Carcinisation
evolution of non-crab-like crustaceans into crab-like forms
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 Lancelot Alexander Borradaile, who described it in 1916 as "the many attempts of Nature to evolve a crab".
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.
Definition
The term was introduced by Lancelot Alexander Borradaile in 1916, with the words:
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:
"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."
Taxonomic range
Carcinisation has occurred independently in at least five groups of extant decapod crustaceans:
Infraorder Anomura:
King crabs, which most scientists believe evolved from hermit crab ancestors, first appearing in the Late Cenozoic
Porcelain crabs, closely related to squat lobsters, first appearing in the Late Jurassic
The hairy stone crab (Lomis hirta)
Hermit crabs:
The coconut crab (Birgus latro)
Patagurus rex
Infraorder Brachyura (true crabs) First appearance: Early Jurassic
The extinct probable crustacean order Cyclida greatly resemble crabs, and probably had a similar ecology.
The pygocephalomorph Iraticaris damianii is significantly carcinised, with a pleon only ⅐ the length of the cephalothorax that is folded ventrally, covering the posterior margin of the final (eighth) thoracic sternite, as well as a roughly oval flattened carapace that is only slightly longer than it is wide. A trend within pygocephalans is reported where later species tend to have a more bent pleon; it is extended or partially bent in Carboniferous taxa, while most Permian species have it fully curved. Iraticaris is the most developed known species in this sense.
The evolution of king crabs (family Lithodidae) from hermit crabs has been well studied, and evidence in their biology supports this theory.
The public source identifies “Carcinisation” as evolution of non-crab-like crustaceans into crab-like forms. This brief keeps that definition visible, then builds a research path around Carcinisation, evolution and non-crab-like.
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This entry incorporates text from “Carcinisation” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.