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 English zoologist Lancelot Alexander Borradaile in 1916, who stated that “Carcinisation 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.”
A carcinised morphology can be defined 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, mostly belonging to the infraorder Anomura. Carcinised crustaceans include hermit crab, porcelain crab, hairy stone crab and king crab, among others. The example of king crabs 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.