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]

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

… the phenomenon which may be called "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."

An important and visually evident marker of difference between true crabs and carcinised Anomura is the number of leg pairs. While Brachyura (true) crabs have four pairs of legs used for locomotion, Anomura possess one much smaller set and therefore three sets of walking legs.

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]

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]}

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.^[17]

Independently arising from multiple ancestral crustacean taxa, the crab-like traits exhibited vary between individual species and taxa. However, all crabs and carcinised organisms are decapods. Correlations between the folding of the pleon tail and widening of the cephalothorax across disparate decapod species suggest similar evolutionary pressures. Some occurrences of carcinisation are derived from convergent but distinct developmental pathways, while others may be instances of homologous parallelism^[15] from shared ancestral body plans.

Most carcinised organisms are descended from the infraorder Anomura, which includes hermit crabs.^[18] Many carcinised Anomura evolved from ancestors with morphologically intermediate forms reminiscent of modern squat lobsters,^[19] not including the King Crab which is hypothesized by researchers to be descended directly from a variety of Pagurid hermit crab. There may be various advantages to adopting brachyuraform (true crab-like) traits.

The adoption of a crab-like body structure can convey a number of selective advantages for crustacean species. Carcinisation is associated with a lowered center of gravity, allowing these creatures to invest in sideways walking.^[20] This evasive adaptation is particularly useful in an ocean environment with forward-moving predators. The pleon is held tightly under the animal’s cephalothorax with reduced musculature, which protects the pleon’s organs from attack.^[20] The smaller and more balanced frame facilitates concealment within rocks and coral. The folding of the pleon below the carapace reduces the crustacean’s exposed surface area, and associated hardening of the pleonal cuticle are all thought to benefit the fitness of this body type.

The caridoid escape reaction is an innate danger response in crustaceans such as lobsters and crayfish, which contracts abdominal flexions and sends the crustacean flying backward in the water.^[21] Brachyura and species which have undergone carcinization have strongly bent and immobile tails, which prevent them from using this evasion strategy. The necessary muscles are no longer developed enough in these species to facilitate the necessary tail flipping. Crabs and false crabs are best suited to escape by ground pursuit in comparison to the quick aquatic escape provided by the caridoid escape reaction.

Porcelain crabs’ closest relatives are squat lobsters, taxa which occupy a morphological middle ground, described by Keiler et. al. as “half-carcinized” due to their partially flexed pleons and carapaces that remain longer than they are wide. Many species do not become fully carcinised but only undergo the crab-like adaptations that are contextually beneficial, to varying degrees.

While most incidences of carcinization are in aquatic Anomura populations, it has evolved in the planet’s largest land-dwelling invertebrate, Coconut crabs. A number of true crab-like features, such as a wide carapace, and a low abdomen with strong supporting legs, allow these crustaceans to wield muscular claws and manipulate their terrestrial environments with greater ease.^[22] The lack of an extended pleon greatly benefits their mobility. In this case, brachyuraform traits accommodate comfortable terrestrial locomotion and are far more pronounced in maturity, after the larval and post-larval stages which remain obligatorily aquatic.^[23] The repeated emergence of carcinised morphological structures suggests selective pressures in various Anomura niches and habitus often favor carcinization, though this may fluctuate and is sometimes reversed by the opposite process of decarcinisation.^[24]

Some crab-shaped species have evolved away from the crab form in a process called decarcinisation. Decarcinisation, or the loss of the crab-like body, has occurred multiple times in both Brachyura and Anomura.^[25][26] However, there are varying degrees of carcinisation and decarcinisation. Thus, not all species can necessarily be distinctly 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 in or lack of domicile necessitates a "semi-carcinised" label.

• List of examples of convergent evolution
• Cretaceous crab revolution
• Mesozoic marine revolution
• Orthogenesis (comparable with convergent evolution but involving teleology)