Sea urchins crawl on tube feet beneath a spiny globe
Sea urchins are class Echinoidea within the echinoderms—about nine hundred fifty seabed species from intertidal rocks to the deep ocean. Typically three to ten centimetres across, their globes bristle with protective spines while tube feet, and sometimes spines, inch them along to graze algae and slow prey.
Adults show fivefold symmetry; pluteus larvae are bilateral, placing them with other Bilateria. They inhabit every ocean and climate from tropics to poles and from shores to hadal depths. The echinoid fossil record reaches the Ordovician. Nineteenth-century embryologists prized their clear embryos; modern genomics still mines their pentaradial oddity and chordate kinship. Aquarium keepers favor slate-pencil urchins as algae patrols; fossils once served as amulets.
Kin include starfish, sea cucumbers, sand dollars, brittle stars, and crinoids—all moving by adhesive tube feet. “Sea urchin” usually means regular, symmetrical globes spanning Euechinoidea and thick-spined Cidaroidea; irregular urchins form a separate euechinoid infra-class. Dried tests reveal the fivefold star living animals hide under spines.
The class name Echinoidea comes from the Greek for spine, while urchin is an old word for hedgehog, and the animals were once called sea hedgehogs; the largest species reach 36 centimetres. Their test, fused calcium carbonate plates under a thin skin, counts as an endoskeleton and is never moulted. It is laid out in ten columns of paired plates, with tube feet poking through tiny pore pairs, and urchins use a nickel-assisted reaction to turn dissolved carbon dioxide into its mineral. Spines lean when a muscle sheath contracts, then lock stiff thanks to collagen that switches from soft to rigid. Under the body, five arrow-shaped pyramids form the jaw.
Irregular forms such as sand dollars break the round mould: oval, domed above and flat below, with a front and a back, a shape that evolved for burrowing through sand.
Source: Sea urchin
