Carcinisation

Carcinisation (American English: carcinization) is a form of convergent evolution in which non-crab crustaceans evolve a crab-like body plan.

[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 crabKeiler et al., 2017 defines a carcinised morphology as follows:[4] An important and visually evident marker of difference between true crabs and carcinised Anomura is the number of leg pairs.

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.

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.

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.

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.

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.