Life's Little Mysteries

Why do animals keep evolving into crabs?

Here we see an orange/red crab with a pale yellow underbelly with its two claws raised in the air. It is on a sandy beach.
Crabs have flat, rounded bodies and a tail that's folded under the body. (Image credit: zahoor salmi via Getty Images)

A flat, rounded shell. A tail that's folded under the body. This is what a crab looks like, and apparently what peak performance might look like — at least according to evolution. A crab-like body plan has evolved at least five separate times among decapod crustaceans, a group that includes crabs, lobsters and shrimp. In fact, it's happened so often that there's a name for it: carcinization.

So why do animals keep evolving into crab-like forms? Scientists don't know for sure, but they have lots of ideas.

Carcinization is an example of a phenomenon called convergent evolution, which is when different groups independently evolve the same traits. It's the same reason both bats and birds have wings. But intriguingly, the crab-like body plan has emerged many times among very closely related animals. 

The fact that it's happening at such a fine scale "means that evolution is flexible and dynamic," Javier Luque, a senior research associate in the Department of Zoology at the University of Cambridge, told Live Science.

Related: Does evolution ever go backward?

Crustaceans have repeatedly gone from having a cylindrical body plan with a big tail — characteristic of a shrimp or a lobster — to a flatter, rounder, crabbier look, with a much less prominent tail. The result is that many crustaceans that resemble crabs, like the tasty king crab that's coveted as a seafood delicacy, aren't even technically "true crabs." They've adopted a crab-like body plan, but actually belong to a closely related group of crustaceans called "false crabs."

The king crab isn't actually a "true crab." (Image credit: lightasafeather via Getty Images)

When a trait appears in an animal and sticks around through generations, it's a sign that the trait is advantageous for the species — that's the basic principle of natural selection. Animals with crabby forms come in many sizes and thrive in a wide array of habitats, from mountains to the deep sea. Their diversity makes it tricky to pin down a single common benefit for their body plan, said Joanna Wolfe, a research associate in organismic and evolutionary biology at Harvard University.

Wolfe and colleagues laid out a few possibilities in a 2021 paper in the journal BioEssays. For example, crabs' tucked-in tail, versus the lobster's much more prominent one, could reduce the amount of vulnerable flesh that's accessible to predators. And the flat, rounded shell could help a crab scuttle sideways more effectively than a cylindrical lobster body would allow.

But more research is needed to test those hypotheses, Wolfe said. She is also trying to use genetic data to better understand the relationships among different decapod crustaceans, to more accurately pinpoint when various "crabby" lineages evolved, and pick apart the factors driving carcinization.

There's another possible explanation: "It's possible that having a crab body isn't necessarily advantageous, and maybe it's a consequence of something else in the organism," Wolfe said. For example, 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, said Luque, who is a co-author of the 2021 paper with Wolfe.

The lobster's tail, which helps it swim and crush prey, is more prominent than a crab's. (Image credit: Jacob Maentz via Getty Images)

For example, a lobster's giant tail can propel the animal through the water and help it crush prey. But it can also get in the way and constrain other features, Luque said. The crab body shape might leave more flexibility for animals to evolve specialized roles for their legs beyond walking, allowing crabs to easily adapt to new habitats. Some crabs have adapted their legs for digging under sediment or paddling through water.

"We think that the crab body plan has evolved so many times independently because of the versatility that the animals have," Luque said. "That allows them to go places that no other crustaceans have been able to go."

The crab-like body plan also has been lost multiple times over evolutionary time — a process known as decarcinization.

"Crabs are flexible and versatile," Luque explained. "They can do a lot of things back and forth."

Wolfe thinks of crabs and other crustaceans like Lego creations: They have many different components that can be swapped out without dramatically changing other features. So it's relatively straightforward for a cylindrical body to flatten out, or vice versa. But for better or worse, humans won't be turning into crabs anytime soon. "Our body isn't modular like that," Wolfe said. "[Crustaceans] already have the right building blocks."

Laurel Hamers
Live Science Contributor

Laurel Hamers is a writer specializing in science, medicine and the environment. Now based in Oregon, she was previously a staff writer at Science News magazine in Washington, D.C. Laurel holds a bachelor's degree in biology from Williams College in Massachusetts and is a graduate of the UC Santa Cruz Science Communication Program.

  • Broadlands
    It seems to have been overlooked that crabs are like other Arthropds. They are "stuck" in a chitinous cuticle exoskeleton that hardens in a process called sclerotization. The animal must molt to grow. That leaves them vulnerable to predators. Other animals did not evolve to have that problem.
    Reply
  • tachyphylaxis
    Broadlands said:
    It seems to have been overlooked that crabs are like other Arthropds. They are "stuck" in a chitinous cuticle exoskeleton that hardens in a process called sclerotization. The animal must molt to grow. That leaves them vulnerable to predators. Other animals did not evolve to have that problem.
    Well, you know how evolution is--"whatever works"!

    As humans, we boldly carry the torch of "satisficing" into the future. ;-)

    Thanks for pointing that out, though. I doubt it would have occurred to me, and it's certainly an important part of the story.
    Reply
  • Broadlands
    "So how can it be taught as fact in the first place?" I believe you are referring to the biochemistry and the mechanisms of evolution, not that biological evolution is a fact. That can easily be taught... One example:

    "Critics of evolution relish in emphasizing the complexity and unsolved problems surrounding its mechanisms. They point to errors made by earlier researchers, and enthusiastically conclude from all of these difficulties that evolution is “only a theory.” They routinely misinterpret or ignore the repeatable, observable, measurable scientific evidence that overwhelmingly supports evolution as a fact. This evidence is abundant. It is the worldwide, sum total of the fossil record. Sedimentary rocks and the fossils preserved in these rocks, even though an incomplete paleontological record, are primary, factual evidence of what has happened back through time. One thing apart from all others presents a compelling case for evolution that is independent of its weaknesses. It is the simple, repeatable observation that deeper, older rocks contain fossils of more primitive, less-evolved organisms than do the younger rocks that rest above them. Consider the magnificent Grand Canyon in Arizona. The rock layers at the base of the gorge are undeniably older than those at the top. No matter how much time was involved in cutting the gorge through all these rocks, there is no way of reasonably denying that as one takes the winding path down the gorge and passes across the layers of rock one also goes back through recorded time. Today we can observe the same thing, obviously on a vastly smaller scale, in the layers of trash at waste disposal sites. The old newspapers, bottles and cans, the less “evolved” cameras and telephones, 78-rpm records, and vacuum-tube radios are preserved among the layers found toward the bottom. The younger more evolved “fossils” are among those found near the top. In the vast Grand Canyon geological “dump” the deeper flat-lying rock layers contain the simpler, more primitive fossils. Gaps in the record notwithstanding, there are shells buried there that record the presence of species not found in the younger rocks above, and no bones of any kind are to be found. The rocks at the top contain fossil remains of still more evolved (developed) animals. All of these thousands of feet of flat-lying rocks rest directly on still older layers. Those below are set at a steep angle. Obviously, substantial time must be allowed for these even older sediments to have been deposited, hardened, turned on edge by mountain-building processes, and later cut down by erosion. In these much older rocks there are precious few fossils of any kind. There are neither shells nor bones. Only a few fossils of simple, microscopic life forms have been found. Geologists find similar rock sequences in many regions of the globe. While these may not have the grandeur and ease-of-visibility that the deeply cut Grand Canyon offers, their included fossils exhibit the same upward changes. I find it hard to escape the conclusion that from this overall geological and paleontological evidence there has been an increase upwards through time in the complexity and sophistication of life. The global fossil record does not present a jumbled mixture of large and small, older and younger, as would be the case after a giant flood of some kind. This record is repeatable, undeniable factual global evidence for a long process of evolution…of biological change over time. The total fossil record reveals these same upward changes, irrespective of the mechanisms that caused them."
    Reply
  • Broadlands
    So, you accept the 4.5 billion year old geological record. . It's clearly NOT just a theory. It's a fact. Right?

    (We know how amino acids might be formed. 20 in the genetic code (plus two others that must have evolved later....Hydroxyproline and hydroxylysine).

    But you continue to deny that natural biological evolution took place? Some obviously intelligent "higher power" designed and managed it all. OK...where does that leave us? What sort of research can be done to test this "higher power" and her design?
    Reply
  • Likenumber86
    Broadlands said:
    So, you accept the 4.5 billion year old geological record. . It's clearly NOT just a theory. It's a fact. Right?

    (We know how amino acids might be formed. 20 in the genetic code (plus two others that must have evolved later....Hydroxyproline and hydroxylysine).

    But you continue to deny that natural biological evolution took place? Some obviously intelligent "higher power" designed and managed it all. OK...where does that leave us? What sort of research can be done to test this "higher power" and her design?
    (We know how amino acids MIGHT be formed. 20 in the genetic code (plus two others that MUST HAVE evolved later....Hydroxyproline and hydroxylysine).

    I appreciate your realistic approach, but this is not a language use of something proven time after time. I thank the people that take realistic approaches, because no, carbon dating is not 100% reliable, & those that put this out there are just being completely honest. It isn't.

    Scientists can be Christians. They just study what God created. Our end all be all is not science alone. There's alot of archeology that supports The Bible. And in instances involves physicalities such as balls of sulfur where Sodom & Gomorrah used to be. They are buried deep in the different layers of dirt, land, & rock. Life in the Earth is possible because math & constants are just right allowing for the right temperatures to permit life, just like in the Universe itself. Although in planets themselves, the temperatures & imbalances do not allow for vegetation, humans, water, nor animals to exist. Not anywhere that we have studied. If any of the conditions are off, it's catastrophic, at least here on Earth. We live because those conditions are perfect. Honestly, it's a little blurry for me to see that the earth was made in seven days or so. But it's more blurry to see that it was all accidental. The fact that us humans struggle with addictions, & sometimes fail to see some as such, means that we are so weak and helpless without a higher power. Simply that we do things that we at times shouldn't, such as being judgmental or being upset. Yet, we've had experiences that unexpectedly made our lives better after we weren't sure that they would; circumstances, dreams, feelings, visions, messages directly or indirectly in ourselves, & in others. We see compassion in others. Therefore, for God to not vow to us, & tell us where we can see that he exists is understandable. Some people will not follow him, no matter what that person sees or has experienced. We are wicked even so, & stubborn. Thus a person that follows him without seeing him physically is more loyal than someone that physically has seen him. By the way, there are also predictions in The Bible that have been accurate, that there was no sensible explanation how they could have been brought up in the first place, or why. Some in involve the technology of today.
    Reply
  • Likenumber86
    Johnnyreddogg said:
    Just taking a look at humanity and the extremely complicated human Proteome, The amount of protein seems to vary anywhere from 10,000 or so, all the way up to 350,000. And, besides our alpha aminos, we need dozens of other aminos just to exist. We take in these aminos from what we eat. Even from being out in the sun! I really find it fascinating, we are truly wonderfully made.

    We can look at photosynthesis that now has been observed to have a quantum element to it. Now all of these things, just the ones discussed in my paragraph, or comment, seems to me to point to something other than random coincidence.

    And, the epochs of time, Periods of time that had spanned hundreds of millions of years, could Not have birthed life along with its extremely complicated infrastructure by just sitting there. Humans never ran around with dinosaurs, and the earth and all life was not created in 6 24-hour days. That is just ludicrous. A person can be a student of science and a student of scripture. I have disagreements with many religious individuals claiming 6 24-hour days. As a matter of fact, the Bible doesn't even mention 6 24-hour days.

    Life is so complicated, It would take humanity and eternity to try and figure it out. Unfortunately, the churches and religious organizations have created a toxic competition with science. That's unfortunate, because science and religion, actually scripture can get along quite well.

    As a matter of fact, science has beyond a doubt prove in that the Earth is round. But you still have individuals that believe the earth is flat. In the book of Job, it talks about the Earth being round and hanging on nothing. This was closer to 3 millenia ago. Way before telescopes.

    Intelligent design makes more sense than stuff just happening by laying around and marinating. What one calls that intellect is up to every individual. They make their own choices.
    Very science oriented and true! Also, very sensible, reasonable, logical, & spiritual!
    Reply
  • Broadlands
    So, you accept the 4.5 billion year old geological and paleontological record. Biological evolution is clearly NOT just a theory. It's a fact. Right? So how can we test this intelligent design... that is just a theory?
    Reply
  • Dawn Lambert
    Johnnyreddogg said:
    You know, if this wasn't so ridiculous it would be a joke. Horseshoe crabs have been around since almost forever. I find it interesting that the opinion of evolution, animals go back and forth back and forth back and forth, why? Why did original single cell organisms evolve from asexual reproduction to sexual reproduction? From meiosis to mitosis? Simpler is easier right? Except it seems that things end up evolving as you claim into much more complex species. And for what reason? Why? There is no chasing the wind evolution, but there is adaptation! Birds with longer beaks can get the nectar, birds with shorter beaks starve. So those genetic traits are passed on to the offspring. That's interspecie selection. Same with insects, same with animals with longer necks and shorter necks, and if things change, so do the animals and other life forms. They produce nothing new, no new species! Again, interspecie selection. So-called evolutionary enlightenment changes week to week month to month and year to year with new so-called discoveries. So how can it be taught as fact in the first place? If life is just an accident, then why is it so diverse? Birds, amphibians, reptiles, mammals, fish, plant life, fungus, mosses, microscopic to macroscopic, and everything in between! Intelligent design, not an accident!
    I think you missed the point of the article. Yes, horseshoe crabs have the crab-like body plan, but they're not crabs! Why did they evolve that body plan? Why has it worked for them for so long? It's clearly successful, but why? Many times in history animals have evolved to look (more or less) like crabs. The question is why?
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  • Broadlands
    Why? "It seems to have been overlooked that crabs are like other Arthropds. They are "stuck" in a chitinous cuticle exoskeleton that hardens in a process called sclerotization. The animal must molt to grow. That leaves them vulnerable to predators. Other animals did not evolve to have that problem."
    Reply
  • Stephan Gunther
    Johnnyreddogg said:
    "Just like why did the apes or chimps evolve into the humanoid species but then again there are still apes and chimps?"
    Creationists really demonstrate their ignorance with that statement. Humans are apes. Apes did not evolve into humans. Chimps didn't evolve into humans. Chimps still exist. Chimps and humans have a common ancestor that diverged 4-6 million yrs ago. Is it really so difficult to grasp a first grade concept ? Go look at an evolutionary tree of apes before making brain dead theistic arguments.

    http://www.jefflewis.net/blog/2017/03/understanding_evolution_-_how_.html
    Note the 2nd diagram that shows the modern great apes, including us, and their evolutionary relation.

    The apes: https://en.wikipedia.org/wiki/Hominidae
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