Why Evolution Keeps Making Crabs: The Strange Case of Carcinization

Carcinization — the repeated, independent evolution of crab-like bodies — has occurred at least five times among crustaceans. That surprising pattern has fascinated scientists because it shows how similar ecological pressures can sculpt unrelated lineages into remarkably similar forms.

What Is Carcinization?

Carcinization is the tendency for diverse crustacean groups to evolve a flattened, short-tailed, broad-bodied “crab” shape. The term traces to the Greek karkinos (crab) and was popularized in zoology by Lancelot Borradaile. Although parallel and convergent evolution are common across life, the number and consistency of independent crab-like transformations are unusually striking.

Why Does It Happen?

Unrelated animals that occupy similar habitats face comparable challenges — from locomotion on complex surfaces to protection from predators and space-efficient body plans. These shared selective pressures can favor similar solutions, so separate lineages often evolve comparable structures and functions. The Encyclopedia Britannica gives a classic parallel: Australian marsupials often resemble placental mammals elsewhere in body form and ecological role despite major reproductive differences.

More Than Skin Deep

The recent paper summarized by BoingBoing goes beyond superficial resemblance. It documents not only external crab-like morphology across five independent events but also parallels in internal systems such as aspects of the nervous system and circulation, while noting which organs and systems differ in shape or scale.

"Some of the internal anatomical characters studied herein are structurally dependent on the external characters of a crab-like habitus. Since morphological coherence can also exist between internal anatomical structures, the coherence chains which can be traced back to the external characters of a crab-like habitus are relatively complex in some cases (indirect coherences)."

This observation highlights a key challenge for researchers: external body plans can influence—and be influenced by—internal anatomy, making it difficult to tease apart cause and effect in the evolutionary process.

Hermit Crabs and King Crabs

Not all crab-like crustaceans started out as typical ‘‘true crabs.’' For example, hermit crabs do not develop a fixed external shell but instead occupy borrowed shells. Intriguingly, the analysis supports the widely discussed evolutionary path by which large, spiny king crabs likely evolved from hermit-crab–like ancestors, illustrating how dramatic morphological shifts can arise from less crab-like beginnings.

Why It Matters

Carcinization is more than a taxonomic curiosity. Each independent transition to a crab-like form is a natural experiment that helps scientists understand how external ecology, functional demands, and developmental pathways interact to produce similar outcomes in very different lineages. Studying these repeated events sheds light on the predictability—and the constraints—of evolution.