Carcinisation

It was stated by L. A. Borradaile in 1916 that:{{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 |oclc=1027015098 |date=1916 |pages=111–126 }}

{{Blockquote|text=… 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:{{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|language=en|volume=121|issue=1|pages=200–222|doi=10.1093/biolinnean/blw031|issn=0024-4066|doi-access=free}}

• "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."

An important and visually evident marker of difference between true crabs and carcinised Anomura is the number of leg pairs. While Brachyura (true) crabs have four pairs of legs used for locomotion, Anomura possess one much smaller set and therefore three sets of walking legs.

Carcinisation is believed to have occurred independently in at least five groups of decapod crustaceans:

• Order Decapoda:
• Infraorder Anomura:
• King crabs, which most scientists believe evolved from hermit crab ancestors,{{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 |s2cid=24458262 }}{{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 }} first appearing in the Late Cenozoic
• Porcelain crabs, which are closely related to squat lobsters,{{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 |s2cid=26260996 }} first appearing in the Late Jurassic
• The hairy stone crab (Lomis hirta){{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 }}
• Hermit crabs:
• The coconut crab (Birgus latro)
• Patagurus rex{{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 }}
• Infraorder Brachyura (true crabs){{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 }} First appearance: Early Jurassic

The extinct probable crustacean order Cyclida are also noted to "strikingly resemble crabs", and probably had a similar ecology.{{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 }}{{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}}

File:The Childrens Museum of Indianapolis - Alaskan red king crab.jpg]]

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, 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.{{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}}{{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 }}{{cite journal |author1=Ling Ming Tsang |author2=Tin-Yam 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= }}{{cite journal |author1=Rafael Lemaitre |author2=Patsy A. 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 }}

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.{{cite journal|author1=Alexandra Hiller|author2=Carlos Antonio Viviana|author3=Bernd 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|url-status=dead|journal=Nauplius|volume=18|issue=1|pages=95–102|archiveurl=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|archivedate=2012-04-25}}

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.

Most carcinised organisms are descended from the infraorder Anomura, which includes hermit crabs.{{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 |url=https://doi.org/10.1093/sysbio/syr063 |journal=Systematic Biology |volume=60 |issue=5 |pages=616–629 |doi=10.1093/sysbio/syr063 |pmid=21835822 |issn=1076-836X|url-access=subscription }} Many carcinised Anomura evolved from ancestors with morphologically intermediate forms reminiscent of modern squat lobsters,{{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 |language=en |volume=276 |issue=1 |pages=1–21 |doi=10.1002/jmor.20311 |pmid=25156549 |issn=0362-2525|url-access=subscription }} not including the King Crab which is hypothesized by researchers to be descended directly from a variety of Pagurid hermit crab. There may be various advantages to adopting brachyuraform (true crab-like) traits.

The adoption of a crab-like body structure can convey a number of selective advantages for crustacean species. Carcinisation is associated with a lowered center of gravity, allowing these creatures to invest in sideways walking.{{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 |language=en |volume=43 |issue=5 |pages=e2100020 |doi=10.1002/bies.202100020 |pmid=33751651 |issn=0265-9247}} 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. The folding of the pleon below the carapace reduces the crustacean's exposed surface area, and associated hardening of the pleonal cuticle are all thought to benefit the fitness of this body type.

File:Caridoid escape reaction.gif

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.{{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 |url-access=subscription }} 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 in comparison to the quick aquatic escape provided by the caridoid escape reaction.

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.

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.{{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 |language=English |volume=60 |issue=1 |pages=13–26 |doi=10.24199/j.mmv.2003.60.3}} 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.{{Cite journal |last1=Wang |first1=Fang-Lin |last2=Hsieh |first2=Hwey-Lian |last3=Chen |first3=Chang-Po |date=2007-01-01 |title=Larval Growth of the Coconut Crab Birgus Latro with a Discussion on the Development Mode of Terrestrial Hermit Crabs |url=https://doi.org/10.1651/S-2797.1 |journal=Journal of Crustacean Biology |volume=27 |issue=4 |pages=616–625 |doi=10.1651/s-2797.1 |issn=0278-0372}} The repeated emergence of carcinised morphological structures suggests selective pressures in various Anomura niches and habitus often favor carcinization, though this may fluctuate and is sometimes reversed by the opposite process of decarcinisation.{{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 |issn=1875-9866}}

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.{{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. |lang=en-AU |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}}{{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 |pages=e2100020 |doi=10.1002/bies.202100020 |pmid=33751651 |s2cid=232325601 |access-date=8 November 2022}} However, there are varying degrees of carcinisation and decarcinisation. Thus, not all species can necessarily be distinctly classified as "carcinised" or "decarcinised". Some examples include the coconut crab, as well as other hermit crabs, that have lost or reduced their outer casing, often referred to as "domiciles". While they retain their crab-like phenotype, their reduction in or lack of domicile necessitates a "semi-carcinised" label.

Beginning in 2019, carcinisation has found popularity as an internet meme. These memes parody 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 these memes may promote misunderstandings of biology and evolution.{{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}}{{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}}

{{Portal|Crustaceans|Evolutionary biology}}

• List of examples of convergent evolution
• Cretaceous crab revolution
• Mesozoic marine revolution
• Orthogenesis (comparable with convergent evolution but involving teleology)

{{reflist|25em}}

Category:Crustaceans

Category:Convergent evolution