]

So, Matt, what does it mean to say when we say that crabs have evolved five times?

[Matthew Wills]

Biologists talk about what, what they call clades or monophyletic groups. And all that means is if you had a model of the tree of life in front of you, kind of made out of plastic, a monophyletic group is one that you can remove just by making one cut. So you get the ancestor and you get all of the descendants within that group. Now, there are other groups that turn out to be what's called polyphyletic. And that's to say if you want to cut them out of the tree, you have to make several snips in several places. What that of course means is they've evolved independently more than once, two, three, however many times. And what if you see one on the desk in front of you, you're going to call, well, that looks like a crab. If you plot that across the tree, you've got several instances of those evolving from things which are actually pretty crab like. So you have to have all the sort of building blocks. You have to already have the. In order to make that so. So that the story has sort of become everything's evolving into crabs. And that doesn't mean that humans are evolving into crabs or trees are evolving into crabs. You have to have, you have to be in an environment where that's an advantage to you. And you also have to have, you know, you also living around clefts in rocks and so forth, such that hunkering down, becoming crab like is an advantage.

[Dara O'Brien]

So it would be. The meme would be less popular, but more accurate. It was like many crab like things, things that resemble crabs.

[Matthew Wills]

It doesn't have the same rings.

[Dara O'Brien]

No, it doesn't.

[Matthew Wills]

Yeah.

[Dara O'Brien]

So we're not going to have one massive clamping arm like a fiddler crab. Yeah.

[Ned Sysat Williams]

Give it 560 million more years and let's see. I mean, crabs have maintained their shape as well. In general, say like we've got a carcinomus, a shore crab at the museum that we show kids and they wave around and drop all the time and that's like a modern day crab and it looks like a crab. But then we've also got a 90 million year old fossil of a crab which looks still like a Crab. So that's 90 million years. That's not a particularly long time. But then within 50 million years you've got animal like dog animals going and becoming whales. So evolution can happen really, really quickly. But despite all that, there are some animals that be like, I am a crab and I'm going to stay as a crab because what I've got going on is clearly working.

[Joanna Wolfe]

That, that kind of gets to the point though actually that these independent evolutions of this body plan didn't all happen at the same time. So, so in the true crabs they were, well, actually probably the second oldest porcelain crabs. Their fossil record suggests that they are older even though they're not in the true crabs. So potentially those were actually the first. I know, it's so ridiculous.

[Dara O'Brien]

I'm sorry. So now we've got pre crabs along with the true crabs and the false.

[Ned Sysat Williams]

Crabs and post crabs and post crabs. Like on the table here in front of me, I've got a specimen of Ronina rhaenina and this was more crab, like. So we were talking about when we were looking at the lobster about the abdomen being splayed out the back. Now at one point in the fossil record, Ronina rhenina, the frog crab, had its abdomen tucked more under and now it's evolving its abdomen to being tucked out. So this is a kind of example of decarcinization. So we're.

[Joanna Wolfe]

Although I would definitely add that it wasn't within that species. Right. This happened over millions of years. So it was.

[Ned Sysat Williams]

Yes, yes, absolutely.

[Hannah Fry]

To describe this, to describe this for the, for the listeners, I mean, it's sort of like lobster had a child with an armadillo, you know, and.

[Joanna Wolfe]

You.

[Hannah Fry]

Know what I mean? That's. Oh, oh no, that side's very, very scorpion.

[Ned Sysat Williams]

Yeah, that's his, that's his bum hole right there at the bottom.

[Hannah Fry]

Delightful. It's, I mean it's, it's quite beautiful in a weird way, but it's sort of halfway between a crab and a lobster.

[Dara O'Brien]

Yeah, yeah, yeah, yeah, yeah. There's less eating in that one.

[Hannah Fry]

There is a lot less eating. There's a lot less eating.

[Joanna Wolfe]

Actually, I think people eat that species.

[Ned Sysat Williams]

Yeah, very much so.

[Matthew Wills]

Very much so.

[Dara O'Brien]

Oh People will eat a lot of things.

[Hannah Fry]

Don't make judgments based on that.

[Dara O'Brien]

Yes. So it's a messier situation. But the fact remains they're just this. Something about this shape. The structure is very good for this environment, that. The environment.

[Joanna Wolfe]

Well, you know, crabs live in a lot of different environments, though. They live in every ocean, pretty much. They also live at every depth. So some species that are crab like are down in hydrothermal vents, you know, thousands of meters below the surface. Crab like and non crab like, live at the shore, and even on land, some of them even go up trees. So what is this environment? Right. What environment is it that they're adapted to? And I honestly do not know the answer.

[Hannah Fry]

Here's the question, though, Matt. So if things evolve into crabs and then stay as crabs, and lots of environments favor the crab shape in the limit, will everything become crabs? I want some truth to this meme somewhere. If you gave infinite time.

[Matthew Wills]

No.

[Hannah Fry]

All right. Damn it.

[Matthew Wills]

Agreed.

[Hannah Fry]

Was there a point where we didn't really understand convergent evolution and thought that this type of crab was actually directly descended from this type of crab? Did we get it wrong initially?

[Matthew Wills]

So before people have molecules they could sequence and look at, and Joe's pushed the bounds of this. What you had to do is you had to do, basically, comparative anatomy. You had to look at your animals under a microscope, you had to code their characteristics, and you had to try to produce a phylogenetic tree using that sort of information. I think the phylogeny of anomerons and brachaeurons was, at least in terms of the two major groups. I think that was fairly solid from morphology. But there are other examples where that's not been the case. And perhaps the best known one is the tree of mammals. So it used to be thought, go back about 20 years, 25 years ago, we kind of thought we knew how the major groups of mammals were related. And then a bombshell dropped, because people started doing phylogenomic analyses based on lots and lots of genes, and they found that the whole tree of mammals completely changed. So it turns out, for example, that we had a group called ungulates, so things that had hooves, cows and horses and so on. It turns out that horses are more closely related to bats. It turns out it's absolutely true. Elephants and Sirenians and.

[Dara O'Brien]

I'm sorry, Sirenians. I'm gonna have to.

[Matthew Wills]

These are manatees and so on. Okay, great.

[Dara O'Brien]

Yeah.

[Matthew Wills]

These are animals that are so different in their ecology, in how they look, their morphology, their anatomy, their size, everything about them. There's Absolutely no way that you would unite them into a group unless you knew about their genomics.

[Dara O'Brien]

The minute we started doing DNA testing, we just found that there were a lot more cousins in the world, a lot more related animals than we previously thought.

[Matthew Wills]

Well, at least the natural groups, the clades, turn out to be things which the comparative anatomists just couldn't see. And these weren't. This wasn't because they were being slapdash. This is really painstaking comparative work going on for decades and decades.

[Hannah Fry]

It's very similar to what happened with the human genetics, really, isn't it? There's a lot more cousins all over the place than.

[Dara O'Brien]

Yeah, family links.

[Hannah Fry]

We weren't expecting bats and horses is quite a stretch though, isn't it, really?

[Dara O'Brien]

Yeah, yeah. I mean, if you've been raised by a bat and you find that you're actually a horse, that would be a difficult conversation for any family.

[Matthew Wills]

But this is where the extent of convergence becomes really apparent, is when you start looking at these molecular trees. Those throw up so many examples that we kind of didn't see before. So moles, for example, so that there are five groups of moles that look remarkably similar. And there's a group that's evolved out of the shrews, There's a group that's actually more closely related to the elephants. There's a marsupial group of moles, again, like a crab. If I put one on the desk in front front of you now, you'd say, well, it's a mole, it behaves in the same way, it does the same sort of thing ecologically, but it's arrived at that end point convergently.

[Hannah Fry]

So tell us about Ned, tell us about crabs then. So, so I mean, in terms of people misunderstanding that the, the path they took through evolution to get here, were. Was there confusion in the past?

[Ned Sysat Williams]

Look, to be honest, I am here at this, on this radio show as a little bit of a shrimp pasta, right. I suffer eternal shrimp Osper syndrome. Right. I'm not like, we're not scientists at the crab museum. We are just a couple of likely lads who love crabs, right? And we've come to it from the outside. But for me, what makes a crab is if it, if it's giving crab. We have the mole on a table. It looks like a mole, it's a mole, like a crab is. A crab is a made up word, like we have horseshoe crabs. They are not crabs, neither are they horses or shoes, but we still recognize them for the wonderful beings that they are.

[Dara O'Brien]

Ned, let me put the question in a slightly different way, why are crabs cool? Right?

[Ned Sysat Williams]

Okay. Well, have you seen them? They're much smarter than we assume they are. For example, they wear hats. Some crabs will live inside the bums of sea urchins. Other crabs will wear jellyfish on their heads upside down, like, as kind of hats. Crabs are underestimated. They're everywhere. They are a hugely important part of the world around us. Take, for example, we mentioned mangroves and sort of tropical environments earlier. The way mangroves are is that they are a sludgy, anoxic environment. There's no oxygen in the mud of a mangrove, and you need to have the crabs to dig their burrows to aerate the roots. Without crabs, mangroves choke and mangroves die. So crabs are ecosystem engineers. They're also just all around us, and they have been for all of our history. We've been talking about crabs from as long as we can, like, scribble down things. They're part of our lives, and I think that we have forgotten how close they are to us and ended up pigeonholing them into a kind of leisure activity for bored kids.

[Dara O'Brien]

Okay, Joe, by the way, when Ned started talking about how cool they were, you and I could see this on the screen. Started doing the crab claw thing as an indication of what you. I mean, you're quite eager to go in on that. Like, whatever.

[Joanna Wolfe]

I just tend to do that on crab zooms because I think it's funny.

[Dara O'Brien]

But it's still pretty. I mean, it's a fairly useful tool to have to be able to grip things.

[Joanna Wolfe]

Yeah, we think it's related to what they eat. And also in some cases, think about fiddler crabs, right? In the males, they have one very enlarged claw, and they use those for sexual displays and sometimes even to battle one another. So there are a lot of different functions that they may have. So those shapes are related often to the function.

[Dara O'Brien]

I'm a big fan of fiddler crabs as well. They're my second favorite. Are they after hermit crab?

[Hannah Fry]

Do you know what I'm noticing, though?

[Joanna Wolfe]

Fiddlers are great.

[Dara O'Brien]

Yeah, they're cool crabs.

[Hannah Fry]

All of these crabs, they're all very similar sizes. I mean, do you get. Could you have a crab the size of a Labrador, for example?

[Dara O'Brien]

Jo?

[Joanna Wolfe]

You can. So there's two ends of the largest crabs. So the largest in terms of, like, leg span is the Japanese giant spider crab. And the name being giant, that's a pretty good indicator. And it can be, like, bigger than my arm span. I mean, I'm not that tall, but.

[Ned Sysat Williams]

The length of a 1997 Nissan Micro.

[Joanna Wolfe]

There you go.

[Dara O'Brien]

Okay.

[Hannah Fry]

New SI unit.

[Joanna Wolfe]

Yeah, yeah, yeah. So that's one way and then the other way. The heaviest and actually the heaviest arthropod ever known is alive with us now. And it's the coconut crab, which is not truly a crab, it's a hermit crab. It's funny with coconut crabs because people have talked about them as always being scary because they're on land with us and they're big. Last year I was in Okinawa, I had the opportunity to meet and basically play with a coconut crab. And I found out they're really slow, they can crush a coconut, but you can move away from them so easily, so there's nothing to be afraid of. They're actually pretty cool.

[Hannah Fry]

Dara, can I show you a picture of this coconut crab? Please do. I'll be honest with you, that looks like something out of a film and.

[Dara O'Brien]

Not a happy film. It's a nightmare. But yeah. That's pretty substantial, isn't it?

[Hannah Fry]

That's pretty substantial.

[Dara O'Brien]

They're your friend, they're not your friend. You can't establish.

[Joanna Wolfe]

It was crawling in my lap like a cat. I don't know what to tell you, Matt.

[Hannah Fry]

Top crab characteristics that you envy.

[Matthew Wills]

Lots of different appendages doing different jobs. And that's the secret to the success of many groups of arthropods. They've managed to subdivide and sub functionalize their limbs, even the pincers. They have a slightly different function for either side of the body. So they tend to have a crushing claw and a cutting claw. And if you look at many species of crabs, it's about 50, 50 right to left. And it depends how they start to use the claw. And it's self reinforcing. If they use one claw a bit more, that becomes the crusher and the one that they use less becomes the slicer. And they're programmed to become asymmetrical.

[Hannah Fry]

So you get left and right handed crabs.

[Dara O'Brien]

I think one of the characteristics which I think you've all adopted, which is interesting, is that while asking you about the crabs, you've not been defensive, not being hard shelled about them in any way, but you have evaded a lot of the crestonly vats. So your sideways movement has been excellent throughout this entire discussion. We've asked you one question and you quickly shifted to left or right and answered a different question. That's very, very impressive. I would have thought that would have been. That looks awkward and looks unusual. Sideways movement is there. What's The. What's going on there?

[Ned Sysat Williams]

Maybe it's surprising, maybe it's the element as well. It's not that. I mean, you'd have thought that maybe sort of predatory fish might have worked out by now, but.

[Hannah Fry]

Well, they've had 90 million years.

[Ned Sysat Williams]

Yeah, yeah, yeah. But slow learners are fishing.

[Joanna Wolfe]

What I can say that's good about sideways walking, regardless of what your body shape may be, you can go equally fast in either left or right direction. Can't go equally fast if you're walking backwards now, can you? Wow.

[Matthew Wills]

There we go.

[Dara O'Brien]

Although I will say, if I come across a crab that suddenly walked towards me, that'd be the next leap because I'd find that surprising.

[Joanna Wolfe]

Spider crab. A spider crab can walk towards you.

[Ned Sysat Williams]

Soldier crabs as well. Soldier crabs kind of go in huge.

[Hannah Fry]

Yeah, Dara.

[Ned Sysat Williams]

And they're big.

[Dara O'Brien]

Again, there's no statement we can make about these things.

[Ned Sysat Williams]

Thousands of them. Thousands of them coming towards you. The little balls and they walk straight towards you.

[Hannah Fry]

No, thank you.

[Dara O'Brien]

Yeah, it's probably nightmare stuff, to be honest.

[Hannah Fry]

I was with you, Ned, when you were saying every interaction you have with a crab, you're lucky that you've had that interaction with them. That one. No, no, thank you.

[Ned Sysat Williams]

Oh, yeah. I mean, but I mean, wouldn't it be cool, though?

[Hannah Fry]

I think this program has been absolutely excellent PR for crabs, I'll tell you that.

[Ned Sysat Williams]

Really?

[Hannah Fry]

Yeah.

[Dara O'Brien]

I mean, there's a crab just clicking their little things. Crab dogs are getting very happy. Very, very good. Castaneting away. Clickety clack, clack, clack.

[Hannah Fry]

Very good.

[Dara O'Brien]

You've been a joy, by the way, all of you.

[Hannah Fry]

Yeah, that's been wonderful.

[Dara O'Brien]

Really, really interesting. It didn't go where we thought it would go. It really didn't. I mean, I think we've solved the issue of converted evolution, but you have fanboyed and fangirled quite consistently and in a way which is really, really enjoyable. Yeah, absolutely. Thank you very much.

[Hannah Fry]

Thank you so much. Joe Wolfe, Matthew Willis and Ned Sissett Williams for helping us get to the bottom of this.

[Matthew Wills]

Thank you very much. Thank you.

[Joanna Wolfe]

Thank you, thank you.

[Hannah Fry]

I've got a newfound appreciation for crabs.

[Dara O'Brien]

I was always, like, faintly pro crab.

[Hannah Fry]

I think I'm anti crab and before now.

[Dara O'Brien]

But now it's turned you around. Well, but whereas I do know the next time I'm on a beach in a beautiful tropical island which is like semi regularly and we're finding hermit crabs, I'll be shouting, you are not a crab. You're a crab of lies, you false crab. You are False. I abjure thee, false crab.

[Hannah Fry]

Let me take away your shell and see you wither. What's your abdomen doing, you weakling?

[Dara O'Brien]

Yeah, stick it underneath you like a proper crab.

[Hannah Fry]

So anyone who is on their honeymoon in the next three to five years. Yeah, just. You'll see Dara Breen on your beach.

[Dara O'Brien]

This is the back of your shot. It's been a wonderful magic occasion. No, you're a liar crab. If you want to be notified as soon as a new episode is released, make sure you're subscribed to Curious Cases on BBC Sounds and have push notifications turned on.

[Joanna Wolfe]

Nature Bang Bang. Hello, hello and welcome to Nature Bang.

[Hannah Fry]

I'm Becky Ripley.

[Joanna Wolfe]

I'm Emily Knight.

[Hannah Fry]

And in this series from BBC Radio 4, we look to the natural world to answer some of life's big questions. Like how can a brainless slime mold help us solve complex mapping problems? And what can an octopus teach us about the relationship between mind and body?

[Joanna Wolfe]

It really stretches your understanding of consciousness.

[Hannah Fry]

Consciousness. With the help of evolutionary biologists, I'm actually always very comfortable comparing us to other species philosophers.

[Ned Sysat Williams]

You never really know what it could be like to be another creature.

[Joanna Wolfe]

And spongologists, is that your job title? Are you a spongologist?

[Matthew Wills]

Well, I am.

[Joanna Wolfe]

In certain spheres, it's science meets storytelling.

[Hannah Fry]

With a philosophical twist.

[Dara O'Brien]

It really gets to the heart of free will and what it means to be you.

[Hannah Fry]

So if you want to find out more about yourself via cockatoos that dance, frogs that freeze and single cell amoebas that design border policies, subscribe to Nature.

[Joanna Wolfe]

Bang from BBC Radio 4, available on BBC Sounds.

[Hannah Fry]

A kidnapped child whispers dark secrets from his past in a language he no longer understands. But a lost cassette will reveal the ugly truth. From Curious Cast and Blanchard House comes a cross continental odyssey to recover a stolen past. This is Stop Rewind, the Lost Boy, available on Spotify, Apple Podcasts, Amazon Music or wherever you find your favorite podcasts.