Carcinisation (or carcinization) is an example of convergent evolution in which a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as “one of the many attempts of Nature to evolve a crab”.
Carcinisation is believed to have occurred independently in at least five groups of decapod crustaceans, most notably king crabs, which most scientists believe evolved from hermit crab ancestors. The other examples are the family Porcellanidae or porcelain crabs (which are closely related to squat lobsters), the hairy stone crab Lomis hirta, the coconut crab Birgus latro, and true crabs (Brachyura). A prehistoric example is the extinct crustacean order Cyclida, which evolved their crab-like morphology before the existence of true crabs.
The example of king crabs (family Lithodidae) evolving from hermit crabs has been particularly well studied, and evidence in their biology supports this theory. For example, most hermit crabs are asymmetrical, so that they fit well into spiral snail shells; the abdomens of king crabs, even though they do not use snail shells for shelter, are also asymmetrical.
An exceptional form of carcinisation, termed “hypercarcinisation”, is seen in the porcelain crab Allopetrolisthes spinifrons. In addition to the shortened body form, A. spinifrons also shows similar sexual dimorphism to that seen in true crabs, where males have a shorter pleon than females.