Evolution continues to make and break crabs, and no one knows why

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Our planet’s convoluted life evolutionary history has spawned countless weird and wonderful creatures, but none excite evolutionary biologists – or divide taxonomists – like crabs.

When researchers attempted to reconcile the evolutionary history of crabs in all their raucous glory just earlier this year, they came to the conclusion that the defining characteristics of grumbling have evolved at least five times in the past 250 million. years.

In addition, the grumbling was lost maybe seven times or more.

This repeated evolution of a crab-like body plan has happened so often that it has its own name: carcinization. (And yes, if you lose grumbling because of evolution, it’s called decarcinization.)

frog crabs (Raninidae) are an unusual example. The crab’s body plan features were also lost en route to the nearly legless Puerto Rican sand crabs (Emerita portoricensis) and various unbalanced hermit crabs – but then red king crabs regained grumpy features at the last minute of evolution.

A sand crab from Puerto Rico. (Michelle Barros Sarmento Gama / iNaturalist / CC BY-NC 4.0)

Why evolution continues to create and shape the crab-like body plane remains a mystery, although evolution must do something right by repeatedly shaping crab creatures.

There are thousands of species of crabs, which thrive in almost every habitat on Earth, from coral reefs and abyssal plains to coves, caves and forests.

Crabs also offer an impressive display of sizes. The smallest, the pea crab (Pinnothera faba), measures only a few millimeters, while the largest, the Japanese spider crab (Macrocheira kaempferi), stretches almost 4 meters (about 12 feet) from claw to claw.

With their species richness, extravagant array of body shapes, and rich fossil record, crabs are an ideal group for studying trends in biodiversity over time. But finding some order in the crab chaos is an ongoing challenge.

What is a crab, anyway?

It gets weirder, because not all crabs are crabs, so to speak. There are “real” crabs, such as mud crabs and swimming crabs. Yet we also have so-called false crabs, like the shy hermit crabs with their spiral abdomens, or king crabs covered with spikes.

The most visible difference between true and false crabs is the number of legs they have: true crabs have four pairs of gangly legs, while false crabs have only three, with another pair of the pint size on the back.

True and false crabs developed their broad, flat, hard upper shells and folded tails independently of each other, from a common ancestor who had none of these characteristics, offers an analysis published in March 2021, edited by evolutionary biologist Joanna Wolfe of Harvard University.

But it was not a simple path after separating the real and false crabs. 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, think Wolfe and his colleagues.

Crabs have long had puzzled taxonomists who have invariably misclassified species like real or false crabs because of their striking similarities.

In addition to figuring out where species belong in the tree of life, understanding exactly how many times evolution has shaped the crab-like body shape and why, could reveal something about what drives convergent evolution.

“There has to be some kind of evolutionary advantage to have this crab form,” Heather Bracken-Grissom, crab expert and co-author of Wolfe. Recount Popular Science in 2020, when carcinization got the internet going.

As with many topics, evolutionary biologists have many ideas, but no firm answers about carcinization. Due to the narrowness of previous research on certain species of crabs, “the sparse history of the evolution of the crab body plan must be reconciled”, the team writing.

To begin with, the trio of researchers compiled data on the crab’s morphology, behavior, and natural history, from living species and fossils, and identified gaps in genetic data that could help resolve issues. confusing evolutionary relationships.

“Almost half of the branches of the crab of life remain dark”, they write.

Most carcinogenic crabs developed hard, calcified shells to protect themselves from predators – a distinct advantage – but some crabs subsequently abandoned this protection, for reasons unknown.

Walking to the side, as silly as it may sound, means the crabs are extremely agile, able to make a quick exit back and forth without losing sight of a predator, if any. But lateral walking is not observed in all carcinized lines (there is spider crabs walking forward) and some noncarrassed hermit crabs may also walk sideways.

The fact that some crabs developed outsized claws to become shell-crushing predators in an ecological arms race also cannot fully explain the timing or successes of the crab’s early evolution.

Taxonomic tree showing lineages of carcinized and decarcinated crabs(Joanna M. Wolfe)

Above: Phylogenetic tree showing examples of carcinated and decarcinated clades, with colored dots noting characteristics on the branches.

Like everything in science, nothing is ever settled and evolution will continue on its way. Although with increasing amounts of genomic information on living and fossilized crab species, rest assured that taxonomists are regularly gathering which makes a crab, a crab.

This “will allow us to resolve the multiple origins and losses of the body forms of” crab “through time and to identify the moment of the origin of the main evolutionary novelties and bodily planes”, said Wolfe.

More than that, the study of crabs offers a tantalizing prospect for evolutionary detectives who believe it might be possible to anticipate the predictable forms evolution creates based on environmental factors and genetic clues.

“Examining the evolution of the crab provides a macro-evolutionary timescale of 250 million years ago for which, with sufficient phylogenetic and genomic data, we might be able to predict the resulting morphology.” , said Bracken-Grissom.

Some form of crab might be a safe bet.

The document was published in Bioassays.

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