Carcinisation (or carcinization) is an example of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

in which a

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

evolves into a

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all th ...

-like form from a non-crab-like form. The term was introduced into

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...

by L. A. Borradaile, who described it as "one of the many attempts of Nature to evolve a crab". Most carcinised crustaceans belong to the infraorder Anomura.

Definition of carcinised morphology

It was stated by Lancelot Alexander Borradaile in 1916 that: Keiler et al., 2017 defines a carcinised morphology as follows: * "The

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

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"

Examples

Carcinisation is believed to have occurred independently in at least five groups of decapod crustaceans: * Order Decapoda: **Infraorder Anomura: ***

King crabs King is the title given to a male monarch in a variety of contexts. The female equivalent is queen, which title is also given to the consort of a king. *In the context of prehistory, antiquity and contemporary indigenous peoples, the ti ...

, which most scientists believe evolved from hermit crab ancestors First appearance: Late

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

*** Porcelain crabs, which are closely related to

squat lobsters Squat lobsters are dorsoventrally flattened crustaceans with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the decapod infraorder Anomura, alongsid ...

First appearance: Late

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

*** The hairy stone crab (''Lomis hirta'') ***

Hermit crabs Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an as ...

: **** The

coconut crab The coconut crab (''Birgus latro'') is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest terrestrial arthropod in the world, with a weight of up to . It can grow to up to in width from the tip ...

(''Birgus latro'') **** ''Patagurus rex'' ** Infraorder Brachyura (true crabs) First appearance: Early Jurassic The extinct probable crustacean order Cyclida are also noted to "strikingly resemble crabs", and probably had a similar ecology.

King crabs

The Childrens Museum of Indianapolis - Alaskan red king crab

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 Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...

, so that they fit well into spiral

snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class ...

shells; the

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

s of king crabs, even though they do not use snail shells for shelter, are also asymmetrical.

Hypercarcinisation

An exceptional form of carcinisation, termed "hypercarcinisation", is seen in the porcelain crab ''

Allopetrolisthes spinifrons ''Allopetrolisthes spinifrons'' is a species of porcelain crab. It displays "hypercarcinisation", whereby the resemblance to a true crab is enhanced by sexual dimorphism of the abdomen. It lives along the Pacific coast of Peru and Chile, as a sym ...

''. In addition to the shortened body form, ''A. spinifrons'' also shows similar

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

to that seen in true crabs, where males have a shorter pleon than females.

Decarcinization

Some crab shaped species have evolved away from the crab form in a process known as decarcinisation.

References