ExtinctZoo - Why All Roads Lead To Crab

Episode Date: June 25, 2026

Crabs, It's always been crabs....

In 2017, behavioral ecologist Mark Ladry documented some pretty odd behavior from coconut crabs in the small island in the Indian Ocean. He had set up cameras to better understand their foraging patterns, and in the middle of the night, one of the cameras showed a coconut crab climbing a tree. And yes, climbing, eventually walking out into a low brand to where a booby, a type of bird, mind you, was sleeping. At that point, it then proceeded to stretch out its claw, locked them around the bird's wing, and boom, snap, breaking its wing and causing it to fall to the ground. Now, the bird tried to fight back, but he might as well have been fighting a living tank, as the crab did not give any quarter and proceeded to smash the heck out of it, ultimately causing the bird become paralyzed. And within 20 minutes of this starting, five more of its bloodthirsty homies had shown up to join the fun.

And by morning, the bird was completely gone. And when I say gone, I mean, Eaton. Now, Mark also surveyed the surrounding islands and found something rather grim, which is that on islands with high coconut crab densities, ground-nesting seabirds, basically do not exist. On the flip side, the one nearby crab-free island in the survey, found ground-nesting birds were happily nesting on the ground like, well, any other self-respecting ground nester. So in other words, an entire seabird community had been bullied, or rather, made locally extinct, by an arthropod. Oh, and, by the way, coconut crabs aren't technically crabs. Yeah,

the name's deceiving. And as it so turns out, coconut crabs are not alone in this. A surprising number of the animals we casually call crabs aren't actually crabs in the taxonomic sense. They just look like them. King crabs, not crabs. They're hermit crabs that gave up on the shell. Horsesland crabs? Not crabs either. They're more close related to squat lobsters.

The hairy stone crab cleaning the rocks in southern Australia, while also looking like a crab and being hairy, also not a crab. In coconut crabs, the nightmare fuel that they are, are as we just established, also not crabs. And the really weird thing isn't just that they aren't crabs. It's that they aren't even closer related to them either. These are completely separate lineages of crustaceans that somewhere along their evolutionary history independently arrived at roughly the same body plan, despite starting from totally different taxonomical places.

And this happens so frequently that there is even a name for it, carcinization, a term coined back in 1916 by an English zoolologist named Lancelette Alexander Boredale, who described it as nature's many attempts at evolving, well, perhaps. And then in 2021 phylogenetic synthesis, researchers, estimated that the fully crab-like body plan has evolved at least five times independently in different lineages, which is why there's a meme floating around the internet that says that everything eventually becomes a crab. Now obviously the meme isn't literally true, evolution doesn't running around with some kind of crab-zapping wand, and humans, fortunately or unfortunately depending on your feelings about

having claws, aren't going to be turning into one either. But the shape really has been reinvented again and again in lineages with completely separate evolutionary histories. And the question of why turns out to be pretty interesting. Now, to actually get into the why, we need to define what becoming a crab really means. Because just saying looks like a crab is doing a lot of work, and not especially helpful. Where the specific changes involved, actually mattering. So to understand this, we need to go back to the basics, the starting point, if you will.

What for all decapod crustaceans is very roughly a more shrimp-like potty plan, meaning elongated body, long muscular abdomen trailing out from behind, and a carapace longer than it is wide, etc. And that is the classic Trimto-Lopster silhouette. And the really key thing about this is that the tail is exposed, and not only exposed, but also extended, and, well, useful. Because you see, lobsters and Trimp use that tail for what's called the caridoid escape reaction, which is basically when they flex the abdomen and tail and rocket themselves backwards to the water like organic jet skis.

Now, it's fast and obviously works, but at the same time, from a predator's perspective, that long muscular tail hanging out from the back is basically a grab-me sign, or perhaps a bite me sign. So to become crab-like, the whole setup has to shift in a few coordinated ways. The shell on top has to flatten and widen, the separate plates on the underside emerged together to form one wide, flat shield, and then the long abdomen has to shrink, fold forward, and tuck up under the body. In fun fact, there's actually a fancy developmental term for the tail-tucking part, and that is called brachiorization, which is a sciencey way of saying the

abdomen folds forward during metamorphosis. And remember that part, as we're going to come back to that in a bit. As the genetic mechanism behind brachiorization turns out to be a lot more interesting than the word makes it sound and is basically one of the keys to this whole becoming a crab thing. But before we go there, quick taxonomic distinction. There are two major groups of crustaceans that we're talking about here. Bracchiora, the true crabs, which originated as one lineage, and stayed that shape with not so much variation, and is also the reason why it's called brachiorization, and then Anomura, a sprawling group that includes hermit crabs, king crabs, porcelain crabs, squat lobsters, mole crabs, and a bunch of other random characters.

Now, the easiest way to tell them apart in person is to count the walking legs, true crabs have one pair of claws up front and four pairs of walking legs behind them. However, on the flip side, many crab-shaped annamurans have one pair of claws and only three obvious pairs of walking legs, because the back pair is reduced and tucked up under the carapace, where it mostly gets used for cleaning the gills. Small anatomical difference, yes, but it's important, as it means a king crab and a dungeoness crab, which is, yes, its actual name, who might be on your plate, are in reality, despite appearing to be cousins at the very least, are separated by hundreds of millions of years of independent

evolution. So yeah, they are more disson related than we are to literally any other living mammal on earth. Anyways, going back to the star players here, the crabs, well, not crab crabs in name and crabs in shape, and, well, you get the point. So king crabs are one of the more dramatic transformations in this story, at least evolutionarily speaking. You see, they aren't crabs, which yes has been established, but get used to hearing this, they're rather hermit crabs that decided to be a bit more social. So hermit crab lineages have spent over 100 million years renting out discarded snail shells. Their entire anatomy has been shaped by that lifestyle. Asymmetric claws, because the larger one has to plug the entrance to the shell like

a cork. A soft, uncalcified abdomen that coals perfectly into the inside of a gastropod shell and a growth ceiling that's basically capped by how big of an empty snail shell they can find on the seafloor, which means the hermit crab's entire life is essentially one, long, stressful search for a home. In a landmark study from 1992, placed king crabs phylogenetically inside the hermit crab genus, pagerus, and pretty much every molecular study since

is confirmed a hermit crab ancestry. So the idea is that somewhere in the past, selection started favoring hermit crabs that could calcify and harden their own abdomens, basically growing their own shells instead of renting one out. And once that shell size constraint relaxed, well, level up, is the lineage produced some absolute giants.

For example, a big red king crab can roughly 28 centimeters or 11 inches across the carapace and then a like span of around 6 feet or 1.8 meters and way up to about 28 pounds or 12 kilograms so in other words basically a small dog with very very long claws and if you flipped one over the abdomen is still subtly asymmetric a leftover from an ancestor that once coiled into a spiral shell porcelain crabs the family in porcelainaday or another case of carcinization but they went in a totally different direction they evolved from a squaw lobster like ancestor and stayed tiny, with some species so flat that they can squeeze into a crack thinner the near finger. What's odd is that their big front claws aren't really

used to assist with eating. That's instead handled by feathery mouth parts as we plinked in out of the water like a biological strainer, whereas the claws are then mainly used for defense and maybe beating up other porcelain crabs. Let me tell you, these guys are hella committed to the idea of not dying. To the point where if something just grabs one by the claw, the crab will just decide to self-amputate itself instead of risking it, essentially cutting off its own claw. And like seriously, it just pops it off, sprints away, and then regress a replacement over the subsequent mold like nothing happened, except perhaps for some lingering mental trauma, which is honestly a pretty elegant solution to the specific problem of having evolved good armor, but also

being bite-sized. And then follow genetically, recent work suggests that the poor Visit BetMGM Casino and check out the newest exclusive. The Price is Right Fortune Pick. BetMDM and GameSense remind you to play responsibly. 19 plus to wager. Ontario only. Please play responsibly. If you have questions or concerns about your gambling or someone close to you,

please contact Connects Ontario at 1-866-531-2,600 to speak to an advisor. Free of charge. BetMGM operates pursuant to an operating agreement with Eye Gaming Ontario. Porcelanidae actually represent one of the older carcinization events within the Annamura, being considerably older than the king crab lineage. So basically the small porcelain crabs came first, and the giant king crabs are the new kids in the block by comparison. The hairy stone crab, Lomas Hertha, which is also my personal favorite, is from another family called Lomissidae.

Now it lives in the rocky shores of southern Australia, maxing out at about 2.5 centimeters, or roughly an inch across the carapace, and then his covered head to claw and a thick coat of brown adorable fun. Obviously the fur here is not for warmth, but rather to give the animal muddy rock like camouflage so it can cling under stones instead of trying to outrun anything. And then there's the coconut crab, Burgus Latro, which we already met committing a felony at the start. Also not a crab, and also descended from hermit crab like ancestors, just like a king crab,

but this time going in a completely different evolutionary direction. As juveniles, coconut crabs actually behave like ordinary hermit crabs, finding and wearing snail shells for protection. Then at some point during growth, they just stop. Instead of looking for a bigger shell, they would just never look back, instead of hardening their own abdomen. And as they get older, they also develop brachio-stegial lungs inside where the gills are, which is basically gill-like tissue would specialize for breathing air instead of water, and essentially makes them land animals in the practical sense.

Now females still do have to return to the sea to release their eggs, but if you held an adult coconut crab underwater for any meaningful amount of time, it would actually drown. Which is kind of wild in itself, a crab that can drown. No wonder it's not a real crab. Am I right? That said, though, they're not the only landlubber crabs in need air, and there are actually some true crabs that have also shun the water. But I digress. Now, honestly, the coconut crab might be the most over-the-top, quote-unquote, crab, period, and by a wide margin of that.

And that's not just regarding their whiteness, pun intended, as there's a lot going on with them beyond just the size. Take their sense of smell, for example. A 2005 study found that coconut crabs have evolved an olfactory system that doesn't really resemble other crustaceans at all. Their olfactory neurons look shockingly similar to that of insects, not only morphologically, but also behaviorally and physiologically, which has been hypothesized as potential consequence of the transition to land. And as is so fitting of their name, another study identified a specific volatile compound called acetoyne as one of, if not, the chemical, that attracts them to food.

And guess where this chemical can be found, among other places? Yeah, in coconuts. So to recap, this is a crab, not crab, that evolved from a not crab to crab, while also picking up an insect-style sense of smell, so convergent evolution within convergent evolution, and coconuts. Then there are the tools of mass destruction, or at least foul destruction. In other words, the claws. And when I say tools of destruction, I mean it.

As the 2016 study, which caught 29 wild coconut crabs, ranging in body weight from about 33 grams, up to around 2.1 kilograms, and basically clamped each one onto a force sensor to see what they could do. And let's just say, they definitely could do. As individual pinches range from roughly 29 newtons at the low end to about 1,765 neutins at the high end. And when the researchers scale their data up to a maximum recorded body weight for the species,

around 4 kilos, they estimated a potential pinching force, if you can still call it that, around 3,300 newtons, which just for context, the average human grip strength is about 400-ish newtons. So we're talking about an animal many times smaller than you, but has a pinch 8 times stronger in your hand. So, uh, maybe don't agree to thumb rustle on these guys. And their predator behavior does not stop at pinching. Coconut crabs will eat or try to eat just about anything, feeding on fruit, carry on, and will even engage in cannibalism, eating their own kind, because apparently chill is lacking despite being named after coconuts. Oh, and, uh, just to top it all off, they have been reported

to be able to live up to 60 years, which for an arthropod is straight up unhinged. Okay, So we've got king crabs, porcelain crabs, hairy stone crabs, and coconut crabs, all as independent carcinization events within Anamura. Plus, of course, the true crabs themselves over in Bracchiera. So multiple separate inventions of basically the same thing. But as it so turns out, becoming a crab doesn't have to be a one-way streak, even for the true crabs. Take the frog crabs. They are true crabs, which means their ancestors got crappified in the standard way, a couple hundred million years ago or so, but somewhere down the line, selection for Burrall, growing and loose sand started favoring a longer, more cylindrical streamlined body, and the

tautail started coming back out. And this is a real phenomena that biologists call decarcinization. I know, shocker. And in that same 2021 synthesis I mentioned, researchers flagged at least seven apparent losses of the crab body plant across the decopods. So evolution made a crab, took a second to look at it, and went, actually, never mind, put that tail back, and a few times over at that. And in between these extremes, you've got animals that show why carcinization is honestly

better thought of the gradient than a binary switch. Squall lobsters are a clean example of this, and the name, like, normal, I guess, is pretty misleading, because they aren't really lobsters. They are unamurrence, but they're more close related to hermit and porcelain crabs than to anything you'd put in abysk. Their carapace is somewhat flattened, but still longer than it is wide. Their claws are often weirdly, almost comically long, and their tails partially folded under, but then not fully tucked. So they kind of looked like someone started building a crab at built a crab workshop, but then got distracted halfway through and decided that it was good enough. which for their environment, it actually was.

And speaking of environments and lifestyles, the crab body plant is pretty much like a Swiss army knife, because it seems to do it all. Without the tiny end of the spectrum, you've got the peat crabs in the family Pinotheraidae, which grow to just less than a centimeter and spend most of their lives living inside oysters, clams, and muscles. Well then at the giant end, you've got the Japanese spider crab,

macrokira, Khym, whose legs span can reach roughly 3.7 meters or about 12 feet from claw tip to claw tip, making it the longest leg span of any living arthropod. Then you have ghost crabs, on the other hand, who are quite small, they can then sprint across beaches at speeds of about 2.1 meters per second, or roughly 6.9 feet per second, all the while having muscle contractile properties and locomotor characteristics that look weirdly similar to those of a mammal, not a crab, which is not a sentence I would ever expect to say about, well, a crab. And then there's a coconut crab, which, as we covered, found the word pairings of ocean and no thanks, with chicken and yes thanks, quite tantalizing. So clearly the crab shape isn't just specialized for one thing, which thus begs the question of why this shape is such a versatile, well, shape in the first place. And the answer is mostly mechanical.

First off, a flat, wide body gives a lower center of gravity that makes the animal more stable on uneven surfaces like intertidal zones, while then also giving it its key ability, the sideways shuffle, which allows the crab to dart left or right, while most of its predators are forward-moving animals. Then, of course, the tux tail removes what would otherwise be a perfect grabbing zone for would-be predators, as you literally cannot grab a crab by the tail, because, duh, there is no tail to grab. Now, interestingly, true crabs likely originated around the Jurassic period based off of fossil evidence, but a lot of their modern lineages diversified during an interval of roughly 145 to 66 million years ago,

and paleontologists call this stretch the Cretaceous crab revolution, and it overlaps of a broader event called the Mesozoic Marine Revolution, where marine predators evolved stronger jaws, more specialized drilling and crushing structures, and more efficient ways to crack, peel, and grind hard prey. And of course, if that happens, the pressure rounds the other way, too, for prey. And so the whole marine ecosystem was basically a slow-motion arms race for millions of years. Now, was it this event that drove the massive diversification event in crabs? Hard to say.

But funny enough, it's often crabs that are included in the list of those shell-cracking predators, not as the prey themselves. So there's definitely something going on here. But regardless, the fossil record from this period also gives us some genuinely strange animals, with combinations of features unlike anything alive today. For example, Kalika Chimera Perplexa, found in Cretaceous Rocks in Columbia, and described in 2019, has been nicknamed the platypus of the crab world, because it combines a crab body plan, but with huge unprotected compound eyes and paddle-like legs for swimming instead of sidewalking,

and all in a body about the size of a quarter. In a 2022 follow-up study looked at its eye anatomy in detail and found that its compound eyes were enormous relative to its body size, and grew rapidly during development, both of which are a class. signatures of an active visual predator. So anyways, we have a decent answer for what carcinization is, when it was happening, and roughly why. But the last question still unanswered is the how. Like, what is it actually happening that makes the same shape somehow so reachable for so many

different lineages? And is it possible in humanity? Just asking for a friend. And so to the answer to this is that a study in the Chinese mitten crab tracked gene expression during the metamorphic transition when the abdomen first folds under and identify the certain genes show decreased expression of exactly that moment, such as SCR and ANTP. Now, these are hox genes, which is the family responsible for laying out the body plan of an animal during development, basically acting like the master architects telling different body segments what to

become. And so, unsurprisingly, these are deeply conserved across animals, like properly ancient as ancient can be, and show up in basically every bilaterian animal, so from flies, all the way to kangaroos. And in crabs, these hoax genes get down-regulated as serious. developmental moments, driving the brachiorization. So what this means is that the entire crab look isn't being built by inventing new genes from scratch in each lineage. It's rather being built, or de-built, by dialing down or up certain genes of the right moments during development. And so what this means is that for any decapod lineage that has these genes lying around, and they all do, because hawk's genes are pretty much universally conserved across animals, becoming crab-shaped is less

about evolving a whole new body plan and more about turning levers on existing developmental instructions. which I guess you could say is the case for many evolutionary quirks, but still. So this is at least a clue for how bractorization happens in true crabs, and is likely similar to what happens in Anamura. Now whether it's the exact same levers, that's an open question. But the takeaway is, as long as Decapodker stations exist, so will crabs. And to answer that also burning question, no crab humans in the foreseeable future. And with that, I'll leave you here.

Thanks for watching, and until next time. Hey, y'all's Kelly Clarkson with Wayfair. Ever order furniture online and wonder what if? Like, what if it doesn't hold up? That sofa was four days old. You should have ordered from Wayfair. With Wayfair, there's no what if. Just style you love and quality you can trust.

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