Evolution Keeps Making Crabs, and No One Knows Why: ScienceAlert

The tangled history of evolving life on our planet has produced countless creatures, both weird and wonderful, but none excites evolutionary biologists — or fission taxonomists — quite like crabs.

When researchers tried to reconcile the evolutionary history of crabs in all their noisy glory in a study published in 2021, researchers concluded that the crab species’ distinctive traits have evolved at least five times in the past 250 million years.

In addition, it was grumpy lost possibly seven times or more.

This repeated evolution of a crab-like physique happened so often that it got its own name: carcinization. (And yes, losing your grumpiness through evolution is called decarcinization.)

Frog crabs (Raninidae) are an unusual example. Characteristics of the crab’s body plan were also lost on the way to the nearly legless Puerto Rican sand crabs (Emerita portoricensis) and various crooked hermit crabs—but then, at the last evolutionary minute, red king crabs took on grumpy features again.

Oval-shaped, armored creature on a sandy beachA Puerto Rican sand crab. (Michelle Barros Sarmento Gama/iNaturalist/CC BY-NC 4.0)

Why evolution continues to create and push the crab-like body plan remains a mystery, although evolution must always get things right in creating surly creatures.

There are thousands of crab species that thrive in almost every habitat on earth, from coral reefs and deep sea flats to streams, caves and forests.

The size of the crabs is also impressive. The smallest, the pea crab (Pinnothera faba), measures only 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, exceptional variety of body forms, and rich fossil record, crabs are an ideal group to study trends in biodiversity over time. But finding order in the crab chaos is a constant challenge.

What is a crab anyway?

It gets even weirder because not every crab is a crab, so to speak. There are “real” crabs, such as mud crabs and swimming crabs. But there are also so-called false crabs, such as the shield-shy hermit crabs with their spiral-shaped abdomen or the king crabs covered with spines.

The most visible difference between real and fake crabs is how many walking legs they have: real crabs have four pairs of lanky legs, while fake crabs only have three, plus another pint-sized pair on the back.

Both true and false crabs independently evolved their broad, flat, hard upper carapace and tucked-in tails from a common ancestor that exhibited neither of these traits, according to an analysis led by Harvard evolutionary biologist Joanna Wolfe published in March 2021 University emerges .

But it wasn’t an easy road after real and fake crabs parted ways. Evolution has repeatedly created crabs over the past 250 million years: once or twice in true crabs and at least three times during evolution of false crabs, Wolfe and colleagues think.

Crabs have long been a mystery to taxonomists, who have consistently misclassified the species as true or false crabs based on their striking resemblance.

In addition to asking where species belong in the family tree of life, understanding exactly how often evolution has created the crab-like body shape and why could reveal something about what drives convergent evolution.

“There has to be an evolutionary advantage to taking on this crab-like form,” crab expert and Wolfe co-author Heather Bracken-Grissom told Popular Science in 2020, as carcinization raged the internet.

As with many subjects, evolutionary biologists have many ideas but no definite answers about carcinization. Because of the narrow focus of previous research on selected crab species, “the unconventional story of the evolution of the crab body plan needs to be reconciled,” the team explained.

First, the trio of researchers collected data on the morphology, behavior and natural history of crabs from living species and fossils, and identified the gaps in the genetic data that could help solve puzzling evolutionary connections.

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

Most carcinated crabs have evolved hard, calcified shells to protect themselves from predators—a definite advantage—but some crabs have abandoned this protection for unknown reasons.

Walking sideways, silly as it may seem, means crabs are extremely agile and can flee quickly in either direction without losing sight of a predator should one appear. However, sidewalking is not observed in all carcinated lineages (forward-walking spider crabs exist), and some noncarcinated hermit crabs can also walk sideways.

That some crabs evolved oversized claws to become mussel-smashing predators in an ecological arms race also cannot fully explain the timing or achievements of early crab evolution.

Taxonomic tree showing carcinated and decarcinated crab lineagesPhylogenetic tree showing examples of carcinated and decarcinated clades, with colored dots marking features on the branches. (Joanna M Wolfe)

Like everything in science, nothing is ever settled and evolution will continue on its merry way. Although more and more genomic information about living and fossil crab species is becoming available, taxonomists can always figure out what makes a crab a crab.

This “will allow us to elucidate the multiple origins and losses of ‘crab’ body forms over time and identify the timing of the emergence of important evolutionary novelties and body plans,” Wolfe said.

Additionally, studying crabs offers an enticing prospect for evolutionists who believe it may be possible to predict the predictable shapes evolution produces based on environmental factors and genetic clues.

“The study of cancer evolution provides a macroevolutionary timescale from 250 million years ago, for which with sufficient phylogenetic and genomic data we might be able to predict the resulting morphology,” Bracken-Grissom said.

A crab-like shape might be a safe bet.

The paper was published in BioEssays.

A previous version of this article was published in January 2022.

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