urchin

Sea urchins are marine invertebrates in the class Echinoidea, within the phylum Echinodermata. They have a hard, calcareous test covered with spines and tube feet. Their bodies exhibit fivefold radial symmetry, with a mouth on the underside that bears a chewing apparatus called Aristotle’s lantern and an anus on the top surface. The spines and pedicellariae provide defense against predators.

Sea urchins occur worldwide in oceans from shallow intertidal zones to deep offshore environments, often on rocky substrates, coral reefs, or seagrass beds. They are mostly herbivores, grazing on algae, but some species scavenge detritus or feed on small invertebrates. They serve as prey for many larger animals and can influence the structure of algal communities on reefs and in kelp forests.

Life cycle and reproduction: Most species are dioecious and reproduce by external fertilization. Eggs develop into free-swimming larvae before metamorphosing into juvenile urchins. Some species display parental care or brooding, but such behaviors are not universal.

Human uses and conservation: Edible sea urchin gonads, known as uni, are valued in East Asian and Mediterranean cuisines. In fisheries, urchin harvest is managed to prevent overexploitation. Climate change and ocean acidification can affect shell formation and survival, and overfishing or habitat loss threaten populations in some regions.

Other uses of the word: The term urchin historically referred to a hedgehog or spiny creature and, by extension, to a mischievous child.