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 among the 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 evolution, which is when different groups independently develop the same traits. It’s the same reason Both bats and birds have wings. But interestingly, the crab-like body plan appeared several times among very 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 ever regressing?
Crustaceans have morphed time and again from having a cylindrical body outline with a large tail—characteristic of shrimp or crabs—to a flatter, rounder, crab-like appearance with a less prominent tail. The upshot is that many crab-like crustaceans, such as the delicious king crab craved as seafood, aren’t even technically a “true crab.” They have adopted a crab-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 remains through generations, it is a sign that the trait is beneficial to the species – this is the basic principle of natural selection. Crab-shaped animals come in many sizes and thrive in a wide range of habitats, from the mountains to the deep sea. He said their diversity makes it difficult to pinpoint a single common benefit to their body plan Joanna Wolfis a research associate in organic and evolutionary biology at Harvard University.
Wolfe and his colleagues offer some possibilities in a paper published in 2021 in the journal Biological sayings. For example, the folded tail of a lobster, versus the more prominent tail of a lobster, can reduce the amount of vulnerable flesh that predators have access to. And the round, flat shell can help the crab pail sideways more effectively than the cylindrical body of the lobster allows.
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 different crustacean decapods, to pinpoint more precisely when the different “crabby” lineages evolved, and to pick out which factors lead to carcinogenesis.
Another possible explanation: “It’s possible that having a crab body isn’t necessarily beneficial, and may be a consequence of something else in the organism,” Wolf said. For example, a crab body scheme might be so successful not because of the shape of the shell or the tail itself, but because of the possibilities that that shape opens up to other parts of the body, said Luque, co-author of a 2021 paper with Wolfe.
For example, the tail of a giant lobster can propel the animal through the water and help it crush prey. It can also get in the way and limit other features, Loki said. The crab’s body shape may leave 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 think that the crab’s body plan evolved several times independently because of the diversity that the animals have,” Lockey said. This allows them to go places no other crustacean has been able to go.”
The crab-like body plan has also been lost several times over the course of evolutionary time—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 swapped out without significantly changing other features. So it is relatively easy for a cylindrical object to be flattened, or vice versa. But for better or worse, humans won’t be turning to crabs anytime soon. “Our body is not standard like that,” Wolf said. “[Crustaceans] Already the right building blocks.”
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