Our planet's convoluted history of evolving life has spawned countless weird and wonderful creatures, but none excite evolutionary biologists – or divide taxonomists – quite like crabs.
With their species richness, extravagant array of body shapes and rich fossil record, crabs are an ideal group to study trends in biodiversity through time. But finding some order in the chaos of crabs is an ongoing challenge.It gets weirder, because not every crab is a crab, so to speak. There are 'true' crabs, such as mud crabs and swimmer crabs. Yet we also have so-called false crabs, such as shell-shy hermit crabs with their spiraling abdomens, or the spike-covered king crabs.
The most visible difference between true and false crabs is how many walking legs they have: true crabs have four pairs of lanky legs, whereas false crabs only have three, with another pint-sized pair at the rear. Both true and false crabs evolved their wide, flat, hard upper shell and tucked tails independently of one another, from a common ancestor that had none of those features,But it wasn't a straightforward path after true and false crabs split. Evolution has made and remade crabs over the past 250 million years: once or twice in true crabs and at least three times during the evolution of false crabs, Wolfe and colleagues think.species as true or false crabs due to their striking similarities.
Besides figuring out where species belong in the tree of life, understanding exactly how many times evolution has crafted the crab-like body form and why, could reveal something about what drives convergent evolution. "There has to be some kind of evolutionary advantage to be this crablike shape," crab expert and Wolfe's co-author Heather Bracken-Grissomtired: convergent evolution is not uncommon especially when species have similar selecting pressures in their environments
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