The Evolutionary Mystery of Crab-Like Animals.
The crab-like body plan has evolved at least five separate times among decapod crustaceans, a group that includes crabs, lobsters and shrimp. Scientists don't know for sure why animals keep evolving into crab-like forms, but they have lots of ideas. The crab-like body plan has emerged many times among very closely related animals, which means that evolution is flexible and dynamic. The fact that the trait is advantageous for the species is a sign that it sticks around through generations, which is the basic principle of natural selection.