Why do animals keep evolving into crabs?

A flat, rounded shell. A tail that's folded under the body. This is what a crab looks like, and apparently what peak performance might look like — at least according to evolution. A crab-like body plan has evolved at least five separate times among decapod crustaceans, a group that includes crabs, lobsters and shrimp. In fact, it's happened so often that there's a name for it: carcinization.

So why do Animals keep evolving into crab-like forms? Scientists don't know for sure, but they have lots of ideas.

Carcinization is an example of a phenomenon called convergent evolution, which is when different groups independently evolve the same traits. It's the same reason both bats and birds have wings. But intriguingly, the crab-like body plan has emerged many times among very closely related Animals. 

The fact that it's happening at such a fine scale "means that evolution is flexible and dynamic," Javier Luque, a senior research associate in the Department of Zoology at the University of Cambridge, told Live Science.

Related: Does evolution ever go backward?

Crustaceans have repeatedly gone from having a cylindrical body plan with a big tail — characteristic of a shrimp or a lobster — to a flatter, rounder, crabbier look, with a much less prominent tail. The result is that many crustaceans that resemble crabs, like the tasty king crab that's coveted as a seafood delicacy, aren't even technically "true crabs." They've adopted a crab-like body plan, but actually belong to a closely related group of crustaceans called "false crabs."

When a trait appears in an animal and sticks around through generations, it's a sign that the trait is advantageous for the species — that's the basic principle of natural selection. Animals with crabby forms come in many sizes and thrive in a wide array of habitats, from mountains to the deep sea. Their diversity makes it tricky to pin down a single common benefit for their body plan, said Joanna Wolfe, a research associate in organismic and evolutionary biology at Harvard University.