Description
Sea urchins or urchins are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to 5,000 meters (16,000 feet; 2,700 fathoms). Their shells (hard shells) are round and spiny, typically 3–10 cm (1–4 inches) across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed mainly on algae, but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish. Like other echinoderms, urchins have fivefold symmetry as adults, but their pluteus larvae have bilateral (mirror) symmetry, indicating that they belong to the Bilateria, the large group of animal phyla that also includes chordates, arthropods, annelids, and mollusks. They are widely distributed across all oceans, all climates from tropical to polar, and inhabit marine benthic (seabed) habitats from rocky shores to hadal zone depths. Sea urchins have a rich fossil record dating to the Ordovician, about 450 million years ago. Their closest relatives among the echinoderms are the sea cucumbers (Holothuroidea); both are deuterostomes, a clade that includes chordates. The animals have been studied since the 19th century as model organisms in developmental biology because their embryos were easy to observe; this has continued with studies of their genomes because of their unusual fivefold symmetry and relationship to chordates. Species like the slate urchin are popular in aquariums, where they are useful for controlling algae. Fossil urchins were used as protective amulets.
