Crab

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Cancer pagurus, the edible or brown crab (Brachyura) Cancer pagurus.jpg
Cancer pagurus , the edible or brown crab (Brachyura)

Crabs are decapod crustaceans, either the Brachyura (the "true crabs") or various groups within the closely related Anomura (sometimes collectively called "false crabs"), characterised by having a heavily armoured shell, their tail segments concealed under the body, the ability to run sideways, and the habit of hiding in rocky crevices. They do not form a single natural group or clade, but have convergently evolved multiple times from the ancestral decapod body plan through the process of carcinisation. As a group they are thus polyphyletic.

Contents

Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 4 m (13 ft). Many crabs are free-living marine omnivores; others are specialist herbivores or carnivores, while some are parasitic. A substantial number of species are adapted to freshwater or other non-marine habitats.

Crabs make up about 20% of the marine crustaceans that are caught or farmed for human consumption. In British cuisine, dressed crab is a traditional seafood meal, while in Goa and Mozambique, crab curry is a typical dish. Crabs feature in Greek and Malay mythology, and as the astrological sign Cancer.

Body plan

Most crabs are members of the Brachyura, sometimes called "true crabs", with around 7,000 species. [1] Several other groups of decapod crustaceans among the Anomura, such as king crabs and porcelain crabs, have a similar appearance; all have convergently evolved through the process of carcinisation to the crab body form and way of life. [2] [3] [4] [5] Many crabs can run swiftly sideways ("crabwise"), though others walk forwards, [6] and some can swim. [7]

The carcinised body form is defined by Keiler and colleagues (2014) as having the following attributes: [8]

Crab body plan, its adaptations illustrated by comparison with a lobster (undersides shown) Crab vs lobster body plan.svg
Crab body plan, its adaptations illustrated by comparison with a lobster (undersides shown)

Diversity

Taxonomic range

Alongside the Brachyura or true crabs, the Anomura contains multiple groups that are called crabs, including the hermit crabs, mole crabs, king crabs, and porcelain crabs. [9] A distantly-related group of arthropods, the horseshoe crabs, with an armoured carapace but a quite different body plan, is a member of the Chelicerata, the group that includes the spiders and scorpions. [10] The crab body form and associated behaviour has arisen independently at different times in multiple groups of decapod crustaceans (boldface in tree). [8]

Arthropoda
Chelicerata

Xiphosura (horseshoe crabs) Horseshoe crab (white background).jpg

Middle Cambrian
Mandibulata
Decapoda
Brachyura

("true" crabs) [11] Liocarcinus vernalis.jpg

Early Jurassic
Anomura
Porcellanidae

(porcelain crabs) [8] PeerJ - Allopetrolisthes punctatus.jpg

Late Jurassic

Munididae (squat lobsters)

Parapaguridae (deep water sea anemone hermit crabs)

Lomisidae (hairy stone crabs) [12] Lomis hirta dorsal.jpg

Aeglidae

Hippidae (mole crabs or sand crabs)

Paguroidea
Lithodidae

(king crabs) [13] [14] Lithodes confundens male dorsal plate black background.jpg

Late  Cenozoic
Paguridae

Birgus latro (coconut crab) Birgus latro.jpg

Patagurus rex [15] Patagurus.jpg

(other hermit crabs) Black Eyed Hermit Crab 20210114 111707 2-6 (cropped).jpg

Late Cretaceous
Late Triassic  

Size and shape

Crabs vary in size from the pea crab, a few millimeters wide, [16] to the Japanese spider crab, with a leg span up to 4 m (13 ft). [17] The coconut crab is the largest terrestrial arthropod, and indeed the largest extant terrestrial invertebrate, at up to 40 cm (16 in) long and weighing up to 4.1 kg (9 lb). [18]

Ecological niche

Many crabs are free-living marine omnivores, feeding on a mixture of algae, small animals such as molluscs, polychaete worms, other crustaceans, and detritus. [19] Others are more specialised: the mottled crab Grapsus albolineatus , for example, is herbivorous, feeding mainly on algae and preferring the more nutritious filamentous algae to leafy (foliose) algae, [20] while the yellow moon crab Ashtoret lunaris is carnivorous. [21] The porcelain crabs are plankton feeders, filtering their prey from seawater using long feathery bristles on their mouthparts. [22] The tiny soft-bodied oyster crab is a kleptoparasite of oysters, living inside the host's shell and eating its food. [23]

The semi-terrestrial tufted ghost crab Ocypode cursor consumes terrestrial animals such as insects. [19] Other species, including the pea crabs (Pinnotheridae), are parasitic, living inside hosts such as bivalve molluscs. [16] The tree crab or Caribbean hermit crab is terrestrial as an adult, [24] only returning to the ocean to spawn. [25] It feeds on plants and by scavenging, [26] and like other hermit crabs, takes over a mollusc shell for protection, breathing air with a lung. [24] Some 1,300 species of crabs in 8 families are adapted to freshwater. [27] Christmas Island red crabs make an annual mass migration to the sea to lay their eggs. [28]

Interactions with humans

Fisheries and food

Crabs make up some 20% of all marine crustaceans caught, farmed, and consumed worldwide, amounting to 1.5 million tonnes annually. One species, the Asian blue crab Portunus trituberculatus , accounts for one-fifth of that total. Other commercially important taxa include Portunus pelagicus , several species in the genus Chionoecetes , the blue crab ( Callinectes sapidus ), Charybdis spp., Cancer pagurus , the Dungeness crab (Metacarcinus magister), and Scylla serrata , each of which yields more than 20,000 tonnes annually. [29]

In Western Europe, much of the crab meat is from the brown crab Cancer pagurus , noted for its sweet, delicate flavour. The United Kingdom hosts significant fisheries of this species, with major operations in Scotland and the South West of England. [30] Dressed crab is a traditional seafood meal in British cuisine made of the meat of the brown crab served in its own shell. [31]

In North America, there are commercial fisheries for the blue crab Callinectes sapidus along the Atlantic coast of the United States, and in the Gulf of Mexico. The fishery was centered on the Chesapeake Bay, but other places are increasing in importance. [32] Crab cakes are traditionally made from Chesapeake Bay crabs. [33] [34]

In Goa and Mozambique, crab curry is a typical dish, flavoured with chilis, garlic, coconut, and spices. [35]

In Ishikawa prefecture, Japan, both the meat and the eggs of the snow crab are served as sushi in wintertime. The male is known as kano-gani, the female as kobako-gani. The short fishing season for the females makes kobako-gani sushi a rare delicacy. [36]

In culture

Both the constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as such. [37] The Moche people of ancient Peru worshipped nature, especially the sea, [38] and often depicted crabs in their art. [39] In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra as it battled the hero Heracles. One of Rudyard Kipling's Just So Stories , "The Crab that Played with the Sea", tells the story of a gigantic crab who made the waters of the sea go up and down, like the tides. [40] The paleontologist Richard Fortey has identified Kipling's giant crab as a horseshoe crab. [41] In Malay mythology, ocean tides were believed to be caused by water rushing in and out of a hole in the Navel of the Seas (Pusat Tasek), where "there sits a gigantic crab which twice a day gets out in order to search for food". [42]

As pets

Hermit crabs are commonly kept as pets and used in the marine aquarium trade. [9] A popular species is the Caribbean hermit crab, Coenobita clypeatus . They can live for 30 years in captivity, if their requirements, simulating a coastal rainforest, are met. The size of tank must be substantial. There must be a substrate of sand and coconut fibre that they can dig in to facilitate moulting. The temperature and humidity of the air must be controlled. A pool of fresh water and a pool of correctly formulated salt water are both necessary. [43]

Hermit crabs in an aquarium Pet hermit crabs (cropped).jpg
Hermit crabs in an aquarium

Meme

The zoologist Joanna Wolfe, writing in Scientific American , notes a popular meme which jokes that crabs are the "ultimate forms" [44] of life as "everything will eventually evolve into a crab". [44] She explains that the meme parodies the genuine process of carcinisation which has taken place in at least five different groups of decapods, but that the process does not apply to humans or other animals. [44] The evolutionary palaeobiologist Matthew Wills comments that 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. [45]

Decapods have repeatedly evolved a crablike body form under the pressures of predation and wave action. A broad low body, an armoured carapace, and the ability to scuttle sideways into a rock crevice all work well in a marine environment. This does not mean that all animals will evolve the same adaptations. Purple rock crabs (Leptograpsus variegatus) lurking in a crevice under Lion Rock.jpg
Decapods have repeatedly evolved a crablike body form under the pressures of predation and wave action. A broad low body, an armoured carapace, and the ability to scuttle sideways into a rock crevice all work well in a marine environment. This does not mean that all animals will evolve the same adaptations.

References

  1. ↑ Joel W. Martin; George E. Davis (2001). An Updated Classification of the Recent Crustacea (PDF). Natural History Museum of Los Angeles County. p. 132. Archived from the original (PDF) on 12 May 2013. Retrieved 14 December 2009.
  2. ↑ Borradaile, Lancelot Alexander (1916). "Crustacea. Part II. Porcellanopagurus: an instance of carcinization". British Antarctic ("Terra Nova") Expedition, 1910. Natural History Report. Zoology. 3 (3): 111–126.
  3. ↑ Martin, J. W.; Abele, L. G. (1986). "Phylogenetic relationships of the genus Aegla (Decapoda: Anomura: Aeglidae), with comments on anomuran phylogeny". Journal of Crustacean Biology. 6 (3): 576–612. Bibcode:1986JCBio...6..576.. doi:10.1163/193724086X00406.
  4. ↑ McLaughlin, P. A.; Lemaitre, R. (1997). "Carcinization in the Anomura - fact or fiction? I. Evidence from adult morphology". Contributions to Zoology. 67 (2): 79–123. doi: 10.1163/18759866-06702001 .
  5. ↑ Scholtz, G. (2014). "Evolution of crabs - history and deconstruction of a prime example of convergence". Contributions to Zoology. 83 (2): 87–105. doi: 10.1163/18759866-08302001 .
  6. ↑ Sleinis, Sally; Silvey, Gerald E. (1980). "Locomotion in a forward walking crab". Journal of Comparative Physiology A. 136 (4): 301–312. doi:10.1007/BF00657350. S2CID   33455459.
  7. ↑ Hazerli, Dennis; Richter, Stefan (2020). "Why "swimming crabs" are able to swim – The importance of the axial skeleton: A comparison between the "swimming crab" Liocarcinus depurator and two other brachyuran crabs (Cancer pagurus, Carcinus maenas) using μCT and 3D-reconstruction". Arthropod Structure & Development. 59 100972. Bibcode:2020ArtSD..5900972H. doi:10.1016/j.asd.2020.100972.
  8. 1 2 3 Jonas Keiler; Stefan Richter; Christian S. Wirkner (2014). "Evolutionary morphology of the organ systems in squat lobsters and porcelain crabs (Crustacea: Decapoda: Anomala): an insight into carcinization". Journal of Morphology . 276 (1): 1–21. Bibcode:2015JMorp.276....1K. doi:10.1002/jmor.20311. PMID   25156549. S2CID   26260996.
  9. 1 2 Bracken-Grissom, Heather D.; Cannon, Maren E.; Cabezas, Patricia; Feldmann, Rodney M.; Schweitzer, Carrie E; et al. (2013). "A comprehensive and integrative reconstruction of evolutionary history for Anomura (Crustacea: Decapoda)". BMC Evolutionary Biology. 13 (1): 128. doi: 10.1186/1471-2148-13-128 . PMC   3708748 . PMID   23786343.
  10. ↑ Garwood, R. J.; Dunlop, J. (13 November 2014). "Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders". PeerJ. 2 e641. doi: 10.7717/peerj.641 . PMC   4232842 . PMID   25405073.
  11. ↑ C. L. Morrison; A. W. Harvey; S. Lavery; K. Tieu; Y. Huang; C. W. Cunningham (2001). "Mitochondrial gene rearrangements confirm the parallel evolution of the crab-like form" (PDF). Proceedings of the Royal Society B: Biological Sciences . 269 (1489): 345–350. doi:10.1098/rspb.2001.1886. PMC   1690904 . PMID   11886621. Archived (PDF) from the original on 10 June 2010. Retrieved 26 March 2010.
  12. ↑ Jonas Keiler; Stefan Richter; Christian S. Wirkner (2016). "Revealing their innermost secrets: an evolutionary perspective on the disparity of the organ systems in anomuran crabs (Crustacea: Decapoda: Anomura)". Contributions to Zoology . 85 (4): 361–386. doi: 10.1163/18759866-08504001 .
  13. ↑ Jonas Keiler; Stefan Richter; Christian S. Wirkner (2013). "Evolutionary morphology of the hemolymph vascular system in hermit and king crabs (Crustacea: Decapoda: Anomala)". Journal of Morphology . 274 (7): 759–778. Bibcode:2013JMorp.274..759K. doi:10.1002/jmor.20133. PMID   23508935. S2CID   24458262.
  14. ↑ Jonas Keiler; Stefan Richter; Christian S. Wirkner (2015). "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". Contributions to Zoology . 84 (2): 149–165. doi: 10.1163/18759866-08402004 .
  15. ↑ "Remarkable new true crab-like hermit discovered". Florida Museum. University of Florida. 13 December 2013. Archived from the original on 25 October 2020. Retrieved 9 December 2020.
  16. 1 2 Baker, Nick (1 February 2023). "Pea crabs guide: what they are and their lifecycle". Discover Wildlife. Retrieved 4 November 2025.
  17. ↑ "Japanese spider crab Macrocheira kaempferi". Oceana North America. Archived from the original on 14 November 2009. Retrieved 2 January 2009.
  18. ↑ Schueman, Lindsey Jean (6 October 2025). "Inside the world of coconut crabs: The largest land arthropod". One Earth. Retrieved 5 November 2025.
  19. 1 2 Chartosia, Niki (2010). "Diet Composition of Five Species of Crabs (Decapoda, Brachyura) that Show a Gradual Transition from Marine to Terrestrial Life". Crustaceana. 83 (10): 1181–1197. Bibcode:2010Crust..83.1181C. doi:10.1163/001121610X533502. JSTOR   41038629.
  20. ↑ Kennish, R; Williams, G. A. (1997). "Feeding preferences of the herbivorous crab Grapsus albolineatus: the differential influence of algal nutrient content and morphology" . Marine Ecology Progress Series. 147: 87–95. Bibcode:1997MEPS..147...87K. doi: 10.3354/meps147087 .
  21. ↑ Yamada, Hideaki; Kobayashi, Masato; Sato, Taku; Kawabata, Yuuki (2013). "Effects of artificial seaweed and water depth on vulnerability of juvenile black-spot tuskfish Choerodon schoenleinii to carnivorous crab Ashtoret lunaris". Nippon Suisan Gakkaishi. 79 (5): 797–803. doi: 10.2331/suisan.79.797 . ISSN   0021-5392 . Retrieved 4 November 2025.
  22. ↑ Allen, Gerald R. (1997). "Anemone crab Neopetrolisthes maculatus". Tropical Marine Life. Periplus Nature Guides. Tuttle Publishing. p. 35. ISBN   978-962-593-157-9.
  23. ↑ Walters, Linda J.; Busch, Sidney J.; Vermeulen, Sophia; Craig, Casey A. (2024). "Entanglement and ingestion of microfibers by the oyster pea crab Zaops ostreum, an endosymbiont of the eastern oyster Crassostrea virginica" . Marine Pollution Bulletin. 201 116251. doi:10.1016/j.marpolbul.2024.116251.
  24. 1 2 Farrelly, C.A.; Greenaway, P. (January 2005). "The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (Coenobita and Birgus): gills, branchiostegal lungs and abdominal lungs". Arthropod Structure & Development. 34 (1): 63–87. Bibcode:2005ArtSD..34...63F. doi:10.1016/j.asd.2004.11.002.
  25. ↑ Nieves-Rivera, Ángel M.; Ernest H. Williams, Jr. (2003). "Annual migrations and spawning of Coenobita clypeatus (Herbst) on Mona Island (Puerto Rico) and notes on inland crustaceans". Crustaceana . 76 (5): 547–558. Bibcode:2003Crust..76..547W. doi:10.1163/156854003322316191. JSTOR   20105594. S2CID   53587978.
  26. ↑ Linton, Stuart; Greenaway, Peter (6 February 2007). "A review of feeding and nutrition of herbivorous land crabs: adaptations to low quality plant diets". Journal of Comparative Physiology B. 177 (3): 269–286. doi:10.1007/s00360-006-0138-z. PMID   17279390. S2CID   23721149.
  27. ↑ Yeo, Darren C. J.; Peter K. L. Ng; Neil Cumberlidge; Célio Magalhães; Savel R. Daniels; Martha R. Campos (2008). E. V. Balian; C. Lévêque; H. Segers; K. Martens (eds.). "Global diversity of crabs (Crustacea: Decapoda: Brachyura) in freshwater". Hydrobiologia . Developments in Hydrobiology, vol. 198. 595 (1). Springer: 275–286. doi:10.1007/s10750-007-9023-3. ISBN   978-1-4020-8258-0.
  28. ↑ "Red crabs – video footage of the migration". Parks Australia. 1 December 2013. Retrieved 3 December 2013.
  29. ↑ "Global Capture Production 1950–2004". Food and Agriculture Organization. Archived from the original on 23 January 2016. Retrieved 26 August 2006.
  30. ↑ "Future Management of Brown Crab in the UK and Ireland (IPF_D123)" (PDF). www.seafish.org. Nautilus Consultants. 2009. Archived from the original (PDF) on 6 September 2011.
  31. ↑ "How to dress a crab". BBC. Retrieved 24 September 2023.
  32. ↑ Cascorbi, Alice (14 February 2004). "Seafood Report: Blue Crab, Callinectes sapidus" (PDF). Seafood Watch. Monterey Bay Aquarium. Archived from the original (PDF) on 28 July 2013. Retrieved 12 September 2012.
  33. ↑ Stern, Jane (4 June 2009). 500 Things to Eat Before It's Too Late: and the Very Best Places to Eat Them. Houghton Mifflin Harcourt. p. 81. ISBN   978-0-547-41644-1.
  34. ↑ "The History of Maryland Crab Cakes - Brookside Inn Restaurant - Oxford". nearsay.com. Archived from the original on 27 October 2022. Retrieved 12 May 2020.
  35. ↑ Ferreira, Luis. "Caril de Caranguejo (Moçambique) - ReceitaseMenus.net". Receitasemenus.net. Retrieved 10 September 2017.
  36. ↑ Osamu, Sawaji (February 2023). "Ishikawa Wintertime Sushi" (PDF). Highlighting Japan. pp. 20–21. Retrieved 6 November 2025.
  37. ↑ Rossi, B. B. (1969). The Crab Nebula: Ancient History and Recent Discoveries. Center for Space Research, Massachusetts Institute of Technology. CSR-P-69-27.
  38. ↑ Benson, Elizabeth (1972). The Mochica: A Culture of Peru. New York, NY: Praeger Press. ISBN   978-0-500-72001-1.
  39. ↑ Berrin, Katherine (1997). The Spirit of Ancient Peru: Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson. p. 216. ISBN   978-0-500-01802-6.
  40. ↑ Kipling, Rudyard (1902). "The Crab that Played with the Sea". Just So Stories. Macmillan.
  41. ↑ "Just So Science: The Crab that Played with the Sea". BBC . Retrieved 6 November 2025.
  42. ↑ Skeat, Walter William (1900). "Chapter 1: Nature". Malay Magic. London: Macmillan. pp. 1–15.
  43. ↑ Sinclair, Melissa Scott (19 August 2025). "It's the Most Misunderstood Pet in America. If Only You Knew Where It Came From—and What It's Capable Of". Slate Magazine . Retrieved 6 November 2025.
  44. 1 2 3 Wolfe, Joanna (27 February 2025). "Crab Memes Amplify Mistaken Ideas about Evolution". Scientific American .
  45. 1 2 Swallow, Bea (17 October 2025). "Why does evolution keep creating 'imposter crabs'?". BBC News. Retrieved 17 October 2025.
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