Carcinisation

Porcelain crabs resemble crabs, but are more closely related to squat lobsters and hermit crabs.^[1]

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".^[2] Most carcinised crustaceans belong to the order Anomura.

Definition of carcinised morphology[]

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

[…] carcinization […] 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 habit of 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 sternites are fused into a wide sternal 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,"

Examples[]

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 (Crabs)^[10] First appearance: Early Jurassic
• Order Cyclida

Prehistoric "crabs"[]

A prehistoric example is the extinct crustacean order Cyclida, which evolved their crab-like morphology before the existence of true crabs.^[11]

King crabs[]

A red king crab in the collection of the Children's Museum of Indianapolis

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.^{[12][13][14][15]}

Hypercarcinisation[]

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.^[16]

See also[]

• List of examples of convergent evolution
• Cretaceous crab revolution
• Mesozoic marine revolution

References[]