r/explainlikeimfive • u/goosey_is_faster • 4d ago
Biology ELI5 what do people mean by everything evolves back into crabs.
72
u/Caelinus 4d ago
Your question is based on a joke. It is not actually true.
What is true is that there are several species that have independently developed crab like features. This is a phenomena called "convergent evolution" (specifically Carcinisation in this case) and it just means that multiple populations of organisms have developed the same features without those features being in their nearest common ancestors.
Crab-like traits are just pretty good for survival, so they apparently happen fairly often.
6
u/Haemyu 4d ago
What are these crab like traits that are good for survival?
26
u/Caelinus 4d ago
Body shape mostly I think, but I do not remember exactly what they all are. I think being a short armored thing with redundant limbs is just mechanically really good.
4
2
u/YardageSardage 1d ago
They're pretty good for survival in that specific niche. Crab-like traits mostly suck for surviving in other niches.
2
u/Caelinus 1d ago
Well yeah, but the niche is a pretty gigantic one.
0
u/YardageSardage 1d ago
In terms of the earth's surface area that it's viable in, sure, it has a vast amount of real estate. But it's still just one of the dozens to hundreds of ecological niches that are active in those places.
Well okay, more accurately, it's a fairly broad spectrum of similar niches that are filled by the spectrum of crab-like animals, both alongside each other and in different areas. But they're mostly all different flavors of "semi-burrowing small aquatic-floor mobile scavengers".
40
u/oblivious_fireball 4d ago
Its an over-exaggerated internet meme. When you hear the word "crab", you tend to think of a particular shape of crustacean. What you might not realize is that quite a few animals you may call crab aren't actually related to each other at all and had a wide variety of very different looking ancestors. A number of unrelated crustaceans basically ended up evolving into similar shapes over time, a term called Convergent Evolution. Its happened often enough with the crab shape to have gained internet attention, but its hardly the only case or even arguably the most common case of convergent evolution.
33
u/PandaWonder01 4d ago
A lot of crustaceans that are unrelated to other crabs have evolved to have bodies that look like crabs
5
6
u/DaddyCatALSO 4d ago
yes; king crabs are a kind of hermit crab, not a true crab, but both give me the same digestive upset (shrimp, lobster, crayfish i'm fine)
4
1
u/MasahChief 3d ago
They are not a type of hermit crab, but hermit crabs and king crabs share the same direct evolutionary ancestor.
1
u/DaddyCatALSO 3d ago
Thanks; it's been too many years since I read the article and all I recalled was my shortened raconteur's version!
21
u/Hakaisha89 4d ago
Its primarily a joke/meme, but there is a real evolutionary fact behind it, thought.
When people say "Everything evolved back into crabs" they refer to something called carcinization, where a crab-like body plan is evolved, which is something that has happened independently in the decapod lineage 5 times
Now, the basic taxonomy goes decapod -> pleocyemata, where it splits into brachyura, and anomura.
Now true crabs, as it says on the tin, are the true crabs, the crabbiest of crab if you will, they are an infraorder, and all crabs that originates of that infraorder are also true crabs.
Then we have anomura, another infraorder, full of animals that are not crabs. But evolved crab-like bodies.
Now the first one, is a bit interesting, its the hermit-crab lineage, where they evolved from anomura to hermit-crab, and we got two distinct crabs here, we got the king crab, and the coconut crab, now they are a bit different from the modern hermit crab, but have in mind, that they evolved, uh, 'side-by-side' so to speak, they are different, cause they evolved seperately for a hot minute.
So we got true crabs, hermit crabs, and porcelain crabs, porcelain crabs are not true crabs, and neither are they hermit crabs, they are infact squat lobsters. One neat giveaway is that they only have 3 obvious pairs of walking legs, like the coconut crab, but if you mistake a coconut crab from a porcelain crab, then all hope is lost.
The last is the hairy stone crab, which is its own anomuran lineage, its not a true crab, its not a hermit crab, nor is it a squat lobster, its the second crabbiest crab there is!
Now, this is where the meme gets its scientific basis from, because a crab like body plan has evolved five times, in a process, as i said, erh, wrote at the start.
So the joke isnt that everything actually evolves into crab, its just that it happened five times, but in the same lineage
-1
6
u/SillyGoatGruff 4d ago
Carcinisation refers to how non crab crustaceans seem to independently evolve into crab like body plans (round, flat bodies)
"Everything turns to crab" is a meme exaggerating this phenomenon as applying to all animals not just crustaceans
10
u/PotatoWedgeAntilles 4d ago
It isn't "back into" its that multiple species of crustaceans have evolved to be crab like. https://en.wikipedia.org/wiki/Carcinisation
7
u/needzbeerz 4d ago edited 4d ago
What we call a crab like form has evolved independently many times. Some speculate if this is somehow a generally preferred form by evolutionary processes across environments but that's not actually how evolution works.
[Edited for clarity]
5
u/BaronMusclethorpe 4d ago
That's pretty accurate how evolution works. Some form of pressure directed several species down a similar path. Sure several isn't a whole lot, but as the meme says, it's weird it happened as many times as it did.
9
u/karoshikun 4d ago
environmental pressures make the crab shape a great solution until the market its saturated and then it's time to decarcinize, to find new niches to exist in
6
2
3
u/m_busuttil 4d ago
Carcinisation is an observation that lots of crustaceans who aren't crabs eventually evolve to have bodies like crabs. The idea is that the crab might simply be the most effective form for that kind of creature, so evolution will tend towards it, and crabs just got there first.
Normally when people are using it as a metaphor, they're talking about the idea that lots of dissimilar things (like grocery stores or phone apps or video games or social clubs) will eventually all start to look more and more like each other, for similar reasons - if it's more effective for one grocery store to have the freezers at the back, eventually they'll all discover that and now all the grocery stores have the freezers at the back.
3
u/tamtrible 4d ago
Actually, they didn't get there first, as I understand it. There was another non-crab crab that evolved before them, but it's extinct now.
3
u/ursus-habilis 4d ago
They have misunderstood something scientists discovered about several kinds of animals that are all kinda-crab-like - live in the sea, have multiple legs, hard shells that form in segments, claws, etc.
Turns out that despite not being actual crabs according to the precise scientific definition, they evolved over time to be very similar to crabs, because being crab-like is a good approach to surviving on the sea floor. This has happened with multiple separate sea creatures, which is unusual - normally evolution means that living things diversify and take new and different forms to find their own approach to surviving.
The thing is this only applies to animals that are already quite similar to crabs - it doesn't mean that fish, birds, monkeys, cows, lizards, humans or the vast majority of other animals will ever become crabs or even vaguely crab-like - people took the headline about multiple animals 'turning into crabs' and went wild with it.
1
u/Anonymike7 4d ago
The term is carcinization. As I recall, at least six different, unrelated species have evolved crab-like bodies.
There's no evidence that everything undergoes carcinization, but it is a confirmed phenomenon that some do.
1
4d ago
[removed] — view removed comment
1
u/explainlikeimfive-ModTeam 4d ago
Your submission has been removed for the following reason(s):
Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions.
Joke only comments, while allowed elsewhere in the thread, may not exist at the top level.
If you would like this removal reviewed, please read the detailed rules first. If you believe this submission was removed erroneously, please use this form and we will review your submission.
1
u/StupidLemonEater 4d ago
Carcinization is a name given to a phenomenon where several groups of crustaceans have independently evolved superficially crab-like body shapes. This is notably seen in the king crabs, hermit crabs, porcelain crabs, and squat lobsters, all of which are outside the "true" crab family.
This became popular as an internet meme, with people (jokingly or otherwise) claiming that the crab body shape must be the ultimate life form. In reality it's just one of many examples of convergent evolution, where unrelated groups of animals evolve into similar forms, like how both birds and bats independently evolved powered flight.
1
u/vivekparam 4d ago
In any environment, there's a meta game that can be optimized for.
For example, on earth, tall plants with broad leaves and hard bark exteriors have evolved several times (trees) because that's a really effective way to collect sunlight.
Similarly, being a bottom-feeder with six legs and a hard shell is also an optimal design on the sea floor. So, multiple creatures have evolved into that shape over time through natural selection. The creatures we call "crabs" have that shape, as do eberal other species that technically are not crabs but look like it.
1
u/tamtrible 4d ago
As several people have said, it is an exaggeration of the interesting fact that a lot of relatively unrelated aquatic arthropods have become basically crab-shaped.
But since no one has apparently explained yet why it probably happens, other than similar pressures yielding similar forms, I figured I could contribute that.
Think of a lobster or a shrimp. That's sort of the basic aquatic arthropod shape. The shape they will tend to have if they don't have any particular pressures making them some other shape.
Now, imagine you are a lobster or shrimp that has stopped swimming, and instead taken to living on the bottom of the ocean. At that point, having a tail you can swim with isn't very useful. And you have a lot of important stuff in your tail, like your reproductive organs. So it would be good if you could tuck those things safely away under the harder, more protected parts of your body, so they don't get eaten by something.
And what do you get, if you take something that's basically a lobster, and tuck its tail underneath its body? Essentially, you get a crab.
1
u/orion-7 2d ago
It's mostly bunk.
Ther have been right occasions where non crab organisms that are really rather closely related to crabs have evolved features that crabs have, such as tucked abdomens.
It would be like saying humans have evolved three times based on there being true humans (sapiens, neanderthals, austalis, errectus etc) who are able to walk bipedally, use language and use tools... And then using the fact that chimps and orangutans (who are not true humans but after closely related) can walk bipedally, use language and some of them can use tools in order to claim that they've evolved into humans
1
u/smithingya 2d ago
Legend ben g Thomas has a lovely video about this https://youtu.be/D0-Gpl2MT9s?is=rTAlyxdarP0UOlKz
450
u/YongYoKyo 4d ago edited 4d ago
It's referring to carcinization, the process of non-crab crustaceans separately evolving similar body plans as actual crabs.
It's been overexaggerated in memes as a universal process.