Evolution Continually Produces Crabs, But This Trait Appeared Only Once

Within the animal kingdom, natural selection seems to repeatedly favor the crab-like physical traits. But curiously, their iconic sideways walk has evolved only once.

Carcinization and Lateral Locomotion

Term introduced in 1916, carcinization designates a form of evolutionary convergence: when unrelated species subjected to similar environmental constraints develop the same physiological, morphological, or behavioral traits. Among crustaceans, it is believed to have appeared in at least five instances, and several cases have also been documented in arachnids.

Providing effective protection against predators, the body plan of crabs also enables them to move with agility and to seize prey with ease. Surprisingly, researchers at Nagasaki University determined that the locomotion so characteristic of many of them had emerged only once, about 200 million years ago.

As part of these studies published in the journal eLife, Yuuki Kawabata and colleagues examined the movement of 50 “true” crabs, and performed genetic comparisons involving 344 crustacean species. It turned out that 35 of these brachyurans moved laterally, and that this trait had arisen in a distant ancestor at the base of the group Eubrachyura, which walked forward.

“This unique event contrasts strongly with the carcinization, which has occurred multiple times among the decapods [crabs, lobsters, shrimp and lobsters],” the team writes.

Walk or Die?

At this stage, the reasons behind such a scenario remain unclear, but it is possible that it started as a defensive strategy, with lateral movements confusing predators.

“Such a trait has probably contributed to their ecological success,” Kawabata surmises. “Thanks to direct behavioral observations and a phylogenetic framework, this work broadens our understanding of how animal locomotion modes diversify and persist through evolution.”

In 2023, a study had revealed that crabs continued to evolve to move from sea to land and back again.