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On Earth, crabs. On other worlds as well?

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Introversion

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Evolution experts were kind enough to answer Ars' questions about alien crabs.

Many organisms on planet Earth aren't crabs. Dogs, for instance—definitely not crabs. Science also suggests that humans are not, in fact, crabs. But a surprising number of creatures either are crabs or look a lot like them. For example, a hermit crab has a distinctly crab-like appearance but is not technically a real crab. Hermit crabs are not alone; over the history of life on Earth, there have been five separate cases in which decapod crustaceans have evolved this way, a process common enough that it has picked up a formal term: carcinization.

Around a year ago, this evolutionary process captured the imagination of the Internet. Headlines like “Why everything eventually becomes a crab” and “Why Does Evolution Keep Turning Everything Into Crabs” popped up. PBS even made a video.

"Everything" is clear hyperbole—the overwhelming majority of things on Earth are not crabs and seemingly have no plans to become them. But if there are benefits to having a crab-like shape on Earth, should we view that as a general rule of life? Could it hold true on other planets? If the process of carcinization operates here, it's not unreasonable to expect that it might happen elsewhere.

Because we take these things far too seriously, Ars spoke to experts on crabs, evolution, and alien life to find out. The answer: it's highly speculative, given that we haven't found life—crab-like or otherwise—anywhere else, but it's not wholly impossible.

OK, but why crabs?​

The reasons creatures evolved crabby features are still unknown, though there are numerous hypotheses. According to Jo Wolfe, a researcher at Harvard University's Department of Organismic and Evolutionary Biology, carcinization could be a mixture of genes and the environment. However, Wolfe—who penned a paperon the topic last March—also noted that there are still no definitive answers.

“There is no clear-cut reason why being a crab is better than not being a crab,” she told Ars.

Yet crab-like forms are common enough that a number of the things we call crabs aren't all that closely related to crabs. For instance, a hermit crab is a decapod crustacean and part of a group called Anomura. Though they evolved to have crab-like features, they are not true crabs, which are in the infraorder Brachyura.

Wolfe said that the similar body plans may evolve because the body shape of a crab could have some advantages. Crab bodies tend to be compact and quite flat, with their abdomens folded up below. This could make them smaller targets for predators and could allow the crabs the ability to run and hide in smaller crevasses.

Say it with me now, Muppet fans: “Crabbbbbs… In… SPAAAAAAAACE!
 

ChaseJxyz

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Carcinization is inevitable and universal. You WILL turn into a crab, you do not have a say in the matter.

Meanwhile, stuff has evolved into parasites at least 200 times, Flight has evolved how many times now? Don't get me started on whales. Convergent evolution is weird and silly, but if it works, it works!
 

Albedo

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No more meme, just crab now.

Margaret Atwood's gunna have to change her definition of science fiction now. It's about space CRABS, not squids.
 
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frimble3

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A brief experiment: find a flat surface -your keyboard will do. Now, cross your hands over each other. Clatter your fingers up and down, as though playing piano. Note how crab-like it looks. Also, there seems to be a mild tendency for the clattering fingers to move the hands sideways.
Carcinisation has begun. Anyone with a crusty rash on the backs of their hands is merely at a somewhat more advanced stage.
Eventually your soft, squishy bits will begin to shrivel and draw up. The process will take a generation or two to complete.
 

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And yet, crabs have been largely absent in my experience from space invader movies or monster movies. Feels like an inexcusable oversight.
 

Helix

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Introversion

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Speaking of serious films where crabs are not monsters, but integral to the plot:

Unstoppable, martial arts film with Don Lee, is worth a watch. If you like butt-kicking. And crabs. And who doesn’t?
 

lizmonster

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Speaking of serious films where crabs are not monsters, but integral to the plot:

Unstoppable, martial arts film with Don Lee, is worth a watch. If you like butt-kicking. And crabs. And who doesn’t?
I LOVE THAT MOVIE.

The crabs are indeed important, but also, the rest of the movie is awesome.
 
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We were camped in a little two-person tent near the beach in Fiji when these crabs making their night trek down to the water ran into our tent. They would feel around it to find an open path. From inside, because the Moon was full, we saw their shadows on the tent as they felt their way along. The shadows looked quite big!

I have another crab story. Sitting in the sand at the water line where the waves would come in but mostly not come up to where we were sitting with every wave. (I don't remember the beach or who I was with.) There were all these little crab holes where they sat with their eye stalks out waiting for bits of food to wash up.

When any little bit would be left behind by the waves the crabs all made a bee-line for it. But when the first one got there, putting its little claw on the item, the rest stopped instantly and went back to their holes. We started throwing little bits out to watch them. So fascinating.

Sorry this goes a bit off-topic but it isn't political, it's about a short story I wrote that is about crab creatures on another planet . Wow, spoiler tags are weird.
I wrote a short horror story where a group of people were stranded on a beach on another planet. They got picked off one by one as they tried to leave the bight. One of the threats was crabs like that but they were giant and they could cut your head off in one clip of the claw.

You can read it here on Kindle preview because it's the first story and the whole story is on the preview Correction, it only goes to a few pages before the end now: :(

Sanitarium Magazine: Stone Games by Virginia May

If anyone reads it and wants to know the ending PM me. :)
 
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Friendly Frog

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So there ARE crab-monster films, even though, I still feel they're somewhat underrepresented...

I happen to have Unstoppable recorded for later viewing. It seems I shall have to move it up the schedule.
 
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Introversion

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I happen to have Unstoppable recorded for later viewing. It seems I shall have to move it up the schedule.
Hope you like it. The interactions with the secondary characters, particularly his wife, pushed the film past so many butt-kicking action films I’ve seen.
 

Introversion

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Do be sure to watch the end-credits to the end. One of my favorite husband/wife moments comes at the very end.
 
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