Sea urchin

Sea urchins make up the class Echinoidea, part of the larger group of echinoderms. Roughly 950 species live on the ocean floor, spread across every ocean and depth, from the shoreline down to the deepest trenches. Their bodies are usually round and encased in a hard, spiny shell called a test, which measures about 3 to 10 centimeters across. These animals creep along slowly, using their tube feet to crawl and occasionally pushing themselves with their spines. Their diet is mostly algae, but they also eat slow-moving or stationary creatures like crinoids and sponges. Predators include sharks, sea otters, starfish, wolf eels, triggerfish, and humans. When predators are scarce, urchins can overgraze an area, creating what are known as urchin barrens—damaged zones stripped of large algae and the animals that depend on them.

Like all echinoderms, adult sea urchins have five-part symmetry, while their pluteus larvae show bilateral (mirror) symmetry. This larval trait places them in the group Bilateria, which also includes chordates, arthropods, annelids, and molluscs. Sea urchins live in every climate, from tropical to polar waters, and occupy benthic habitats ranging from rocky shores to the hadal zone. Their fossil record stretches back to the Ordovician period, about 450 million years ago. Their closest echinoderm relatives are sea cucumbers (Holothuroidea), and both are deuterostomes—a clade that includes chordates.

Scientists have studied sea urchins since the 1800s as model organisms in developmental biology, because their embryos are easy to observe. This research continues today with genome studies, driven by 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.

**Diversity**

Sea urchins belong to the phylum Echinodermata, which also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids. Like other echinoderms, they 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 in a dried test. The term “sea urchin” specifically refers to the “regular echinoids”—symmetrical, globular animals—and includes several taxonomic groups. There are two subclasses: Euechinoidea (modern sea urchins, including irregular ones) and Cidaroidea (slate-pencil urchins, which have thick, blunt spines often covered with algae and sponges). The “irregular” sea urchins form an infraclass within Euechinoidea called Irregularia, which includes Atelostomata and Neognathostomata. These irregular echinoids include flattened sand dollars, sea biscuits, and heart urchins.

Together with sea cucumbers (Holothuroidea), sea urchins make up the subphylum Echinozoa, defined by a globoid shape without arms or projecting rays. Sea cucumbers and irregular echinoids have secondarily evolved different shapes. Though many sea cucumbers have branched tentacles around their mouths, these tentacles come from modified tube feet and are not homologous to the arms of crinoids, sea stars, or brittle stars.

**Description**

Urchins usually range from 3 to 10 centimeters across, but the largest species can reach up to 36 centimeters. They have a rigid, mostly 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 sits at the bottom (the oral surface) and the anus at the top (the aboral surface). However, some sea urchins—like sand dollars—are oval, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these irregular forms, the upper body is slightly domed, the underside is flat, and the sides lack tube feet. This body shape evolved to help them burrow through sand or soft materials.

**Systems**

**Musculoskeletal**

The internal organs are enclosed in a hard test made of fused calcium carbonate plates, covered by a thin dermis and epidermis. The test is considered an endoskeleton rather than an exoskeleton, even though it encloses almost the entire urchin, because it is covered with a thin layer of muscle and skin. Sea urchins also do not molt like invertebrates with true exoskeletons; instead, the plates of the test 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 consists of two sets of plates, for a total of 20 columns. The ambulacral plates have pairs of tiny holes through which the tube feet extend. All plates are covered with rounded tubercles that anchor the spines. Spines are used for defense and locomotion 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 series of spines—primary (long) and secondary (short)—distributed over the body, with the shortest at the poles and the longest at the equator. The spines are usually hollow and cylindrical.