A flat, round shell. Tail folded under the torso. This is what crabs look like, and what peak performance looks like, at least according to evolution. Crab-like body plans evolved at least five times among decapod crustaceans, including crabs, lobsters, and shrimps. In fact, this happens so often that it gets the name “charring”.
So why do animals continue to evolve into crab-like forms? Scientists don’t know for sure, but they have many ideas.
Carbonization is an example of a phenomenon called convergent evolution, in which different groups evolve the same trait independently. It’s the same reason bats and birds have wings. Interestingly, however, crab-like body plans have emerged many times among very closely related animals.
The fact that it’s happening on such a fine scale “means that evolution is flexible and dynamic,” Javier Luque, senior researcher at Cambridge University’s Department of Zoology, told Live Science.
Crustaceans have repeatedly changed from the cylindrical body shape with the large tail characteristic of shrimp and lobster to a flatter, rounder, crab-like appearance with a less prominent tail. As a result, many crab-like crustaceans, such as the delicious red king crab, a coveted seafood delicacy, aren’t even “true crabs,” strictly speaking. Although they have a crab-like body, they actually belong to a closely related group of crustaceans called “pseudocrabs”.
When a trait appears in an animal and persists through generations, it indicates that the trait is beneficial to that species. This is the basic principle of natural selection. Crab-like animals come in many sizes and thrive in a wide range of habitats, from mountains to the deep sea. Their diversity makes it hard to pinpoint just one common benefit to their body plans, said Joanna Wolfe, a researcher in biological evolutionary biology at Harvard University.
In a 2021 paper published in BioEssays, Wolfe et al. For example, a lobster’s tail protrudes farther, whereas a crab’s tail is tucked in, which may reduce the amount of vulnerable meat accessible to predators. And a flat, round carapace may help the crab escape to the side more effectively than a cylindrical lobster body.
But more research is needed to test these hypotheses, Wolf said. She will also use the genetic data to better understand the relationships among different decapod crustaceans, better pinpoint when different “stubborn” lineages evolved, and try to isolate the factors that cause charring. and
There is also another possible explanation. “It’s also possible that having a crab body isn’t necessarily an advantage, perhaps it’s a result of something else in the organism,” Wolff said. For example, the crab body plan may have been so successful not because of the shell and tail shape itself, but because of the potential for this shape to open up to other parts of the body, according to a 2021 paper. Co-author Luque said. Paper with Wolf.
For example, a lobster’s huge tail helps propel it through the water and crush its prey. But it can get in the way and constrain other features, Luque said. The crab’s body shape leaves more flexibility for the animal to evolve the specialized role of its feet beyond walking, potentially allowing the crab to adapt easily to new habitats. Some crabs have adapted their legs for digging under sediments and paddling through water.
“We believe that the crab’s body structure has evolved independently many times because of the versatility of this animal,” Luque said. “It allows them to go where other crustaceans couldn’t.”
The crab-like body plan has also been lost many times over the course of evolution. This is a process known as decarburization.
“Crabs are flexible and versatile,” Luque explains. “They can go back and forth a lot.”
Wolfe thinks of crabs and other crustaceans like Lego pieces. These crustaceans have many different components that can be replaced without significantly altering other functions. It is therefore relatively easy to flatten a cylindrical body and vice versa. But for better or worse, humans don’t turn into crabs anytime soon. “Our bodies aren’t that modular,” Wolff says. “[Crustaceans] You already have the right components. “
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