Carcinisation

Carcinisation (American English: carcinization) is a form of convergent evolution in which non-crab crustaceans evolve a crab-like body plan. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as "the many attempts of Nature to evolve a crab".^[2]

Definition of carcinised morphology

It was stated by Lancelot Alexander Borradaile in 1916 that:^[3]

... carcinisation … consists essentially in a reduction of the abdomen of a macrurous crustacean, together with a depression and broadening of its cephalothorax, so that the animal assumes the general body of a crab

Keiler et al., 2017 defines a carcinised morphology as follows:^[4]

• "The carapace is flatter than it is broad and possesses lateral margins."
• "The
  plastron
  which possesses a distinct emargination on its posterior margin."
• "The
  pleon is flattened and strongly bent, in dorsal view completely hiding the tergites
  of the fourth pleonal segment, and partially or completely covers the plastron."

Carcinisation is believed to have occurred independently in at least five groups of decapod crustaceans:^[4]

• Order Decapoda:
  • Infraorder Anomura:
    • King crabs, which most scientists believe evolved from hermit crab ancestors^[5][6]
      • First appearance: Late Cenozoic
    • Porcelain crabs, which are closely related to squat lobsters^[7]
      • First appearance: Late
        Jurassic
    • The hairy stone crab (Lomis hirta)^[8]
    • Hermit crabs:
      • The coconut crab (Birgus latro)
      • Patagurus rex^[9]
  • Infraorder Brachyura (true crabs)^[10] First appearance: Early Jurassic

The extinct probable crustacean order Cyclida are also noted to "strikingly resemble crabs," and probably had a similar ecology.^[11][12]

King 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