Round flat shell. Tail tucked under the body. This is what crabs look like, and what peak performance looks like — at least according to development. The crab-like body plan has evolved at least five times in the decapod crustaceans, a group that includes crabs, lobsters, and shrimp. In fact, it so often happens that there is a name for it: carcinoma.
So why do animals keep evolving into crab-like shapes? Scientists don’t know for sure, but they have a lot of ideas.
Carcinogenesis is an example of a phenomenon called convergent evolutionThat is, when different groups develop the same traits independently. It’s the same reason Bats and birds have wings. But oddly enough, the crab-like body outline has been shown up again and again in closely related animals.
The fact that it happens on such a fine scale “means that evolution is fluid and dynamic”, Javier Lockea research associate in the Department of Zoology at the University of Cambridge told Live Science.
Related: Is evolution backwards?
Crustaceans changed over and over from a cylindrical body outline with a large tail—characteristic of shrimp or crabs—to a more rounded, rounded, lobster-like appearance with a less prominent tail. The upshot is that many crustaceans that look like crabs, like the delicious king crab that is foamed up as seafood, aren’t even technically a “true crab.” They’ve adopted a lobster-like body plan, but in fact they belong to a closely related group of crustaceans called “false crabs.”
When a trait appears in an animal and persists through generations, it is a sign that the trait is beneficial to the species – this is the basic principle of natural selection. Angry-shaped animals come in many sizes and thrive in a wide range of habitats, from the mountains to the deep sea. Their diversity makes it difficult to pinpoint one common benefit to their body plan, he said Joanna WolfResearch associate in organic and evolutionary biology at Harvard University.
Wolfe and his colleagues present some possibilities in a 2021 article in the journal Bioassays. For example, the folded tail of lobsters, compared to the more prominent lobsters, can reduce the amount of vulnerable flesh that predators have access to. And the flat, rounded carapace can help the crab move horizontally more efficiently than the lobster’s cylindrical body.
But Wolf said more research is needed to test these hypotheses. It is also trying to use genetic data to better understand the relationships between the various crustacean decapods, to determine more precisely when the different “crabby” subspecies evolved, and to characterize the factors that cause carcinogenesis.
Another possible explanation: “It’s possible that having a crab body isn’t necessarily beneficial, and may be the result of something else in the body,” Wolf said. For example, a crab body plan may be so successful not because of the shape of the shell or the tail itself, but because of the possibilities that the shape opens up for other parts of the body, said Luque, co-author of the 2021 paper with Wolfe.
For example, the tail of a giant lobster can propel the animal through the water and help crush its prey. It can also get in the way and limit other features, Lockie said. The crab’s body shape could allow more flexibility for the animals to develop specialized roles for their legs beyond walking, allowing the crab to easily adapt to new habitats. Some crabs have adapted their legs for digging under sediment or paddling in water.
“We believe that the crab’s body plan evolved several times independently due to the diversity of the animals,” Lockey said. “It allows them to go places no other crustacean has been able to go.”
The crab-like body outline has also been lost several times during evolution – a process known as decarcinogenesis.
“Crabs are flexible and versatile,” Locke explained. “They can do a lot of things back and forth.”
Wolf thinks of crabs and other crustaceans like Lego creations: they have many different components that can be interchanged without radically changing other properties. So it is relatively easy for a cylindrical body to flatten, or vice versa. But for better or worse, humans aren’t turning to crabs anytime soon. “Our body is not stereotyped like that,” Wolf said. “[Crustaceans] Indeed the right building blocks. »
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