resemble crabs, but are more closely related to squat lobsters and hermit crabs. Carcinisation (or carcinization) is an example of convergent evolution in which a crustacean evolves into a crab-like form from a non-crab-like form. The term was introduced into evolutionary biology by L. A. Borradaile, who described it as "one of the many attempts of Nature to evolve a crab". PDF Most carcinised crustaceans belong to the order Anomura.
Definition of carcinised morphology
It was stated by Borradaile in 1916 that:Borradaile LA 1916 Crustacea. Part II. Porcellanopagurus: An instance of carcinization Natural History Report, Zoology 3: 111-126. Klier 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,"
Carcinisation is believed to have occurred independently in at least five groups of decapod crustaceans:
King crabs, which most scientists believe evolved from hermit crab ancestors First apperance: Late Cenozoic
Porcelain crabs, which are closely related to squat lobsters First appearance: Late Jurassic
The hairy stone crab (Lomis hirta)
The coconut crab (Birgus latro)
True crabs (Brachyura) First appearance: Early Jurassic
A prehistoric example is the extinct crustacean order Cyclida, which evolved their crab-like morphology before the existence of true crabs.
in the collection of the Children's Museum of Indianapolis. 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, so that they fit well into spiral snail shells; the abdomens 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 also shows similar sexual dimorphism to that seen in true crabs, where males have a shorter pleon than females.