Carcinization is a phenomenon where certain crustaceans independently evolve into a crab-like shape with a flat carapace and tucked-in tail. The shape has evolved at least five times within the Anomura order, and researchers speculate that there may have been a common ancestor that was not yet carcinized. Carcinization is a form of convergent evolution, where unrelated species develop similar traits. The reasons behind carcinization remain unknown, but theories suggest that the crab shape provides advantages in burrowing, maneuverability, and predator avoidance. Studying carcinization can provide insights into natural selection and how organisms adapt to their environment.