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Carcinisation (American English: carcinization) is a form of convergent evolution in which non-crab evolve a crab-like body plan. The term was introduced into evolutionary biology by Lancelot Alexander Borradaile, who described it in 1916 as "the many attempts of Nature to evolve a crab". PDF
Carcinisation has been observed most often in species of infraorder Anomura, and is characterized by a flattened and widened carapace, fused sternites, and a bent and flattened pleon. It is hypothesized to offer the selective advantages of protecting vital organs and allowing organisms to more easily escape predators on the ocean floor.
An obscure phenomenon for much of the time since its discovery, carcinisation has become more widely known since 2019, having been the subject of a popular internet meme.
Keiler et al., 2017 defines a carcinised morphology as follows:
The extinct probable crustacean order Cyclida also "strikingly resemble crabs", and probably had a similar ecology.
The evolution of king crabs (family Lithodidae) from hermit crabs has been well studied, and evidence in their biology supports this theory. For example, most hermit crabs are asymmetrical, and fit well into spiral snail seashell; the pleon 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, A. spinifrons shows similar sexual dimorphism to that seen in true crabs, where males have a shorter pleon than females.
Most carcinised organisms are descended from the infraorder Anomura, which includes . Many carcinised Anomura evolved from ancestors with morphologically intermediate forms reminiscent of modern .
The adoption of a crab-like body structure can bring several selective advantages for crustacean species. Carcinisation yields a lowered center of gravity, allowing these creatures to invest in sideways walking. 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. 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 and crayfish, which contracts abdominal flexions and sends the crustacean flying backward in the water. 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.
' 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, . 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. 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 Crustacean larva and post-larval stages which remain obligatorily aquatic. The repeated emergence of carcinised morphological structures suggests that selective pressures in various Anomura niches and habitats often lead to carcinization, though the opposite process of decarcinisation also exists.
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