Our planet's convoluted history of evolving life has spawned many weird and wonderful creatures, but none that excited evolutionary biologists or taxonomists.
The defining features of crabbiness have evolved at least five times in the past 250 million years, according to researchers who tried to reconcile the evolutionary history of crabs in all their glory just earlier this year.
Crabbiness may have been lost seven times or more.
The body plan that has evolved many times is called carcinization. If you lose crabbiness to evolution, it's called decarcinization.
Frog crabs are an example. Red king crabs regained features of their crab body plan at the last evolutionary minute, after they were lost to Puerto Rican sand crabs and various lop-sided hermit crabs.
A crab is from Puerto Rican. The CC BY-NC 4.0 is written by the iNaturalist/CC by-NC.
The mystery of why evolution keeps crafting and shafting the crab-like body plan remains, though evolution must be doing something right in fashioning crabby creatures time and time again.
There are thousands of crab species, which thrive in almost every habitat on Earth.
There is an impressive display of sizes. The Japanese spider crab is the largest and spans over 4 meters from claw to claw.
Crabs have a rich fossil record and are an ideal group to study the trends in biodiversity through time. Finding order in the chaos of crabs is a challenge.
What is a crab?
It gets weirder because not every crab is a crab. Mud crabs and swimmer crabs are true crabs. We also have false crabs, such as shell-shy hermit crabs with their spiraling abdomens, or spike-covered king crabs.
True crabs have four pairs of walking legs, whereas false crabs only have three, with another small pair at the rear.
An analysis published in March of 2021, led by evolutionary biologists from Harvard University, suggests that both true and false crabs evolved from a common ancestor that had none of the features.
After true and false crabs split, it wasn't a straight path. Over the past 250 million years, evolution has made and remake crabs at least three times, according to Wolfe and colleagues.
Crabs have long stumped taxonomists who have often misclassified species as true or false due to their striking similarities.
Understanding how many times evolution has created 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," Heather Bracken-Grissom, Wolfe's co-author, told Popular Science in 2020.
convergent evolution is not uncommon when species have similar selecting pressures.
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Everything is a crab.
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October 13, 2020
Evolutionary biologists have a lot of ideas, but no firm answers on crab body plan evolution.
To start, the trio of researchers compiled data on crab behavior and natural history from living species and fossils, and identified the gaps in genetic data which might help to resolve puzzling evolutionary relationships.
Half of the crab tree of life is dark.
Some crabs have abandoned their protection of hard, calcified shells, which is a clear advantage, for reasons unknown.
Crabs are able to make a quick exit in either direction without losing sight of a predator, if you walk sideways. There are some crabs that can walk sideways, but not all of them.
It is not possible to explain the timing or success of early crab evolution.
Joanna M. Wolfe is a person.
There are examples of carcinized and decarcinized clades in the tree.
Evolution will continue on its merry way, like anything in science. Taxonomists are still trying to piece together what makes a crab, despite the fact that there is more information on crab species.
This will allow us to resolve the multiple origins and losses of 'crab' body forms through time and identify the timing of the origin of key evolutionary novelties and body plans.
Evolutionary sleuths think it might be possible to anticipate the predictable shapes evolution makes based on environmental factors and genetic cues, if they study crabs.
"Examining crab evolution provides a macroevolutionary timescale of 250 million years ago for which, with enough phylogenetic and genomic data, we might be able to predict the morphology that would result."
A crab-like shape is a good bet.
The paper was published in a journal.
