Crab

Lore & Background

Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 3.8 m. 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. They have appeared in art in media including pottery, paintings, blouse panels, and book illustrations. Hermit crabs are often kept in aquariums and as pets. A popular meme jokes that everything will evolve into crabs, based inaccurately on the genuine evolutionary trend within the decapods.

In Their Own Story

The tide retreated, revealing a mosaic of wet sand and scattered shells. Under the cloak of twilight, a blue crab surged from its burrow, raising claws in a silent warning against unseen threats. The water bit cold, heavy with the scent of brine and decaying kelp. It scuttled sideways with practiced precision—one claw snapping at a fleeing shrimp while the other sifted through grit for buried clams. Above, the moon cast silver ripples across the surface, glinting off its armored back. Suddenly, it froze. A shadow loomed overhead: a heron hunting in the shallows. In an instant, the crab vanished beneath the sand, burying itself until only its stalked eyes remained visible, watching. Silence reclaimed the shore, broken solely by the rhythmic crash of waves against the pilings.

Reader's Guide

Crabs are not a single taxonomic group. Instead, alongside the Brachyura or true crabs, are multiple groups of the Anomura that are called crabs, including the hermit crabs, mole crabs, king crabs, and porcelain crabs. 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 crab body form and associated behaviour have arisen independently at different times in multiple groups of decapod crustaceans. True crabs (Brachyura) are generally covered with a thick exoskeleton composed primarily of highly mineralized chitin. Porcelain crabs (Porcellanidae) are small flattened decapods that hide under rocks. The hairy stone crab (Lomisidae) is a slow-moving crab of the Australian shore. Hermit crabs (Paguroidea) have heavily armoured crab-like claws, but inhabit empty scavenged gastropod mollusc shells. King crabs (Lithodidae) live mainly in cold deep water. The coconut crab (Birgus) is a large terrestrial hermit crab. Patagurus rex is a specialised hermit crab from Polynesia. Many crabs can run swiftly sideways, though others walk forwards, and some can swim.

Did You Know?

• Crabs are polyphyletic, meaning they have multiple evolutionary origins.
• The coconut crab is the largest terrestrial arthropod, up to 1 m long and weighing up to 4.1 kg.
• Some 1,300 species of crabs in 8 families are adapted to freshwater.
• The crab body form has evolved independently multiple times through carcinisation.