Sea urchin

Sea urchins make up the class Echinoidea, a group within the echinoderms. There are about 950 species living on the ocean floor, found in every ocean and at every depth, from the shoreline to the deep sea. Their bodies are usually round and covered with a hard, spiny shell called a test, which typically measures 3 to 10 cm (1 to 4 inches) across. These animals move slowly, using tiny tube feet to crawl and sometimes pushing themselves along with their spines. Their diet is mostly algae, but they also eat slow-moving or stationary animals like crinoids and sponges. Predators include sharks, sea otters, starfish, wolf eels, triggerfish, and humans. If predators are scarce, urchins can overgraze an area, creating barren zones where large algae and the creatures that depend on them disappear.

Like all echinoderms, adult sea urchins have a five-part symmetrical body plan, but their larvae have bilateral (mirror) symmetry. This larval trait places them in the group Bilateria, alongside chordates, arthropods, annelids, and molluscs. Sea urchins live in every climate, from tropical to polar waters, and on all kinds of seafloor habitats, from rocky shores to the deepest hadal zones. Their fossils date back to the Ordovician period, around 450 million years ago. Their closest echinoderm relatives are sea cucumbers (Holothuroidea), and like them, sea urchins are deuterostomes—a group that also includes chordates.

Scientists have studied sea urchins since the 1800s as model organisms in developmental biology because their embryos are easy to observe. Research continues today, especially on their genomes, due to their unusual fivefold symmetry and their evolutionary link to chordates. Some species, like the slate pencil urchin, are popular in aquariums, where they help control algae. Fossil urchins have also been used as protective amulets.

The phylum Echinodermata also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids. Like other echinoderms, sea urchins have fivefold symmetry (pentamerism) and move using hundreds of tiny, transparent, adhesive tube feet. This symmetry isn't obvious in a living urchin but is easy to see on a dried test. The term "sea urchin" specifically refers to the "regular echinoids"—symmetrical and globular forms—which include two subclasses: Euechinoidea (modern sea urchins, including irregular ones) and Cidaroidea (slate-pencil urchins with thick, blunt spines that often host algae and sponges). The "irregular" sea urchins are an infraclass within Euechinoidea called Irregularia, which includes Atelostomata and Neognathostomata. Irregular echinoids include flattened sand dollars, sea biscuits, and heart urchins. Together with sea cucumbers, sea urchins form the subphylum Echinozoa, characterized by a globoid shape without arms or projecting rays. Sea cucumbers and irregular echinoids have evolved different shapes secondarily. Although many sea cucumbers have branched tentacles around their mouths, these come from modified tube feet and are not the same as the arms of crinoids, sea stars, or brittle stars.

Urchins usually range from 3 to 10 cm (1 to 4 inches) in size, but the largest species can reach up to 36 cm (14 inches). They have a rigid, usually spherical body with movable spines, which gives the class its name Echinoidea (from the Greek *ekhinos*, meaning "spine"). The word "urchin" is an old term for hedgehog, which sea urchins resemble; they were once called sea hedgehogs. The name comes from Old French *herichun*, from Latin *ericius* ("hedgehog"). Like other echinoderms, sea urchin larvae start with bilateral symmetry but develop fivefold symmetry as they mature. This is most obvious in regular sea urchins, which have roughly spherical bodies with five equal parts radiating from a central axis. The mouth is on the bottom (oral surface) and the anus on top (aboral surface). Some sea urchins, like sand dollars, are oval with distinct front and rear ends, giving them some bilateral symmetry. In these irregular forms, the top is slightly domed, the underside is flat, and the sides lack tube feet—an adaptation for burrowing through sand or soft materials.

Inside the body, organs are enclosed by a hard test made of fused calcium carbonate plates, covered by a thin dermis and epidermis. This test is considered an endoskeleton, not an exoskeleton, because it is covered by a thin layer of muscle and skin, and sea urchins do not molt. Instead, the plates grow as the animal grows. The test is rigid and divided into five ambulacral grooves separated by five wider interambulacral areas. Each of these ten longitudinal columns has two sets of plates, making 20 columns total. The ambulacral plates have tiny paired holes through which the tube feet extend. All plates have rounded tubercles where spines attach. Spines are used for defense and movement and come in various forms. The inner surface of the test is lined by peritoneum. Sea urchins convert aqueous carbon dioxide into the calcium carbonate of their test using a catalytic process involving nickel. Most species have two types of spines: long primary spines and short secondary spines, distributed across the body, with the shortest at the poles and the longest at the equator. The spines are usually hollow and cylindrical.