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Crab: the story on HearLore | HearLore
Crab
The Japanese spider crab, with a leg span reaching up to 12 feet, represents the extreme end of a body plan that has evolved independently at least five times across the animal kingdom. This is not a single family of creatures but a collection of unrelated lineages that have converged on the same solution to survival: the crab. While the horseshoe crab, an ancient arthropod from the late Ordovician period around 445 million years ago, belongs to the Chelicerata group alongside spiders and scorpions, the true crabs and their many cousins belong to the decapod crustaceans. These animals have developed a heavily armoured shell, a flattened body, and the ability to run sideways, a trait known as carcinisation. This process has created a polyphyletic group, meaning crabs do not share a single common ancestor that was also a crab, but rather have evolved similar characteristics to hide in rocky crevices and defend against predators in wave-swept environments. The body form is defined by a carapace that is flatter than it is broad, with lateral margins, and a sternum fused into a wide plastron that hides the tail segments. This specific anatomy allows them to scuttle sideways into rock crevices, a defensive mechanism that has proven so effective that it has been adopted by diverse groups ranging from the tiny pea crab to the massive coconut crab.
The Hidden Diversity
Beneath the familiar image of the brown crab lies a vast and strange world of crustaceans that defy simple categorization. The porcelain crabs, small flattened decapods that hide under rocks, often shed limbs to escape predators, a sacrifice that allows them to survive encounters with hungry fish. The hairy stone crab of the Australian shore moves slowly, camouflaged with brown hair that blends into the rocky seabed, while the king crabs live in the cold deep water, resembling true crabs but sharing a closer evolutionary history with hermit crabs. The coconut crab, the largest terrestrial arthropod on Earth, can grow up to 18 inches long and weigh up to 9 pounds, yet it is essentially a hermit crab that has abandoned the ocean for the islands of the Pacific and Indian oceans. Some species, like the pea crabs, are parasitic, living inside the shells of bivalve molluscs and feeding on their host's food supply. The tree crab, or Caribbean hermit crab, lives as an adult on land, breathing air with a lung and taking over a mollusc shell for protection, only returning to the ocean to spawn. Even the Patagurus rex, a specialised hermit crab from Polynesia, lives at a depth of 400 metres, possessing a unique carapace that makes it look crab-like despite its deep-sea existence. This diversity extends to feeding methods as well, with some crabs being herbivores that prefer filamentous algae, while others are carnivores that hunt small animals, and still others are plankton feeders that filter prey from seawater using long feathery bristles on their mouthparts.
The Japanese spider crab has a leg span reaching up to 12 feet. This species represents the extreme end of a body plan that has evolved independently at least five times across the animal kingdom.
When did the horseshoe crab first appear in the fossil record?
The horseshoe crab is an ancient arthropod from the late Ordovician period around 445 million years ago. It belongs to the Chelicerata group alongside spiders and scorpions.
How much crab meat is consumed worldwide each year?
Crabs make up approximately 1.5 million tonnes of marine crustaceans caught, farmed, and consumed worldwide annually. This amount represents approximately 20% of all marine crustaceans caught globally.
What is the maximum lifespan of a Caribbean hermit crab in captivity?
The Caribbean hermit crab Coenobita clypeatus can live for 30 years in captivity if their requirements are met. This lifespan requires a substantial tank with specific substrate, temperature, and humidity controls.
Which artist painted a meticulous image of the crab Eriphia verrucosa in 1495?
The German artist Albrecht Dürer made a meticulous painting of the crab Eriphia verrucosa in 1495. Dürer likely painted the animal when he visited Venice, as the species lives on the Adriatic coast.
What is the scientific name for the process where unrelated lineages evolve crab-like features?
The process is known as carcinisation and has created a polyphyletic group of crabs. This evolutionary trend has occurred independently in at least five different groups of decapods to help animals hide in rocky crevices and defend against predators.
Crabs make up approximately 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, while other commercially important taxa include the blue crab Callinectes sapidus, the Dungeness crab Metacarcinus magister, and the brown crab Cancer pagurus. In Western Europe, much of the crab meat comes from the brown crab, noted for its sweet, delicate flavour, with major fisheries operating in Scotland and the South West of England. Dressed crab, a traditional seafood meal in British cuisine, consists of the meat of the brown crab served in its own shell. In North America, the fishery for the blue crab is centered on the Chesapeake Bay, where crab cakes are traditionally made, though other places are increasing in importance. In Goa and Mozambique, crab curry is a typical dish, flavoured with chilis, garlic, coconut, and spices. In Ishikawa prefecture, Japan, both the meat and the eggs of the snow crab are served as sushi in wintertime, with the male known as kano-gani and the female as kobako-gani. The short fishing season for the females makes kobako-gani sushi a rare delicacy. These fisheries support economies and cultures across the globe, from the Atlantic coast of the United States to the Gulf of Mexico, and from the shores of Britain to the islands of the Pacific.
The Mythic Shell
The constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as such, but the creature's presence in mythology often begins with a secondary role. In Greek mythology, Karkinos was a crab that came to the aid of the Lernaean Hydra as it battled the hero Heracles, though the crab is at best a secondary character in the myth and sometimes omitted altogether. This has been explained by the suggestion that it was introduced into the myth by Mesopotamian influence, as astrologers tried to associate the Labours of Hercules with the twelve signs of the Zodiac. Crabs have appeared in art since ancient times in many different media, from the Moche people of ancient Peru who worshipped nature and often depicted crabs in their art, to the German artist Albrecht Dürer who made a meticulous painting of the crab Eriphia verrucosa in 1495. Dürer likely painted the animal when he visited Venice, as the species lives on the Adriatic coast. A quite different artistic portrayal is the monumental crab table setting charger created by the Barbizet Studio that made glazed earthenware pottery between 1850 and 1890. In China, Gao Qipei painted Crabs and Chrysanthemums in ink and light watercolour on paper, while in Panama, the Guna people of the San Blas Islands make Mola appliqué blouse panels decorated with motifs such as crabs from the waters of the Caribbean. 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, and the paleontologist Richard Fortey has identified Kipling's giant crab as a horseshoe crab. 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, where there sits a gigantic crab which twice a day gets out in order to search for food.
The Pet Paradox
Hermit crabs are commonly kept as pets and used in the marine aquarium trade, with the Caribbean hermit crab Coenobita clypeatus being a popular species. They can live for 30 years in captivity if their requirements, including simulating a coastal rainforest, are met. The size of tank must be substantial, and 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, and a pool of fresh water and a pool of correctly formulated salt water are both necessary. Despite their popularity, keeping hermit crabs as pets is a complex endeavor that requires significant knowledge and resources. The crab's life cycle involves moulting, a process where they shed their exoskeleton to grow, and this requires a specific environment to be successful. The crab's need for a shell to protect its soft body is a key aspect of its care, as they must be provided with a variety of shells to choose from as they grow. The crab's social behavior is also important, as they are often kept in groups, but this can lead to competition for resources and space. The crab's diet must be varied, including both plant and animal matter, to ensure they receive all the necessary nutrients. The crab's lifespan of up to 30 years in captivity is a testament to their resilience, but it also means that keeping them as pets is a long-term commitment that requires dedication and care.
The Evolutionary Joke
The zoologist Joanna Wolfe, writing in Scientific American, notes a popular meme which jokes that crabs are the ultimate forms of life as everything will eventually evolve into a crab. Sara Kiley Watson, writing in Popular Science, comments that the joke comes from an actual truth, that decapods span multiple crab-like groups, including the true crabs but not limited to them. Wolfe 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. 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. This does not mean that all animals will evolve the same adaptations, as the crab meme wrongly suggests. The process of carcinisation has created a body form that is so effective in certain environments that it has been adopted by diverse groups, but it is not a universal goal of evolution. The crab's body plan is a solution to specific problems, such as predation and the need to hide in crevices, and it is not a path that all animals will follow. The meme is a humorous exaggeration of a real evolutionary trend, but it is important to understand the limitations and context of the process. The crab's body form is a testament to the power of natural selection, but it is not a blueprint for all life.