Various Animals Are Turning Into Crabs, And Scientists Say More Could Follow

Crabs are well-known for their flat and rounded shells, fearful sharp pincers, and a tail that is most often folded underneath their body. What’s most interesting, though, is their particular body set-up appears to be evolutionarily desirable. Indeed, scientists know for a fact that this configuration has evolved a minimum of five times among decapod crustaceans, a category that includes crabs and similar species like lobsters and shrimp. This process has been titled carcinization, and it keeps happening. But now boffins are wondering whether other species of animals will effectively turn into crabs in the millennia to come. Could we one day see crab-like humans?

It all started with a meme

Interest in crabs and indeed carcinization really skyrocketed back in October 2020 thanks largely to a very 21st-century way of communicating ideas — and most often jokes — online. We are, of course, talking about memes.

Well, this particular entry into the increasingly deep meme catalog impishly hinted that due to the process of carcinization, all of Earth’s living things will eventually evolve to become crabs — not excluding us human beings!

A meme with some basis in reality

But whereas many memes have little basis in fact or reality, this particular one — and many of those that were inspired by it and followed in its wake — does have a degree of scientific merit. Evolution, it seems, is really proud of its handiwork in creating the body of the crab, and wants to replicate it again and again.

But could it really happen? Could human beings eventually evolve into being crabs? Whiloe you’re probably right not to be entirely convinced on that point, equally this evolutionary trend is really fascinating. So, let’s take a look at the history of carcinization, and where it all started.

Defining carcinization

To begin with, we should properly define what carcinization is. Or more accurately, we’ll let an expert do that for us, as perhaps we are not quite the evolutionary science and biology boffins that we might wish to be!

Carcinization is a form of convergent evolution. That is, a process “whereby distantly related organisms independently evolve similar traits to adapt to similar necessities,” as scientist Masanori Kasahara wrote in his 2010 study Progress in Molecular Biology and Translational Science. Think of bats and birds: they’re clearly quite different animals, but they both have shared characteristics such as wings.

Origin of the term

The term itself can be traced back over 100 years to 1916 when English biologist and zoologist L.A. Borradaile first introduced it into the scientific lexicon. Borradaile studied crabs and the morphological evolution of decapods into “false crabs.”

Borradaile noted that carcinization was “one of many attempts by nature to evolve a crab.” The carcinization process has also been referred to as “brachyurization,” but the former seems to have superseded that term in defining the process Borradaile described.

What are Brachyura?

So, what does Brachyura mean then? Why it is the infraorder of crabs, within the greater decapod crustacean category. Translated from the Greek, it means “short tail,” which is what crabs have hidden under their thorax.

But in the context of carcinization, the term Brachyura essentially means “true crabs,” which the decapods who evolve to be like them — known as Anomura — are not. You will thus see them labeled as “false crabs.” The frauds!

Wider public consciousness

Anyway, carcinization has been a topic of great interest to zoologists and evolutionary biologists since Borradaile’s time, or just before; namely the beginning of the 20th century. But it was not widely known or in the public consciousness until that aforementioned meme went viral in October 2020.

Then, from that moment everything changed, and carcinization was something ordinary folk became interested in. Would all living things eventually evolve into crabs? Really? What kind of deranged horror movie is this?

Describing the meme

The meme itself that went super-viral and dwarfed all of those that came before it originally seemed innocuous enough. It came from a Twitter/X user called Amy A @lolennui, who was formerly known as @cableknitjumper on the site at the time of her post.

On October 13, 2020, above a screenshot of a Wikipedia page describing the process of carcinization, Amy A simply wrote, “Crustaceans have evolved into the shape of a crab at least 5 independent times. I do not like this.”

Discussion, debate, and more memes

Little did she know that her seemingly jokey comment would go big on the web. Yes, Amy A’s slightly fearful post sparked off a lot of debate and discussion on the then-largely unknown topic of carcinization.

Not only that, a flood of memes followed in its wake, with some really funny ones too, such as one on the crab cycle which noted, “There is only one step, and it is crab.” And as of February 2024 Amy A’s initial post had racked up at least 71,000 likes, 14,000 retweets, and over 900 replies.

Carcinization cult

Later on the day of her initial post, Amy A jokingly commented a few more times. First she wrote, “I feel like it would be easy to start a cult based on this.” Perhaps she was starting to realize the immense interest her post had quickly gained.

Next, Amy A joked, “I’m guessing the minor deity of evolution was really phoning it one week,” in reference to the supposedly unsophisticated crab body compared with humans and other animals. But her last zinger was arguably the best, in which she wrote, “Me, a foolish ape: nooo you can’t just keep inventing crabs you’ve already done it 5 times. Evolution: haha crab printer go brrrrrrrrr”

Funny – and serious – replies

Amy A’s post very quickly received some humorous responses, as well as some concern for a crab-dominated world. One funny response came from Twitter/X user Andrew Olson, who wrote above a picture of a crab, “This is the ideal male body. You may not like it, but this is what peak performance looks like.”

Several others referenced an episode of South Park, in which crabs take over. Max wrote, “Matt Stone and Trey Parker might have been on to something.” Indeed, perhaps the creators of South Park had been brushing up on carcinization some time before it went big via Amy A’s meme?

Viral video

Anyway, YouTube channel PBS Eons decided to cash in on all that sudden interest in carcinization, and it released a video on the phenomenon on October 28, 2020. The video helped to properly educate people on the subject, and the history of crabs.

To say that it attracted interest would be an understatement. By mid-November of 2020, around 16 days after it had been published, PBS Eons’ video had racked up an astonishing 3.2 million views, and over 10 million as of February 2024.

Decapods 365 million years old

As the PBS Eons video attests, crabs have been around a lot longer than that meme. Yes, the crustaceans — which live in every ocean on Earth and also on land and in saltwater — have been around since the Jurassic Period.

Indeed, crab-like creatures first appeared on the face of the planet around 365 million years ago. These included Palaeopalaemon, a decapod crustacean — named for its ten feet — that looked much like a lobster, and probably searched the ocean floor for food like its modern counterpart too.

The decapods split

As the narrator on the PBS Eons video explains, it was around 250 million years ago that the decapods split — or evolved — into two different groups: the Anomura and the Brachyura. Otherwise known as the “fake crabs” and the “real” or “true crabs.”

What was the first “true crab” to ever exist, you might be wondering? Well, PBS Eons informs us that it was Eoproson, which roamed around this planet some 185 million years ago. Scientists did argue for some time over whether it constituted a “true crab” due to it having some un-crab-like traits including an elongated abdomen, but it is now generally accepted to have been the genuine article.

Evolving into a classic crab shape

Anyway, evolution would see both of these decapod categories begin to take on the more familiar crab shape that we know and love. Or maybe you don’t exactly love it, but at least recognize it! This came for both types after a long period in a lobster shape.

Then, in that long evolutionary process, the familiar round-and-flat shape of a crab formed. And as the narrator explains, scientists have been befuddled as to why this has happened. For over 100 years they have been trying to figure it all out.

Natural selection

Yes, evolution — and more specifically natural selection — is sometimes so strange that even seasoned biologists and zoologists are left scratching their heads about what is happening and why. So, with that in mind, what chance does Joe Public have when confronted with such things if the brightest and best don’t know what’s going down?

Therefore it should be no surprise that many people freaked out a little bit when they first heard or read about the process of carcinization after that 2020 meme. Maybe you were a bit braver, reader! But you’ve got to admit, the way in which numerous creatures are continually evolving into crabs is kind of scary.

Lobsters’ tails and changing carapaces

But what exactly is the change that occurs through carcinization? Well, the first person to theorize it was the aforementioned British boffin Borradaile. As he explained it, a non-Brachyura crustacean such as a lobster has a long, visible pleon, or tail. But over evolutionary time periods and through the process of carcinization, that tail becomes shorter.

Effectively, then, this shorter tail gets tucked under the body, which eventually resembles that of the “true crabs.” Not only that, the narrower carapace part of the lobster — or a similar decapod — evolves to become flatter, and rounder, so that it takes on what we are going to call the classic crab look.

Crab-based confusion

To summarize, what ends up happening after carcinization is that there are a bunch of actually unrelated crustaceans that wind up looking as though they are all part of the same crab species. But they are not “true crabs,” as we have discovered, but Anomura that have become more crab-like by carcinization.

To make matters worse and much more complicated than they arguably need to be, there are actually numerous decapods that are called crabs that aren’t actually crabs. Step forward — or perhaps sideways — hermit crabs, king crabs, and horseshoe crabs. Oh for goodness sake, why can’t they just call them something else?

Five times

As we’ve alluded to, carcinization has not constituted a freakish isolated incident or anomaly, but has been happening repeatedly in nature. Indeed, scientific research — such as the March 2021 study by Joanna M. Wolfe, Javier Luque, and Heather D. Bracken-Grissom entitled How to become a crab: Phenotypic constraints on a recurring body plan — has illustrated that the process has happened five times, at least.

That’s five times that we know about, anyway. There may well be many more that have so far gone undocumented. Those crustaceans that have carcinized to develop a classic crab-like body shape have included “true crabs” and “false crabs” among their number.

Common ancestor

That Wolfe, Luque, and Bracken-Grissom study looked at both living crustaceans and fossilized ones. It found that carcinization had occurred three times within the Anomura order, the one to which lobsters and shrimp belong.

Dr. Luque told the American Association for the Advancement of Science, “This evidence suggests that indeed carcinization evolved independently in those groups.” The researchers also hypothesized that there was a common ancestor between the Anomura and the Brachyura which had not yet gone through the process of carcinization.

Crab explosion

The obvious question in all this is, ‘Why?’ Why do decapods consistently go from being longer and narrower in body shape to being flatter and rounder, the classic crab shape? It’s a head-scratcher, but looking at the long history of the process in detail might help us understand it, so let’s do that.

The aforementioned PBS Eons video points to a 1980s study that analyzed all of the fossil crustaceans through the Mesozoic Era — which stretched from 251-66 million years ago — and the scientists discovered that there was “an explosion” of crab-shaped crustaceans then.

Cretaceous crab revolution

A later study cited by PBS Eons focused on the Cretaceous Period, when this phenomenon really took off; it might even be called a “Cretaceous crab revolution.” Approximately 80 percent of all the “true crabs” around today emerged during those two periods.

Significantly, the Mesozoic Era saw a notable shift in the diversity of the decapods towards more classic-crab shaped species over the long-bodied “false crabs.” Not only that, the “true crabs” seemed to have been able to exploit numerous different habitats on earth for their needs and survive in them more effectively than the other decapods.

Better mobility

Clearly, there must be some evolutionary reason for this. Did the crab’s flatter shape allow it to maneuver around its habitat more easily, giving it greater mobility? It certainly seems that way, and observing true crabs burrow, walk, run, and swim in contrast to the more clumsy and limited movements of the lobster seems to bear this out.

Lobsters can only really scuttle along the seafloor and swim, whereas some decapods with crab-style bodies can even climb trees — hello, the fearsome coconut crab — and you’ve never seen a lobster do that, have you reader? Still, we’d like to steer clear of scary tree-climbing crabs if we can, thanks.

Advantage of the true crab shape

Another theory for carcinization put forward by boffins is that the removal or reduction of the pleon meant that the decapod had one fewer part of its body that was desirable for predators. Also, there was now one less thing that they could potentially grab hold of to snaffle that meal.

So, in short, the carcinization towards the classic-crab shape occurred because it was an evolutionary way to succeed better in avoiding predators. Seemingly, evolution had tried both shapes and classically shaped crabs fared better in avoiding being eaten, could move more effectively across several different habitats and do more things effectively, like burrowing and scuttling sideways. It was evolutionarily advantageous.

Is a crab-like body really advantageous?

Yet this is not universally agreed on by scientists: there is still some debate and need for further testing. What’s more, several other hypotheses have been put forward, including one which Dr. Wolfe wrote about in that 2021 paper.

Dr. Wolfe told Live Science, “It's possible that having a crab body isn't necessarily advantageous, and maybe it's a consequence of something else in the organism.” Her colleague on that paper, Dr. Luque, stated, “The crab body plan might be so successful not because of the shell or tail shape itself, but because of the possibilities that this shape opens up for other parts of the body”

Jury still out

So, the jury is still out on just why carcinization happens, it seems. While scientists have learned a lot in the last few decades, there is always more to know. Science, and indeed evolution, never stops.

For her part, Dr. Wolfe has set about investigating genetic data of different decapod crustaceans, in order to get a deeper understanding of the relationships between them. If she succeeds, perhaps it will be possible to determine precisely when various ‘crabby” decapod lineages evolved, and fully understand the factors that are driving carcinization.


Now, here is the curveball you didn’t expect — and likely didn’t want — just to make things ever stranger, because clearly things weren’t complicated enough as it is! So, carcinization of decapods into the classic crab shape isn’t necessarily an everlasting transformation.

No, in actual fact, on occasion, crustaceans have gone through an evolutionary phase where they’ve ditched the crab-like body plan they morphed into via carcinization. Scientists call this process “decarcinization” and it has happened at least seven times.

Going back and forth

Just why that has happened is a question you might be pondering right now. Well, Dr. Luque of that aforementioned study told Live Science, “Crabs are flexible and versatile. They can do a lot of things back and forth.”

Well isn’t that just great? Don’t worry about confusing the heck out of all us then, crabs and assorted decapods, as you go from non-crab-like to crab- like and crab-like to non-crab-like just as you please. You evil geniuses!

Crabs or crab-people takeover?

At this point, you might find yourself both fascinated and annoyed by carcinization and decarcinization. Granted, it is a mighty interesting form of convergent evolution, but all that changing around from one body-type to another is enough to make your head burst!

But anyway, what about us humans? Is there really going to be a complete crab takeover of the planet, with crab people forming as South Park seemed to hilariously predict some years ago? Are any other species of animal going to become all crab-like? Will we one day see, say, lions with huge shells and pincers?

Exclusive to crustaceans

Not any time soon, it seems, we’re pleased — and not gonna lie, kind of relieved — to report. No, from the copious amounts of scientific study so far conducted, it has been deduced that the process of carcinization appears to be exclusive to crustaceans.

So, we are safe! Haha! For now, at least. Who knows exactly what will happen in the distant future, with this weird thing we call evolution? But unless we start living exclusively in crab habitats such as sandy beaches and rocky water pools over evolutionary timescales, we’re unlikely to develop similar bodies.