From the Vault: The Glorious Hermit Crab, Part 1
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Hey, welcome to Stuff to Blow Your Mind. This is Robert Lamb. Today is Saturday, of course, so I have a vault episode for you. And oh, this one published a year ago this very day. This published on 1-4-2024. It is part one in our series on the hermit crab. I hope you enjoy. Welcome to Stuff to Blow Your Mind, a production of iHeartRadio. Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.

Hey, welcome to Stuff to Blow Your Mind. My name is Robert Lamb. And my name is Joe McCormick. And we're back after the holidays. I guess you've already been back, Rob. I'm back for the first time now.

Yeah, that's right. I left a little early and then I came back to scramble together a few episodes. But now it's time to return proper with a true core episode of Stuff to Blow Your Mind. But we're ringing in the new year once more with crabs because, of course, crab content is suitable for any holiday and there is an abundance of it.

You know, I don't know if it's just the name of Christmas Island that created this correlation, but I think we tend to do crab content in the winter. Have you noticed that? That's not on purpose, at least not on my part. No, this is great. I think the Christmas Island crab thing kicked it off a while back. That's part of it for sure. There's something maybe holiday-centric about crabs.

It also helps that sometimes my family and I travel during the winter break and go somewhere where crabs are abundant. And yeah, I'm excited to talk about crabs once more because over the holidays, my family and I were fortunate enough to once more visit Glover's Reef, a partially submerged atoll located off the southern coast of Belize, California.

Tremendous fun. We stayed on a small island all week. There were other humans, certainly, but we spent as much time as possible getting into the water to snorkel, to check out the fish and the coral. But the predominant land organism was, without a doubt, the terrestrial hermit crab. So this is a holiday with crabs underfoot. Yes. I mean, I don't think anyone ever stepped on one, but they were everywhere.

Most hermit crab species are aquatic, as we'll probably touch on several times during this episode. But I'm going to be talking mostly about the terrestrial variants, like those that I encountered on this island, that still depend on the ocean for reproduction, but which live most of their lives on the ocean.

on land. And that comes with various complications and innovations. And also, they're very visible. You know, you don't need a snorkel. You don't need a dive suit to engage with the world of the hermit crab because on places like this, they are everywhere and they are widespread. You also don't have to go to Belize to encounter hermit crabs, terrestrial hermit crabs even. They are out

there. So roughly how big were or how big was the range of the ones you were seeing? Are we talking like silver dollar size or hand size? Like what are we dealing with here? Yeah, the size differential is one of the things that makes hermit crab watching in the wild so fun because you never know exactly what size you're going to encounter. Like some of the very small ones, you know, oh so cute. It's the size of a dime or something, you know, it's very small. But other times it'll be like I'm

I'm probably exaggerating to say the size of a catcher's mitt, but at least the size of a very large fist, you know, just a big chonker of a hermit crab. And as we were going about on the little pathways on the island...

you know, they would be moving around pretty much all the time. They're very busy. They're constantly trooping about. They're scavenging. They're competing for shells, though I don't think we ever directly observe this, but clearly it is happening. And if you come across one while it's, say, crossing the sandy path,

they'll suddenly stop. And then if you get a little bit closer, they'll retreat into their shells. And as they do that, that'll cause them to roll over onto their back. And then of course they cap the shell opening with their larger claw and then give them enough time and they get back up and they continue on with their business. Mm-hmm.

So, yeah, there's just something magical about hermit crabs, at least to someone like me who doesn't get to observe them all the time. But even the team on the island here, they seem to find a certain amount of joy in the creatures, despite how accustomed they were to their presence. I heard that one of the cooks brought...

some shells for the crabs with her from the mainland, uh, with the, with the names of each of her children written on them. And then, and then would enjoy like running into various crabs who would claim these shells and make them their homes. Oh, that's funny. It's like when people write funny things on a dollar, you know, it's like, are you expecting to encounter this again? Yeah. Yeah.

Yeah. And yeah, there's just something they're like little like bio machines. You know, there's just something about how industrious and relentless they are as they scavenge the terrain. For instance, this place that we were staying, this place called Off the Wall, the communal dining area at the place has a sand floor. And Jim and Kendra, who run the place, told us that first thing in the morning when you go in there, the sand is...

is completely devoid of human footprints because during the night, the crabs have come, the hermit crabs have come. And so in the morning, they are only crab tracks and not even the slightest crumb left behind because they have come and claimed everything. Crab wipe. Yeah.

So I love it. So after experiencing all that, I was like, well, we've got to find some more things to talk about with hermit crabs. We've talked about them before, but now we're going to go in a little more depth. Well, it turns out I think hermit crabs are very interesting and there is a lot we can talk about. So I guess we will start with the basics. What are hermit crabs? Are hermit crabs crabs? The answer is no.

Hermit crabs are decapod crustaceans. That means decapod, they're tin-legged invertebrates with a hard exoskeleton that grow by molting. So they shed their old exoskeleton and emerge soft and then a new exoskeleton hardens when they need to get bigger, when they grow bigger.

But they are considered distinct from what are called true crabs. True crabs belong to the infraorder Brachyuria. Hermit crabs belong to the related infraorder Anomura, A-N-O-M-U-R-A. They're close cousins to true crabs, but different. Other Anomurans or false crabs include the Lithodoidea, or the king crabs, and the Porcelinidae, which are the porcelain crabs.

And one thing anamurans generally have in common is that

OK, so these decapods all have five pairs of legs, 10 legs total. And in the anamurans, the last pair of legs farthest away from the head is fun sized. So these animals are still decapods, but a lot of them look like they have only eight legs instead of 10 or maybe six legs and two claws. Those front front legs, the cheli or the claws are legs. But in crabs, they're often claw shaped.

But they look like they have only eight legs instead of 10 because the hindmost pair is tiny and often hidden or tucked away under another body part. Yeah. And we'll get back to this particular detail later on in our discussion, if not this episode, then perhaps a second episode.

So, animurans have ten legs. A lot of times it looks like they only have eight. Hermit crabs belong in the taxonomic superfamily Paguroidea, and with a few exceptions, they are mostly notable for exactly what you're thinking of. Their dependence on externally scavenged material for armor and shelter.

Usually this armor is a shell that once belonged to a gastropod, such as a snail, a periwinkle, or a whelk.

Though there are some animals in the family that don't need scavenge shelter at all. We can talk about those in a minute. And there are a few that rely on things other than mollusk shells. One example cited in a paper that I'll get to in a minute is the Discorsopigurus schmitty, which takes up shelter not in a gastropod shell, but in empty polyheat worm tubes.

The hermit crabs that do rely on external mollusk shells have bodies that are actually shaped by this need. So while true crabs have hard exoskeletons covering their entire bodies, hermit crabs have what you might call non-calcified abdomens. So the front-facing part of a hermit crab's body, the head, the thorax, and the front pairs of legs and claws are

These all have hard exoskeletal coating like any other crab, but the back part of the hermit crab, the abdomen and what you might call the tail or the telson,

This doesn't really resemble a crab body at all. In a lot of species, it looks kind of like a curly fat worm and it does not have a hard exoskeleton. The abdomen is flexible, soft and vulnerable. It is covered in an external coating, but it's just very thin and soft. It's very un-crab.

This is the part of a hermit crab that curls up inside the externally sourced shell. Yeah. So if you're just encountering hermit crabs out in the wild or even seeing them in an enclosure somewhere, you're probably not going to see this part of the organism. With its abdomen inside the shell, as we've been discussing, it actually walks on its second and third pairs of legs. The first pairs are modified to form pinchers.

And its fourth and fifth pairs of legs are small and specialized to grip the inside of the shell. It also has appendages called uropods at the end of its abdomen to aid in securing that shell. So, you know, all these things aiding to sort of grip and hold on to that shell that it has taken on as its shelter.

The larger left uropod hooks the central post of a shell, and they can also use this uropod to hold onto other things when they are out of the shell, like I read about how they can attach to, say, like a tree or something, and it can also use it to maintain balance.

Yeah. So I think it's interesting that the hermit crab is an animal. Its body is shaped in a, in a, for double purpose. Uh, so the front end of it is shaped for facing the external world. It has the Chelly, it has the, the claws like, uh, many other crabs you would think of. Uh, but the back end of it is shaped entirely for holding on to this piece of mobile shelter and the concept of mobile shelter while not, uh,

completely unique, is pretty unique in what makes the hermit crab interesting. To read a quote from a highly cited paper on hermit crabs that I was looking at by a biologist named Brian Haslett. The paper is called The Behavioral Ecology of Hermit Crabs. Haslett writes, quote, Many animals utilize exogenous shelters, but almost all 800 species of hermit crabs are mobile while sheltered.

The combination of mobility and protection afforded by this lifestyle must contribute to the large numbers of these crustaceans found in virtually all marine environments, as well as in tropical terrestrial shores.

So Haslitt is sort of saying the hermit crab plan is a successful plan, clearly shown by the diversity of the species found all around the world. This is a plan that works because sourcing shelter from outside the body that you can take with you when you move works really well.

But it also comes with costs, and we'll talk about those costs as we go on. One thing I think to understand is that in general, while a hermit crab can leave its externally acquired shell and it can survive for some time outside of its external shell,

The acquisition of a shell for external armor is not optional for a hermit crab. It's not like a nice-to-have. It is essential for survival in the wild, and the hermit crab's evolution has been shaped by the need for these externally sourced shells.

Yeah, yeah. This is not just somewhere it goes to sleep at night. Like, it has to have this with it. And if it does not have the shell, then it is highly vulnerable to predation, exposure. It very likely will not survive without it. Right.

So I want to turn to a paper I was reading for a general overview of hermit crabs and related species. This was an overview published in the journal Current Biology called Hermit Crabs by Mark Brifa and Sophie L. Moles in 2008.

And they're going to paint the picture here of the kind of hermit crab family tree. So there are currently more than 800 known species of hermit crabs. Most of these species live in the ocean. They're marine, with a few exceptions. One is a species called Clibonarius fonticola. And this is the only known freshwater hermit crab, which lives in Vanuatu. That's a volcanic archipelago in the Pacific Ocean.

And there are other freshwater animurans, but they are not hermit crabs. So this is the one freshwater hermit crab out there. I should throw in that the hermit crabs that we were observing in the wild, I believe, are the Caribbean hermit crab. And this is a variety that's

common to the West Atlantic, Belize, southern Florida, Venezuela, and the West Indies. Yes. And the ones you were observing, you were saying, were terrestrial or semi-terrestrial, right? They spend a lot of time on land. Yeah, these were land boys. We did see one variety, and I didn't identify this one, one variety of aquatic hermit crab out while snorkeling. But yes, the ones that I'm mostly talking about here are the land hermit crabs, the terrestrial hermit crabs.

So there are about a dozen species of land-dwelling or semi-land-dwelling hermit crab descendants in a family called Cenobitids. This family includes one famous species that has shed its need for an externally acquired shell altogether, and that is the coconut crab or robber crab. We've talked extensively about these before, but the coconut crab is the largest land-dwelling invertebrate in the world.

It can have a leg span of up to one meter and can weigh almost five kilograms or about 10 pounds. They live mostly in coastal areas throughout the Indian and Pacific Oceans. I think we talked a good bit about coconut crabs in our series on the Christmas Island crabs that we did several years back.

This was the animal where we were talking about the field notes of Darwin from the Voyage of the Beagle, where he's like, you know, they make really good eating. You boil the fat under their tail and it makes a quart bottle of limpid oil. And then he also tells a story about how they like locked one inside of a biscuit tin with wire and using its claws. It was able to essentially like rip the tin out and escape the box. Yeah.

Oh, man. There's a whole horror movie for you right there. It would be Charles Darwin and crew on the Beagle having to fight off the killer coconut crab. They come and they just find the empty box. They're like, it's loose. No one knows where it is.

You know, as long as we're throwing around Charles Darwin quotes, there's another place in The Voyage of the Beagle where he mentions hermit crabs. This is not a particularly insightful quote. It's just a mention, but I still had to drag it out. Quote, in every part one meets hermit crabs of more than one species carrying on their backs the shells which they have stolen from the neighboring beach. Stolen. And yes, there is a fair amount of shell theft as we'll get into.

As long as I'm throwing around quotes unrelated, I want to throw in this quote from Aristotle from the history of animals. Is he going to tell us where they come from? Yes. Yes. He will reveal the at the time, you know, non-controversial hypothesis on crab origins.

Quote, the hermit crab grows spontaneously out of soil and slime and finds its way into untenanted shells. As it grows, it shifts to a larger shell, as, for instance, into the shell of the nerites or of the strombus or the like, and very often into the shell of the small syrix. After entering a new shell, it carries it about and begins again to feed. And by and by, as it grows, it shifts again into another larger one.

OK, I give I give Aristotle half credit on this. I think he's a bit wrong on the spontaneous generation out of slime and soil. But he correctly observes the shell shifting behavior of hermit crabs, which is a a major feature of hermit crab society that we'll have to talk about later in the episode.

Yeah. So he's absolutely wrong on spontaneous generation. We don't really have to drive that home. But in a weird way, like in a very general way, the idea of like matter becomes crabs, nature becomes crabs, maybe not that that far off as we'll discuss later on.

But apart from the coconut crab, there are various other species of hermit crab that live basically would live their adult lives on land, though they still usually live out part of their life cycle in the water with females releasing larvae into the sea. And apart from these exceptions, hermit crabs are marine species. Absolutely. We'll come back to some of the ramifications of this in a bit. ♪

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Now, according to Brifa and Moles, there are five families of hermit crabs, and it breaks down like this. You've got Paguridae, which are the right-handed hermit crabs, and you've got Diogenidae, which are the left-handed hermit crabs. Alright.

Right-handed, left-handed, what does that mean? We'll come back to that. Then you've got Cenobitidae, which are the land hermit crabs. You've got Parapaguridae, which are the deep sea hermit crabs. And you've got Pilocellidae, which are symmetrical hermit crabs. Though the more than 800 species of hermit crabs display different local adaptations and behaviors, for the most part, it seems like hermit crabs are

are take what you can get foragers, which is true of many true crabs as well. But hermit crabs, for the most part, they will eat bits of dead organic matter, both animal and vegetable. They will eat live prey when they can catch it. Like true crabs, most hermit crabs are not very picky about food. Whatever they can get in their mouth, they're probably going to eat. Yeah, yeah. Both times that I was in Belize, there was a lot of fun to be had, especially with the kids there.

of leaving something out for them with permission, of course, like I'm talking about the, like, like a cracked coconut. And then seeing the hermit crabs eventually swarm over the material. Yeah.

So while foraging for food is, of course, essential for survival, so is foraging for shelter. We will have more to say about this as we go on, but obviously a huge part of the hermit crab's survival revolves around acquiring a good shell to live in. And the majority of these shells come from, as we've said, mollusks like snails, but her

Hermit crabs don't have to fight the snail for its shell. They generally move into the shell that is left behind after a snail dies. Also, finding a shell is not a one-time pursuit. Hermit crabs grow larger throughout their lives, which means they need to trade up for bigger shells, which can lead to very interesting mass behaviors. More on that later.

One thing the authors of this overview point out is that the gastropod shell filled by a hermit crab is

is not only a hard surface to protect the soft part of the body, the soft abdomen, it also forms a kind of shelter against the external environment. Now, what kind of shelter against the environment would a hermit crab need? One example I recall from some documentary footage I saw years ago was the idea that, you know, a hermit crab that's on land is under the hot sun and

And if it's got soft body parts exposed outside of the shell, it could quickly sort of bake and dry out without the shelter and moisture provided by a shell.

Yeah, absolutely. Yeah, there again, we have to keep in mind that even though they are terrestrial, they are linked inherently to the ocean. And especially when you're dealing with tropical heat environments. Yeah, they've got to have they have to use that shell also to protect themselves and carry around some moisture. So I think it makes sense to look at a hermit crab's external shell in two different ways that both have some truth to them. And

On one hand, you could look at the external shell as an outsourced exoskeleton. It is, you know, it's like doing the role that would normally be done by the hard chitinous outer skeleton of a true crab.

But another way to think of it is that it's like a portable burrow. And this is the point that Brian Haslett was making in that quote I read earlier about the idea of mobile shelter. Lots of animals find holes to hide in and rocks or other objects with recesses to provide a protective home. Hermit crabs find protective recesses that can actually come along anywhere with them.

Now, I wanted to come back to the concept of asymmetry, which we already mentioned. Rob, you brought it up earlier, and it came up in the idea that there are these different families of hermit crabs, the right-handed hermit crabs, the Pegyridae, and the left-handed hermit crabs, the Diogenidae.

Yeah, this is all very important because we think about the asymmetry of the crab in this case. And this lines up with the asymmetry of the shells that they are probably going to be inhabiting. Right. So one thing is that hermit crabs have a directional curve in the abdomen. The abdomen can kind of

curl in a spiraling direction one way or the other. But there's also an important difference in the size of the claws. And from what I could tell, it was the claw asymmetry that was primarily used to sort these animals into the different families. One claw is often bigger than the other in hermit crabs. Now, why would the animals be asymmetrical in this way? Well, so the gastropod shells are

Most often inhabited by hermit crabs also have right-handed or left-handed spirals, as you're saying, Rob. And the abdomens are curled so that they fit into the chiral shell. Meanwhile, one claw is often bigger than the other, so it can function as what the authors call an operculum.

which in general means a structure that closes an opening or an aperture. But in the context of gastropods like snails, it has a specific meaning. A lot of snails are able not only to retract the soft parts of their bodies easily,

into their shells when threatened, they actually have a movable hard plate that they can use to close the door, essentially to block the opening of their shell behind them after they retract like a solid trap door. And this is the snail's operculum. And the interesting thing is it seems that hermit crabs

evolved claw asymmetry, at least in part, to fulfill the same function as the operculum of the snails that formed the shells that the hermit crabs take over after the snails die. So Briefe and Moles write that a hermit crab can use its larger claw to close off the aperture of its shell after it retreats when threatened. And this connects to what you were talking about seeing, Rob, where the hermit crabs, you know, they might fall

flip over on their back and then cover up the opening of the shell with one claw, maybe the bigger claw. Yeah, exactly. Yeah, I got to see this happen so many times. It's like, imagine some sort of futuristic crab that has evolved to use abandoned human port-a-johns or portable toilets like you see at concert venues as its home. And then it has evolved...

its larger pincher to be the exact shape needed to serve as the door of that portage on. I thought you were going to say it has evolved a claw that can say either vacant or in use. I guess it would never want it to say vacant. So it says in use on its claw. Yeah. I mean, maybe it wants people to go. That's how you get mimics. That's exactly how you get mimics.

So anyway, you end up with these asymmetries. You have right-handed and left-handed hermit crabs, the pagyrids and the diogenids, respectively.

There are also what are known as symmetrical hermit crabs called the pilochelids, which actually do, unlike the others, they have hardened exoskeletons on their abdomens. Though they do still hide in burrows, like in recesses in wood or rock, but are thought to leave these burrows to feed. So they're related animals, but they live somewhat different lifestyles.

Now, we need to talk more about the selection of shells among asymmetrical hermit crabs and how that important biological need shapes hermit crab behavior and even what you might call hermit crab society. One thing you might naturally wonder about with hermit crabs and the selection of shells is how.

Is one shell as good as the next? You know, is any shell just as good as another? And the answer is no, not at all. In multiple ways. One big factor is, of course, size. A shell that is too small or too large will greatly reduce a hermit crab's fitness.

And studies show that when a hermit crab's shell is too small, for one thing, it just increases mortality. Too small a shell means the hermit crab is more likely to die. But it also has less room to grow. And females with smaller shells produce fewer offspring. So it is not good for a hermit crab to have too small of a shell. However, it doesn't just want the biggest shell possible because if the shell is too big, that increases the energy cost of carrying it.

Yeah, absolutely.

So finding a shell of just the right size is crucial, and that right size will change throughout the hermit crab's life as it grows. Yeah, so let's get into this a bit more. It's such an important aspect of the hermit crab, and it's the first thing we think about, but it really defines almost everything about their social interaction.

And it's interesting to think about hermit crab society or at least, you know, think of them as social organisms because they're

We call them hermit crabs, which is a bit misleading because in human history and in just sort of common usage, what is a hermit? A hermit is someone who withdraws from society and or civilization. So it might lead you to believe that hermit crabs are also loners. And in a certain sense, they are. But, you know, but that doesn't mean that they they they don't have interactions with others of their kind. In fact, they have a lot of interactions and they're very complex. So don't be too literal in thinking that a hermit crab is.

is an actual hermit in the same way that you wouldn't think that a king crab actually rules over decapods or something. Right. And issues decrees. Yeah. Yeah. As Mark E. Lydra pointed out in an article for Natural History magazine back in, I believe, 2019, he

Hermit crabs live actually highly social lives, absolutely full of drama. Just way more drama than you'd expect your local human hermit to have, I'd wager. Yeah, I mean, I guess unless hermits were like constantly fighting one another to try to trade clothes. Yeah, or a pillar. You know, if you have your pillar developers, they'd be like, no, I want the taller pillar. Yeah, exactly. I'm going to bang my pillar on your pillar until you give me your pillar, unless I decide I want my old pillar back.

So the various drama that they encounter and instigate includes, but is not limited to, according to Lyra, commotion of social aggregations, intergenerational inheritance of homes, life or death competitive struggles.

So the author points out that in contrast to a lot of other social organisms you might think about, terrestrial hermit crabs socialize with non-relatives. And this is crazy to think about. The reason, he points out, is that while they spend most of their lives on land, we're talking about terrestrial hermit crabs again, not the aquatic ones, they spend most of their lives on land, but they are born in the sea.

That's where the larvae are released. They become mixed in the ocean, and then they land on far-flung shores via the tides. So I want to read this quote from Lyra. He says, quote, By the time an immature crab first arrives on land, it is therefore far away from any of its relatives, encountering instead only an assortment of non-kin. Moreover, because the abundance of terrestrial hermit crabs is

on short stretches of beach often measures in the hundreds of thousands or even the millions. Each crab is but a stranger within a vast crowd. However, you know, if you think of a human analogy, imagine a bunch of very selfish humans are thrown into a mix of, you know, they're all living together beside one another and not amongst their kin, just among strangers. And, you know, maybe imagine they're not very nice people. They're not very inclined to be helpful to others. They

They still might find reasons to hang out around one another, even if they're mostly selfish. And one of those reasons might be the need for trade or an economy of sorts. Exactly. And what are they going to have an economy of sorts about? Well, it's going to be the shell, of course. I also like how this idea, it kind of matches up with the huge stereotype that one encounters of big city life, particularly in movies from like the 70s.

And I guess into the 80s as well. Like nobody's related to each other in the city. You go to the city, it's just everybody's for themselves. You're going to lose your shell in that big city. And I guess it's kind of that way with the craps here.

But anyway, the shells that they use, like we've been saying, these are scavenged from dead mollusks. But this is also key. They have been remodeled. These are remodeled homes, Lydra points out. The crabs, when they have a fresh shell that is going to, like, this is, okay, something has died. A snail has died, and now I'm going to make this shell into a home. You can't just put it on and wear it out. No, no, you need to use chemical secretions to weaken the shell's calcium carbonate.

as well as additional physical sculpting via your appendages, you're going to change the shell into something that absolutely suits you and absolutely suits your purposes. Right. So this is another way in which one shell is not necessarily as good as any other. Size matters a lot, which is a sort of inherent feature of the shell, but also the remodeling condition of the shell matters a lot.

There are shells that have been recently renovated, and that's much more desirable than a shell that is a real fixer-upper.

Right, right. And this is unique to terrestrial hermit crabs. And we'll get more into why in a second. But as Lydra points out, it's absolutely necessary for terrestrial Hermes because they can't depend on water buoyancy to help them carry that shell around. It's just the basic reality of living out of the water. And some of these shells can be quite hefty for the crab.

And so by reducing the shell mass, they're lowering the energy cost of just traveling across land, and they do a lot of traveling. Additionally, shell remodeling increases the internal space in the shell, allowing not only more room for the crab, but more room for increased water reserves to keep the organism from drying out. Like we mentioned earlier, the importance of this being a way to help them sustain themselves when they are living at times in very hot environments.

He also points out that while aquatic hermit crabs would conceivably benefit from shell remodeling as well, because, you know, even though you have buoyancy, lighter shells could still be a benefit, right? More internal space could still be a benefit. However, aquatic hermit crabs have to contend with shell expert predators in the ocean, organisms that are highly evolved to bypass shell protections.

Therefore, it would be a mistake to sacrifice any of your shell's protection in order to get any of these benefits. Meanwhile, on land, the terrestrial hermit crabs have far fewer shell specialists to contend with. And Lyra points to various research that's shown that like your average

predator that would be messing with a hermit crab is just not going to have like the bite power or just not going to have the tools necessary to crack open even a partially degraded, a partially remodeled hermit crab chosen shell.

Now, Joe, I included a photograph here. This is from Lyra's paper. And this is an example of an unmodified versus a modified shell. The central axis is often removed, but the shell, again, retains protection against the bite strength of terrestrial predators. Yeah. And one thing you can see in this picture is that a lot of the effort in the remodeling seems to be focused on the interior of the shell, sort of like expanding the interior cavity and making more room there and smoothing it out.

Yeah. And certainly, you know, removing some of the additional protection of the shell. But again, it's apparently not going to matter for terrestrial variants.

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Now, like we said, one of the huge factors in hermit crab societies is, of course, the need for these shells and the fact that as the crab grows and molts, they'll need to abandon smaller shells in favor of larger shells. And as Lydra points out, there's something really interesting about all of this. Quote, over time, remodeled shells have come to dominate the housing market of terrestrial hermit crabs.

Exhaustively sampling these housing markets, I have found few unremodeled shells and also few shells that are still in the process of being remodeled. Most shells have already been completely remodeled.

Remodeled shells present a superior home, and as with any superior resource, it is understandable that terrestrial hermit crabs should prefer remodeled shells over unremodeled shells. However, over evolutionary time, a mere preference has transformed into an absolute dependence. As remodeled shells accumulated, terrestrial hermit crabs came to specialize in living in them, to the point that life in unremodeled shells became nearly impossible.

My field experiments revealed that after an early life stage, most terrestrial hermit crabs cannot survive in unremodeled shells for even a single day. Wow. Okay. I did not realize that.

Yeah, yeah, because, you know, what's a hermit crab to do? If you need a shell, well, you could remodel your own shell, technically, but most crabs simply can't do this anymore, in large part because they end up too big to access the inner parts of a fresh shell. So only the smallest immature hermit crabs actually can get in there and do this, which paradoxically often means that they're left with oversized shells. But as such, all terrestrial hermit crab individuals can

highly prefer a remodeled shell over a fresh one. Even if they can fit inside that fresh shell and conceivably remodel it and make it their own, remodeling the shell is still a last resort. Lydra stresses that this is the only factor that ensures the continued creation of remodeled shells, that some hermit crabs are going to be put in a position where they have no choice.

Wow. Because, I mean, they can do other things too, it should note. Like if a crab, a hermit crab is left without anything and we'll touch on how this occurs in a moment, they may use something like a bottle cap and that might buy them a little time, but they need a shell. The thing is, you don't just find many emptied remodeled shells anywhere.

He points out that on a given beach, the only empty remodeled shells you're likely to find are the ones that have been physically compromised in one way or another. So like they're broken. They don't actually protect the hermit crab anymore. Or they're clogged with a rock, something like that. They're no longer functional. The competition for remodeled shells is intense. And you're just not going to, as a hermit crab, you are not going to find them out there unless you're extremely lucky.

As such, hermit crabs have to remain vigilant at all times, always looking out for a bigger shell because either you need one or you're about to need one. So you need to always be looking. And also you have to be peeking over your shoulder because there's probably someone eyeing your shell.

that is looking for a slightly bigger shell, and they're trying to decide if it's worth trying to take it from you. Right, so this leads to the fact that hermit crabs very often get into... Well, I guess there's some debate over how exactly to characterize these encounters, whether they are...

purely agonistic fights or whether you might consider them in some way a kind of trade or negotiation. Maybe we can talk about that some more in the next episode. But they get into these encounters that lead to shell swapping. Yeah, they're the very least aggressive negotiations. Yeah. Yeah.

But, you know, so I'll continue to categorize them as battles. But yeah, with that caveat in mind, basically what seems to be happening is one crab will attempt to flip the other on its back and force its opponent out of their shell.

But as Lydra points out, this kind of thing can last hours and it might just end in a stalemate. It might just end in everyone just going home or carrying on with the shell they had two hours ago. And that alone is pretty fascinating. Most animal competitions and struggles are fascinating. But it's the onlookers that make this even more interesting. Lydra writes, quote, as bystanders gather at the site and

And as the moment of eviction nears, order emerges out of the chaos of commotion in the aggregation. The crabs array themselves literally into a line, each holding the shell of the crab ahead of it. This social formation emanates from the pair of antagonists, with the line of crabs thereafter being size-ordered from biggest to smallest. Untactable.

unquote, because if a crab is forced to give up its shell, the winner will leave its shell and this will set off what's called a vacancy chain. All of the onlookers will have a shot at leveling up their current shell situation to a new one that is slightly bigger. You can find video of this online, by the way. There are multiple documentaries that have

captured versions of this. And it's amazing to see. They literally do just organize themselves in a size-ordered line where they're each grasping the shell of the slightly bigger one, like feeling around on it, trying to make sure that it's what they want. Because the whole process, by the way, in which hermit crabs are

assess a shell for the qualities they want is interesting on its own. There seem to be some visual processes going on. They, you know, look at a shell to assess from a distance whether it's something they would want, but that's never enough. They have to inspect it physically. They like feel all over it with their antennae and their, and their celli, you know, the claws and the legs and,

And then usually they will want to sort of like dip into it with their legs and maybe their abdomen to see if they fit right. And they may end up changing their mind and wanting to go back to the previous shell, though obviously that could be a tricky thing if a bunch of hermit crabs are lined up all trying to level up at the same time. It is an amazing thing to see.

Yeah. To your point about sizing up the shell, trying it on, sometimes you have a situation where you have, remember the asymmetry of shells and hermit crabs here. And sometimes you have a left-handed crab trying to get into a right-handed shell or vice versa. And that's going to be a situation where the crab is going to quickly realize this does not fit. This is not the shell I want to live in.

We may come back with more details on that regarding the left-right handedness in Hermit Crabs and Shells. Now, Lyra, again, points out that all this competition is based, again, not on kinship, but on selfishness and competition.

And he has this wonderful part in the paper where he says that it's what's kind of the antithesis of something like a social ant, you know, where all the ants in a colony are kin and they work together to ensure the success of the colony, the success of their genetic line.

But that is not the case with the hermit crab. And Lydra writes the following, quote, Now,

there have been plenty of other researchers and also science journalists and writers and so forth that have commented on all of this. Elizabeth Preston wrote about this topic for the New York Times in 2019 in an article titled, Even Hermit Crabs Have Wealth Inequality. This was based on a 2020 study. This is because the New York Times paper came out a month before the 2020 study in a December publication. So that's why the dates change.

would seem to be in the wrong order here. But that particular paper was A Comparison of Wealth Inequality in Humans and Non-Humans by Chase et al. This was a study that, as I recall, received a fair amount of mainstream attention at the time because, of course, wealth inequality is always a topic of interest among human beings, and crabs are inherently interesting. And there's this...

You can throw hermit crabs into any study and it's going to be fascinating, even if there's not really connective tissue there. Well, is there connective tissue here?

To a certain extent, it seems to be. I mean, yeah. So in the study, Chase and his co-authors gathered around 300 hermit crabs on Long Island Beach and took away all their shells, which, OK, for science, I will allow this. But, you know, obviously I can't help but sympathize because this is literally all these crabs care about. The researchers weighed and measured the various shells, and then they considered how they were distributed across the sample population of hermit crabs.

And this is a quote here from that article by Preston. Quote, the distribution curve they found peaked around medium-sized shells, then dropped as the shells got larger before tapering off very gradually through the largest shells of all. This matches the shape of wealth distribution curves in many human societies.

So it's interesting. But at the same time, the New York Times article citing anthropologist Monique Mulder points out that we shouldn't get too carried away comparing hermit crabs to humans in this scenario, because, first of all, there are plenty of other factors involved in human inequality. You know, it's one of those things.

It's often described as a wicked problem. You know, it's complex enough that you can't just point to necessarily. There are certainly large factors, but there are multiple factors. And Mulder here, who, again, is an anthropologist, speculated that vacancy chains are probably also not the only factor in terrestrial hermit crabs as well. I mean, you know, there are other factors as well into how they are dealing with each other, even on the basis of their shells.

Still, you know, we can't help but compare ourselves to the hermit crabs. Again, part of it is just that hermit crabs are that interesting. And we as human beings are that self-absorbed. Like we can't help but see ourselves in the crabs. We can't help but anthropomorphize the crabs.

And especially when we see them in a very complex fashion struggle over limited resources like this. But again, I think if you're just casually aware of hermit crabs, you might not realize that just any shell, it's not a situation where any shell will do.

And it is a situation where the shells have been augmented. And therefore, it's competition not just for random garbage left over by dead snails. It's competition for mostly previously augmented shells, like the products of a hermit crab civilization, if you will.

That's funny. You think of it as like more fighting over fighting over a limited pool of finished goods rather than creating new goods. It's almost like a human post-apocalyptic scenario. It's kind of Mad Max.

It's kind of Fallout, right? It reminds me that in at least some of the Fallout games, you do encounter a giant hermit crab that is using, what, some sort of like a bus or something, like a school bus for its abdomen. I can't remember offhand, but I remember when I would play the most recent Fallout game, I would occasionally encounter that creature. And yeah, yeah. I mean, it's interesting. Where are they getting all these school buses? What are they changing about? I guess they're taking all the seats out. Yeah.

Is it a right-handed bus or a left-handed bus?

I don't know. Yeah. And are they sizing up from different buses? Like they start with a, like a shorter bus, then they go to a full size school bus and they're going to like a mega bus. Then it's a double decker bus. Those are the only, that's only in fallout London, probably, but you know, you could go wild with it, with the scenario. Well, Hey, we've got more hermit crab stuff to talk about. So join us again next time. That's right. We have some more content lined up regarding the hermit crabs. We're going to fish around for a bit more because I'm a hermit crabs.

There's a lot of research out there. That's one of the reasons I felt pretty secure in going in and discussing them again, even though we've discussed them in the past, because there's constantly new hermit crab research coming out. And there are some remaining mysteries and controversies about hermit crabs. So we'll get into all of that in the next episode. In the meantime, we'll remind you that Stuffed to Blow Your Mind is a science podcast podcast.

with core episodes on Tuesdays and Thursdays. That's the main event. You can also tune in for a short episode on Wednesday, a listener mail episode on Mondays. And on Fridays, we set aside most serious concern to just talk about a weird movie on Weird House Cinema.

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I'm Jason Alexander. And I'm Peter Tilden. And together, our mission on the Really Know Really podcast is to get the true answers to life's baffling questions like why the bathroom door doesn't go all the way to the floor, what's in the museum of failure, and does your dog truly love you? We have the answer. Go to reallyknowreally.com and register to win $500, a guest spot on our podcast, or a limited edition signed Jason bobblehead. The Really Know Really podcast. Follow us on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.