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Nature really likes crabs and keeps trying to evolve more of them.
Carcinisation is the name of a process of convergent evolution, in which crustaceans evolve into crab-like forms. In decapod crustatceans, there have been at least 5 distinct times when crab-like animals have evolved.
Coconut crabs, king crabs, and hermit crabs are not types of true crab, and are only distantly related.
Convergent evolution is really cool.
Sperm whales and copepods use the same mechanism to control their buoyancy.
@InvaderXan 
humans: "We are the pinaccle of evolution. This world is all about us."
nature: "make moar crabs"
@polychrome @InvaderXan nature has a crabby sense of humor
@InvaderXan it's the crab cycle
@InvaderXan Crabs: they just work!
@InvaderXan I remember hearing about this before, it's super interesting. Must just be a real damn comfy niche to fill.
Do you know somewhere I can read more about convergent evolution?
@RedFuture I guess it's just that crab-shaped animals are especially good at what they do!
Convergent evolution... I can't think of any good authors offhand, but the Wikipedia articles about this are actually pretty good!
@InvaderXan Crabs: excellent at crabbing it up
Mari 
@InvaderXan now I'm curious as to how horseshoe crabs fit into this. I think they're much older?
@bouncinglime They are, yes. They're also arachnids, more closely related to spiders.
I think carcinisation refers specifically to decapod crustaceans (crabs, lobsters, shrimp, etc) but there are certainly some crab-like animals among the insects and arachnids too!
@InvaderXan ah! I knew there would be some sort of distinction for them. But I just love horseshoe crabs so much and had to bring them up. ^_^
hmmm maybe a custom emoji is needed...
@bouncinglime They really are lovely creatures, I agree!
@InvaderXan I agree with nature on this one, crabs are pretty neat
@troubleMoney What's not to love, amiright?
@InvaderXan crabs are like "this is the ideal body shape. you may not like it, but this is what peak performance looks like."
@FuchsiaShock Pinchy perfection!