In evolutionary biology, carcinisation [kar-sin-i-say-shun] is a hypothesised process whereby a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced by 20th century English zoologist L. A. Borradaile, who described it as ‘one of the many attempts of Nature to evolve a crab.’ Many animals that are called crabs – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, and crab lice – are not true crabs (infraorder Brachyura).
Carcinisation is believed to have occurred independently in at least five groups of decapod (ten-footed) crustaceans, most notably king crabs, which most scientists believe evolved from hermit crab ancestors. 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, it also developed sexual dimorphism that matches true crabs, where males have a reduced pleon (abdomen) compared to females. Porcelain crabs likely evolved from squat lobsters
The Daily Omnivore 