Carcinisation

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 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."

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

A red king crab

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, and fit well into spiral snail shells; the abdomens of king crabs, even though they do not use snail shells for shelter, are also asymmetrical.[13][14][15][16]

Some crab shaped species have evolved away from the crab form in a process known as decarcinisation. Decarcinisation, or the loss of the crab-like body, has occurred multiple times in both Brachyura and Anomura.[18][19] However, there are varying degrees of carcinisation and decarcinisation. Thus, not all species can necessarily be classified as "carcinised" or "decarcinised." Some examples include the coconut crab as well as other hermit crabs that have lost or reduced their outer casing, often referred to as "domiciles." While they retain their crab-like phenotype, their reduction or lack of domicile necessitates a "semi-carcinised" label.

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