Crab

• 1. Introduction

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  Crab is a word that covers a surprising amount of ground. What we casually call a crab is not a single family, not a single lineage, but a shape that evolution has independently invented at least five separate times. The pea crab, only a few millimeters wide, and the Japanese spider crab, with a leg span reaching 4 meters, are both called crabs. They share armoured shells, tails tucked under the body, and a talent for running sideways. Yet they arrived at that body plan by entirely different routes. How does nature keep reinventing the same creature? And what does this restless convergence tell us about life in the ocean?

• 2. Carcinisation

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  The process responsible for the repeated invention of the crab is called carcinisation, and it describes how unrelated groups of decapod crustaceans keep arriving at the same solution. Keiler and colleagues, writing in 2014, set out the specific criteria that define the carcinised body: a carapace that is broader than it is flat, sternites fused into a wide sternal plastron, and a pleon so strongly bent that it completely hides certain tail segments from view. Those are the engineering specifications of something built to wedge into rocky crevices, resist wave action, and carry its armour with minimal drag.

  The hermit crabs, king crabs, and porcelain crabs all carry this body form, yet none of them sits inside the Brachyura, the true crabs. They belong instead to the Anomura, a related but distinct group. King crabs in particular resemble true crabs closely enough to fool a casual observer, yet they are more closely related to hermit crabs than to anything in the Brachyura. The evolutionary palaeobiologist Matthew Wills has pointed out that the pressures driving this convergence are legible: defence against predators, life in crevices, and the constant battering of waves all favour a broad compact body, solid armour, and the ability to scuttle sideways.

  Because crabs have multiple origins, biologists describe the group as polyphyletic, meaning it has no single common ancestor that is exclusively ancestral to all crabs. A popular internet meme jokes that everything will eventually evolve into a crab, parodying this real trend. The zoologist Joanna Wolfe, writing in Scientific American, noted that the meme exaggerates the pattern but does point at something genuine about decapod evolution.

• 3. Size, Diet, and Habitat

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  The coconut crab, Birgus, is the largest terrestrial arthropod alive today, reaching up to 40 cm in length and weighing up to 4.1 kg. It lives on islands in the Pacific and Indian oceans and is technically a terrestrial hermit crab, which means it shares more ancestry with hermit crabs than with the true crabs it physically resembles.

  Far out in the open ocean, many crabs take a more generalist approach, feeding on algae, molluscs, polychaete worms, other crustaceans, and whatever detritus drifts past. The mottled crab Grapsus albolineatus goes further in one direction, feeding mainly on algae and showing a preference for the more nutritious filamentous varieties over leafy ones. The yellow moon crab Ashtoret lunaris goes the other way entirely, hunting as a carnivore. Porcelain crabs have refined feeding into something almost passive: long feathery bristles on their mouthparts filter plankton directly from the water.

  Some crabs have cut out foraging altogether by becoming parasites. The tiny soft-bodied oyster crab lives inside an oyster's shell and simply eats the food the host collects. The Pinnotheridae, the pea crabs, pursue the same strategy inside bivalve molluscs. On Christmas Island, the red crabs make an annual mass migration to the sea for a very different purpose, traveling overland to lay their eggs in the ocean. Some 1,300 species across 8 families have made the move into freshwater, a reminder that the ocean is not the only home crabs have claimed.

• 4. Ancient Lineages

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  Horseshoe crabs are not true crabs at all. They belong to the Chelicerata, the same broad group as spiders and scorpions, and their fossils have been found in the late Ordovician of Canada, around 445 million years ago. Their body plan is built on an anterior prosoma, a fused head and thorax covered by a semicircular carapace, plus a posterior opisthosoma. Five pairs of walking legs and a pair of pincer-like chelicerae emerge from the underside of the prosoma.

  The paleontologist Richard Fortey identified the giant crab in Rudyard Kipling's Just So Story "The Crab that Played with the Sea" as a horseshoe crab, a reading that fits both the animal's ancient lineage and its imposing scale. Kipling's creature was imagined making the tides rise and fall, and the horseshoe crab's deep time on Earth gives it a kind of mythic weight that makes the identification feel apt.

• 5. Crab in Culture and Myth

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  Albrecht Durer painted the crab Eriphia verrucosa in 1495, capturing the animal with a precision that suggests he worked from life. Since that species lives on the Adriatic coast, he most likely painted it during a visit to Venice. The German artist's meticulous attention to the subject sits in a long tradition of human fascination with crabs across many cultures.

  The Moche people of ancient Peru, who worshipped nature and especially the sea, depicted crabs repeatedly in their art. In China, the painter Gao Qipei, who lived from 1672 to 1734, painted Crabs and Chrysanthemums in ink and light watercolour. The Barbizet Studio, which made glazed earthenware pottery between 1850 and 1890, produced a monumental crab table setting charger that sits in a completely different register, ornamental rather than naturalistic. In Panama, the Guna people of the San Blas Islands sew Mola applique blouse panels with crab motifs drawn from the Caribbean waters around them.

  In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra during its battle with Heracles. The crab's role in that story is minor and sometimes dropped from the telling altogether. Scholars have suggested the creature was inserted into the myth by Mesopotamian influence, as astrologers worked to match the Labours of Hercules to the twelve signs of the Zodiac. The constellation Cancer and the astrological sign share that origin. In Malay mythology, the tides themselves were explained by a gigantic crab living at the Navel of the Seas, Pusat Tasek, climbing out twice a day to search for food.

• 6. Crabs as Food

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  Crabs account for around 20% of all marine crustaceans caught, farmed, and consumed worldwide, adding up to roughly 1.5 million tonnes every year. A single species, the Asian blue crab Portunus trituberculatus, makes up one-fifth of that entire harvest on its own.

  The brown crab Cancer pagurus, valued for its sweet and delicate flavour, dominates the market in Western Europe, with major fisheries operating in Scotland and the South West of England. Dressed crab, its meat served inside the animal's own shell, is the traditional British preparation. In North America, commercial fishing for the blue crab Callinectes sapidus runs along the Atlantic coast and into the Gulf of Mexico; the fishery was historically centred on the Chesapeake Bay, and crab cakes made from Chesapeake Bay crabs are a recognized regional tradition.

  In Ishikawa prefecture in Japan, both the meat and the eggs of the snow crab appear as sushi during winter. The male snow crab is known as kano-gani; the female is kobako-gani. The short fishing season for the females makes kobako-gani sushi genuinely rare. In Goa and in Mozambique, crab curry flavoured with chilis, garlic, coconut, and spices is a typical dish, drawing on coastal traditions from two different ocean basins.

• 7. Hermit Crabs as Pets

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  The Caribbean hermit crab, Coenobita clypeatus, is one of the most commonly kept pet crabs and a staple of the marine aquarium trade. In the right conditions, including a tank that simulates a coastal rainforest environment, these animals can live for 30 years in captivity.

  The requirements are more demanding than many new owners expect. The substrate must contain sand and coconut fibre deep enough for the crab to dig into during moulting. Temperature and humidity need active management. Two separate water pools are necessary: one of fresh water and one of correctly formulated salt water. The Caribbean hermit crab is also the tree crab, a terrestrial adult that returns to the ocean only to spawn, breathing air through a structure that functions as a lung. That dual life, land animal by day, ocean-dependent by breeding season, makes the animal a vivid example of how far from the sea some crabs have migrated.