Carcinisation

{{Short description|Evolution of crustaceans into crab-like forms}} {{distinguish|Carcinogenesis}} {{use British English|date=April 2026}} [[File:South eastern Pacific species of Petrolisthes, Allopetrolisthes, and Liopetrolisthes (Porcellanidae).jpg|thumb|Porcelain crabs resemble true crabs, but are more closely related to squat lobsters and hermit crabs.<ref>{{Cite journal |last=Baeza |first=J. Antonio |date=2016-03-10 |title=Molecular phylogeny of porcelain crabs (Porcellanidae: Petrolisthes and allies) from the south eastern Pacific: the genera Allopetrolisthes and Liopetrolisthes are not natural entities |journal=PeerJ |volume=4 |article-number=e1805 |doi=10.7717/peerj.1805 |pmc=4793318 |pmid=26989636 |doi-access=free }}</ref>]]

'''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".<ref>{{cite journal |first1=Patsy A. |last1=McLaughlin |first2=Rafael |last2=Lemaitre |year=1997 |title=Carcinization in the Anomura – fact or fiction? I. Evidence from adult morphology |journal=Contributions to Zoology |volume=67 |issue=2 |pages=79–123 |doi=10.1163/18759866-06702001 |doi-access=free }} [http://decapoda.nhm.org/pdfs/11727/11727.pdf PDF] {{Webarchive|url=https://web.archive.org/web/20120402131842/http://decapoda.nhm.org/pdfs/11727/11727.pdf |date=2012-04-02 }}</ref>

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:<ref name="Borradaile 1916">{{cite journal |first=L.A. |last=Borradaile |title=Crustacea. Part II. Porcellanopagurus: an instance of carcinization |url=https://www.biodiversitylibrary.org/page/31659763 |journal=British Antarctic ("Terra Nova") Expedition, 1910–1913. Natural History Report. Zoology |volume=3 |issue=3 |publisher=British Museum (Natural History) |oclc=1027015098 |date=1916 |pages=111–126 }}</ref>

{{ quote|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:<ref name=biolinnean>{{Cite journal |last1=Keiler |first1=Jonas |last2=Wirkner |first2=Christian S.|last3=Richter |first3=Stefan |date=2017-05-01 |title=One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea) |journal=Biological Journal of the Linnean Society |volume=121 |issue=1 |pages=200–222 |doi=10.1093/biolinnean/blw031 |doi-access=free}}</ref>

* "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."

[[File:Crab vs lobster body plan.svg|thumb|center|upright=2.5|Carcinised (crab) body plan, its adaptations illustrated by comparison with a lobster (undersides shown) ]]

== Taxonomic range ==

Carcinisation has occurred independently in at least five groups of decapod crustaceans:<ref name=biolinnean/>

* Infraorder Anomura: ** King crabs, which most scientists believe evolved from hermit crab ancestors,<ref>{{cite journal |author1=Jonas Keiler |author2=Stefan Richter |author3=Christian S. Wirkner |year=2013 |title=Evolutionary morphology of the hemolymph vascular system in hermit and king crabs (Crustacea: Decapoda: Anomala) |journal=Journal of Morphology |volume=274 |issue=7 |pages=759–778 |doi=10.1002/jmor.20133|pmid=23508935 |bibcode=2013JMorp.274..759K |s2cid=24458262 }}</ref><ref>{{cite journal |author1=Jonas Keiler |author2=Stefan Richter |author3=Christian S. Wirkner |year=2015 |title=The anatomy of the king crab Hapalogaster mertensii Brandt, 1850 (Anomura: Paguroidea: Hapalogastridae) – new insights into the evolutionary transformation of hermit crabs into king crabs |journal=Contributions to Zoology |volume=84 |issue=2 |pages=149–165 |doi=10.1163/18759866-08402004 |doi-access=free }}</ref> first appearing in the Late Cenozoic ** Porcelain crabs, closely related to squat lobsters,<ref>{{cite journal |author1=Jonas Keiler |author2=Stefan Richter |author3=Christian S. Wirkner |year=2014 |title=Evolutionary morphology of the organ systems in squat lobsters and porcelain crabs (Crustacea: Decapoda: Anomala): an insight into carcinization |journal=Journal of Morphology |volume=276 |issue=1 |pages=1–21 |doi=10.1002/jmor.20311|pmid=25156549 |bibcode=2015JMorp.276....1K |s2cid=26260996 }}</ref> first appearing in the Late Jurassic ** The hairy stone crab (''Lomis hirta'')<ref>{{cite journal |author1=Jonas Keiler |author2=Stefan Richter |author3=Christian S. Wirkner |year=2016 |title=Revealing their innermost secrets: an evolutionary perspective on the disparity of the organ systems in anomuran crabs (Crustacea: Decapoda: Anomura) |journal=Contributions to Zoology |volume=85 |issue=4 |pages=361–386 |doi=10.1163/18759866-08504001 |doi-access=free }}</ref> ** Hermit crabs: *** The coconut crab (''Birgus latro'') *** ''Patagurus rex''<ref>{{cite web |title=Remarkable new true crab-like hermit discovered |url=https://www.floridamuseum.ufl.edu/science/remarkable-new-true-crab-like-hermit-discovered/ |website=Florida Museum |date=13 December 2013 |publisher=University of Florida |access-date=December 9, 2020 |archive-date=October 25, 2020 |archive-url=https://web.archive.org/web/20201025134235/https://www.floridamuseum.ufl.edu/science/remarkable-new-true-crab-like-hermit-discovered/ |url-status=live }}</ref> * Infraorder Brachyura (true crabs)<ref>{{cite journal |author1=C. L. Morrison |author2=A. W. Harvey |author3=S. Lavery |author4=K. Tieu |author5=Y. Huang |author6=C. W. Cunningham |year=2001 |title=Mitochondrial gene rearrangements confirm the parallel evolution of the crab-like form |journal=Proceedings of the Royal Society B: Biological Sciences |volume=269 |issue=1489 |pages=345–350 |url=http://www.biology.duke.edu/cunningham/pdfs/Morrison%20et%20al.pdf |doi=10.1098/rspb.2001.1886 |pmid=11886621 |pmc=1690904 |access-date=2010-03-26 |archive-date=2010-06-10 |archive-url=https://web.archive.org/web/20100610111946/http://www.biology.duke.edu/cunningham/pdfs/Morrison%20et%20al.pdf |url-status=live }}</ref> First appearance: Early Jurassic

{{clade |label1=Decapoda |1={{clade |label1=Brachyura |1=('''"true" crabs''') 50 px |label2=Anomura |2={{clade |1={{clade |1=Porcellanidae ('''porcelain crabs''') 50 px |2=Munididae (squat lobsters) }} |2={{clade |1={{clade |1=Parapaguridae (deep water sea anemone hermit crabs) |2={{clade |1=Lomisidae ('''hairy stone crabs''') 50 px |2=Aeglidae }} }} |2={{clade |1=Hippidae (mole crabs or sand crabs) |label2=Paguroidea |2={{clade |1=Lithodidae ('''king crabs''') 50 px |label2=Paguridae |2={{clade |1=''Birgus latro'' ('''coconut crab''') 50px |2='''''Patagurus rex''''' 50px |3=(other hermit crabs) }} }} }} }} }} }} }}

The extinct probable crustacean order Cyclida also "strikingly resemble crabs", and probably had a similar ecology.<ref name="Schweigert">{{cite journal |author=Günter Schweigert |year=2007 |title=''Juracyclus posidoniae'' n. gen. and sp., the first cycloid arthropod from the Jurassic |journal=Journal of Paleontology |volume=81 |issue=1 |pages=213–215 |doi=10.1666/0022-3360(2007)81[213:JPNGAS]2.0.CO;2 |url=http://decapoda.nhm.org/pdfs/30426/30426.pdf |citeseerx=10.1.1.490.9065 |s2cid=131620349 |access-date=2020-08-30 |archive-date=2018-07-21 |archive-url=https://web.archive.org/web/20180721071911/https://decapoda.nhm.org/pdfs/30426/30426.pdf |url-status=live }}</ref><ref>{{Citation|title=Introduction to Brachyura|date=2015-01-01|url=https://brill.com/view/book/edcoll/9789004190832/B9789004190832_003.xml|journal=Treatise on Zoology – Anatomy, Taxonomy, Biology. The Crustacea, Volume 9 Part C (2 Vols)|pages=3–9|editor-last=Castro|editor-first=Peter|publisher=BRILL|doi=10.1163/9789004190832_003|isbn=978-90-04-19083-2|access-date=2021-11-04|editor2-last=Davie|editor2-first=Peter|editor3-last=Guinot|editor3-first=Danièle|editor4-last=Schram|editor4-first=Frederick|url-access=subscription}}</ref>

The evolution of king crabs (family Lithodidae) from hermit crabs has been well studied, and evidence in their biology supports this theory. For example, most hermit crabs are asymmetrical, and fit well into spiral snail shells; the abdomens of king crabs, even though they do not use snail shells for shelter, are also asymmetrical.<ref>{{cite journal |author1=C. W. Cunningham |author2=N. W. Blackstone |author3=L. W. Buss |s2cid=4257029 |year=1992 |title=Evolution of king crabs from hermit crab ancestors |journal=Nature |volume=355 |issue=6360 |pages=539–542 |doi=10.1038/355539a0 |pmid=1741031 |bibcode=1992Natur.355..539C}}</ref><ref>{{cite journal |author1=Patsy A. McLaughlin |author2=Rafael Lemaitre |author3=Christopher C. Tudge |year=2004 |title=Carcinization in the Anomura – fact or fiction? II. Evidence from larval, megalopal and early juvenile morphology |journal=Contributions to Zoology |volume=73 |issue=3 |pages=165–205 |doi=10.1163/18759866-07303001 |doi-access=free }}</ref><ref name="Tsang 2011">{{cite journal |last1=Tsang |first1=Ling-Ming |first2=Tin-Yam |last2=Chan |author3=Shane T. Ahyong |author4=Ka Hou Chu |year=2011 |title=Hermit to king, or hermit to all: multiple transitions to crab-like forms from hermit crab ancestors |journal=Systematic Biology |volume=60 |issue=5 |pages=616–629 |doi=10.1093/sysbio/syr063 |pmid=21835822 |doi-access= }}</ref><ref>{{cite journal |first1=Rafael |last1=Lemaitre |first2=Patsy A. |last2=McLaughlin |year=2009 |title=Recent advances and conflicts in concepts of anomuran phylogeny (Crustacea: Malacostraca) |journal=Arthropod Systematics & Phylogeny |volume=67 |issue=2 |pages=119–135 |doi=10.3897/asp.67.e31692 |doi-access=free }}</ref>

An exceptional form of carcinisation, termed "hypercarcinisation", is seen in the porcelain crab ''Allopetrolisthes spinifrons''. In addition to the shortened body form, ''A. spinifrons'' shows similar sexual dimorphism to that seen in true crabs, where males have a shorter pleon than females.<ref name="Hiller">{{cite journal |first1=Alexandra |last1=Hiller |first2=Carlos Antonio |last2=Viviana |first3=Bernd |last3=Werding |year=2010 |title=Hypercarcinisation: an evolutionary novelty in the commensal porcellanid ''Allopetrolisthes spinifrons'' (Crustacea: Decapoda: Porcellanidae) |url=http://www.crustacea.org.br/artigos/728_25_18(1)_95-102_-_Hiller,_Viviani,_Werding.pdf |journal=Nauplius |volume=18 |issue=1 |pages=95–102 |archive-url=https://web.archive.org/web/20120425232740/http://www.crustacea.org.br/artigos/728_25_18%281%29_95-102_-_Hiller%2C_Viviani%2C_Werding.pdf|archive-date=2012-04-25}}</ref>

== Selective pressures and benefits ==

Independently arising from multiple ancestral crustacean taxa, the crab-like traits exhibited vary between individual species and taxa. However, all crabs and carcinised organisms are decapods. Correlations between the folding of the pleon tail and widening of the cephalothorax across disparate decapod species suggest similar evolutionary pressures. Some occurrences of carcinisation are derived from convergent but distinct developmental pathways, while others may be instances of homologous parallelism from shared ancestral body plans.<ref name="Tsang 2011" />

Most carcinised organisms are descended from the infraorder Anomura, which includes hermit crabs.<ref>{{Cite journal |last1=Tsang |first1=Ling Ming |last2=Chan |first2=Tin-Yam |last3=Ahyong |first3=Shane T. |last4=Chu |first4=Ka Hou |date=2011-08-10 |title=Hermit to King, or Hermit to All: Multiple Transitions to Crab-like Forms from Hermit Crab Ancestors |journal=Systematic Biology |volume=60 |issue=5 |pages=616–629 |doi=10.1093/sysbio/syr063 |pmid=21835822}}</ref> Many carcinised Anomura evolved from ancestors with morphologically intermediate forms reminiscent of modern squat lobsters.<ref name="Keiler 2015">{{Cite journal |last1=Keiler |first1=Jonas |last2=Richter |first2=Stefan |last3=Wirkner |first3=Christian S. |date=January 2015 |title=Evolutionary morphology of the organ systems in squat lobsters and porcelain crabs (Crustacea: Decapoda: Anomala): An insight into carcinization |url=https://onlinelibrary.wiley.com/doi/10.1002/jmor.20311 |journal=Journal of Morphology |volume=276 |issue=1 |pages=1–21 |doi=10.1002/jmor.20311 |pmid=25156549 |bibcode=2015JMorp.276....1K |url-access=subscription }}</ref>

The adoption of a crab-like body structure can bring several selective advantages for crustacean species. Carcinisation yields a lowered center of gravity, allowing these creatures to invest in sideways walking.<ref name="onlinelibrary.wiley.com">{{Cite journal |last1=Wolfe |first1=Joanna M. |last2=Luque |first2=Javier |last3=Bracken-Grissom |first3=Heather D. |date=May 2021 |title=How to become a crab: Phenotypic constraints on a recurring body plan |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.202100020 |journal=BioEssays |volume=43 |issue=5 |article-number=e2100020 |doi=10.1002/bies.202100020 |pmid=33751651 |url-access=subscription }}</ref> This evasive adaptation is particularly useful in an ocean environment with forward-moving predators. The pleon is held tightly under the animal's cephalothorax with reduced musculature, which protects the pleon's organs from attack. The smaller and more balanced frame facilitates concealment within rocks and coral.<ref name="onlinelibrary.wiley.com"/><ref name="ABC News 2022"/><ref name="Wolfe 2021"/>

alt=Animated crustacean moving backwards by activating the caridoid escape reaction|thumb|Animated depiction of the caridoid escape reaction in profile

The caridoid escape reaction is an innate danger response in crustaceans such as lobsters and crayfish, which contracts abdominal flexions and sends the crustacean flying backward in the water.<ref>{{Cite journal |url=https://journals.biologists.com/jeb/article/201/11/1771/7750/Tail-Flip-Mechanism-and-Size-Dependent-Kinematics |access-date=2024-05-10 |journal=Journal of Experimental Biology |doi=10.1242/jeb.201.11.1771 |title=Tail-Flip Mechanism and Size-Dependent Kinematics of Escape Swimming in the Brown Shrimp ''Crangon Crangon'' |date=1998 |last1=Arnott |first1=Stephen A. |last2=Neil |first2=Douglas M. |last3=Ansell |first3=Alan D. |volume=201 |issue=11 |pages=1771–1784 |pmid=9576888 |bibcode=1998JExpB.201.1771A |url-access=subscription }}</ref> Brachyura and species which have undergone carcinization have strongly bent and immobile tails, which prevent them from using this evasion strategy. The necessary muscles are no longer developed enough in these species to facilitate the necessary tail flipping. Crabs and false crabs are best suited to escape by ground pursuit.<ref name="onlinelibrary.wiley.com"/><ref name="ABC News 2022"/><ref name="Wolfe 2021"/>

Porcelain crabs' closest relatives are squat lobsters, taxa which occupy a morphological middle ground, described by Keiler et. al. as "half-carcinized" due to their partially flexed pleons and carapaces that remain longer than they are wide. Many species do not become fully carcinised but only undergo the crab-like adaptations that are contextually beneficial, to varying degrees.<ref name="Keiler 2015"/>

While most incidences of carcinization are in aquatic Anomura populations, it has evolved in the planet's largest land-dwelling invertebrate, coconut crabs. 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.<ref>{{Cite journal |last=Greenaway |first=P. |date=2003 |title=Terrestrial adaptations in the Anomura (Crustacea: Decapoda) |url=https://museumsvictoria.com.au/collections-research/journals/memoirs-of-museum-victoria/volume-60-issue-1-2003/pages-13-26/ |journal=Memoirs of Museum Victoria |volume=60 |issue=1 |pages=13–26 |doi=10.24199/j.mmv.2003.60.3}}</ref> The lack of an extended pleon greatly benefits their mobility. 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.<ref>{{Cite journal |last1=Wang |first1=Fang-Lin |last2=Hsieh |first2=Hwey-Lian |last3=Chen |first3=Chang-Po |date=2007 |title=Larval Growth of the Coconut Crab Birgus Latro with a Discussion on the Development Mode of Terrestrial Hermit Crabs |journal=Journal of Crustacean Biology |volume=27 |issue=4 |pages=616–625 |doi=10.1651/s-2797.1 |bibcode=2007JCBio..27..616H}}</ref> The repeated emergence of carcinised morphological structures suggests that selective pressures in various Anomura niches and habitats often lead to carcinization, though the opposite process of decarcinisation also exists.<ref>{{Cite journal |last=Scholtz |first=Gerhard |date=2014-03-26 |title=Evolution of crabs – history and deconstruction of a prime example of convergence |url=https://brill.com/view/journals/ctoz/83/2/article-p87_1.xml |journal=Contributions to Zoology |volume=83 |issue=2 |pages=87–105 |doi=10.1163/18759866-08302001 |doi-access=free }}</ref>

== Decarcinisation ==

Some crab-shaped species have evolved away from the crab form in a process called ''decarcinisation''. Decarcinisation, or the loss of the crab-like body, has occurred multiple times in both Brachyura and Anomura. However, there are varying degrees of carcinisation and decarcinisation. Thus, not all species can necessarily be distinctly classified as "carcinised" or "decarcinised". For example, hermit crabs have lost or reduced their outer casings or "domiciles". While they retain their crab-like phenotype, their reduction in or lack of domicile necessitates a "semi-carcinised" label.<ref name="ABC News 2022">{{cite news |date=2022-09-17 |df=dmy-all |title=Hermit crabs aren't real crabs |quote=There are hundreds of other crustacean pretenders living right under our noses. |series=ABC News |publisher=Australian Broadcasting Corporation |url=https://www.abc.net.au/news/science/2022-09-18/crab-convergent-evolution-crustacean-australia-marine-biology/101388282 |access-date=2023-09-15}}</ref><ref name="Wolfe 2021">{{cite journal |last1=Wolfe |first1=Joanna M |last2=Luque |first2=Javier |last3=Bracken-Grissom |first3=Heather D. |date=9 March 2021 |title=How to become a crab: Phenotypic constraints on a recurring body plan |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.202100020 |journal=BioEssays |volume=43 |issue=5 |article-number=e2100020 |doi=10.1002/bies.202100020 |pmid=33751651 |s2cid=232325601 |url-access=subscription }}</ref>

== Cultural impact ==

Beginning in 2019, carcinisation has found popularity as an internet meme. This parodies carcinisation, purporting that crabs possess the "ideal body plan" and conceptualizing the evolution of other animal groups, especially vertebrates, of eventually developing crab-like bodies (often being examples of speculative evolution). There are concerns that this may promote misunderstandings of biology and evolution.<ref>{{Cite web |last=Wolfe |first=Joanna |date=2025-02-27 |title=Crab Memes Amplify Mistaken Ideas about Evolution |url=https://www.scientificamerican.com/article/crab-memes-amplify-mistaken-ideas-about-evolution/ |url-status=live |archive-url=https://web.archive.org/web/20250227231154/https://www.scientificamerican.com/article/crab-memes-amplify-mistaken-ideas-about-evolution/ |archive-date=2025-02-27 |access-date=2025-03-22 |website=Scientific American}}</ref><ref>{{Cite web |last= |first= |last2= |first2= |date=2021 |title=Carcinization |url=https://knowyourmeme.com/memes/carcinization |url-status=live |archive-url=https://web.archive.org/web/20250315132858/https://knowyourmeme.com/memes/carcinization |archive-date=2025-03-15 |access-date=2025-03-22 |website=Know Your Meme}}</ref> The evolutionary palaeobiologist Matthew Wills comments that while multiple groups have converged on a crablike body plan, and despite the meme, humans are not about to evolve the same way. This is because all the crabs are decapods, and the evolutionary pressures apply in a marine environment where defence, living in crevices, and being wave-swept favour armoured protection, a broad compact body, and the ability to scuttle sideways.<ref name="Swallow 2025">{{cite news |last1=Swallow |first1=Bea |title=Why does evolution keep creating 'imposter crabs'? |url=https://www.bbc.co.uk/news/articles/cly7d7erz80o |access-date=17 October 2025 |work=BBC News |date=17 October 2025}}</ref>

== See also == {{Portal|Crustaceans|Evolutionary biology}}

* {{anl|Cretaceous crab revolution}} * {{anl|List of examples of convergent evolution}} * {{anl|Mesozoic marine revolution}} * {{anl|Orthogenesis}}

== References ==

{{refs}}

Category:Crustaceans Category:Convergent evolution Category:Crabs