Why are more and more animals evolving to become crabs?

Crab-like bodies are so evolutionarily favorable that they have evolved at least five different times.

Wednesday, June 21, 2023

Our planet's complex evolutionary history has spawned countless strange and wonderful creatures, but none have caused great curiosity to evolutionary biologists - or analysts. divisive learning - like crabs.

When researchers tried to analyze the evolutionary history of crabs in a study published in 2021, they came to the conclusion that trait-defining traits, crab-like body structure, evolved at least five separate times among ten-legged crustaceans in the last 250 million years.

The iterative evolution of a crab-like body plan happens so often that it has its own name: carcinization.

The traits that determine the trait, the crab-like body structure have evolved at least 5 times.

Carcinization is an example of a phenomenon known as convergent evolution, which is when different groups of organisms independently evolve similar traits. That's why both bats and birds have wings. But interestingly, the evolution of a crab-like body has appeared many times among very closely related animals.

Why evolution continues to build and adapt the bodies of many creatures to resemble crabs remains a mystery, but we do know that there are thousands of species of crabs that thrive in almost any habitat. on Earth, from coral reefs and abyssal plains to creeks, caves and forests.

The crab is also a very diverse species in terms of body size, the smallest being the bean crab (Pinnothera faba), measuring just a few millimeters, while the largest, the  Japanese spider crab  (Macrocheira kaempferi), is almost 4 feet long. meter when measuring between the forks.

With their species richness, body shape diversity and rich fossil record, crabs are an ideal group of organisms to study biodiversity trends over time.

The crab is also a very diverse species in terms of body size.

Crustaceans have repeatedly switched from a cylindrical body shape with a large tail - typical of shrimp or lobsters - to a flatter, rounder, more crab-like shape with a tail tucked below the abdomen.

As a result, many crustaceans have evolved to look a lot like crabs, such as king crabs, which have a crab-like body structure, but actually belong to a group of closely related crustaceans. called "false crab".

The most obvious difference between "real crabs" and "fake crabs" is the number of legs they walk: real crabs have four pairs of legs, while fake crabs have only three.

Both real and fake crabs evolved a broad, flat, hard upper shell and a tail tuck independently of each other, from a common ancestor without those traits, according to an analysis published in March 2021, led by evolutionary biologist Joanna Wolfe of Harvard University.

Many crustaceans have evolved to look a lot like crabs.

As with many other topics, evolutionary biologists have a lot of ideas to explain, but at the moment there is no definite answer about carcinization.

When a trait occurs in an animal and persists for generations, it is a sign that the trait is beneficial to the species. That is the basic principle of natural selection. Crab-like animals come in many sizes and thrive in a variety of habitats, from mountains to the deep sea. Their diversity makes it difficult to identify a single common benefit for their body structure, says Joanna Wolfe.

Wolfe and colleagues have proposed several possibilities in a 2021 paper in the journal   BioEssays. For example, the retracted tail of a crab, compared to a lobster's, can reduce the amount of vulnerable meat that predators can access. And the round, flat shell can help the crab flip to the side more efficiently than the cylindrical lobster body.

But more research is needed to test those hypotheses, Wolfe said. She is also trying to use genetic data to better understand the relationships between different ten-legged crustaceans, to more accurately determine when different lines of "crabs" evolved and differentiated. factors that promote carcinization.

The crab's body shape could give animals more flexibility to develop specialized roles for their legs beyond walking, allowing crabs to more easily adapt to new habitats. Some crab species have adapted their legs to dig under sediment or paddle in water.

Most species that have undergone carcinization develop hard, calcified shells to protect them from predators - an obvious advantage - but then there have been some crab species that have abandoned the protection. this for unknown reasons.

Sidewalking, as silly as it may seem, nevertheless means that crabs are extremely agile, able to exit quickly in either direction without losing sight of predators, should they appear. But traversing is not observed in all lines of crabs undergoing carcinization (there are spider crabs that go forward ) and some hermit crabs that do not undergo carcinization may also traverse.