Sea urchins are marine invertebrates, instantly recognizable by their dense covering of spines. They belong to the phylum Echinodermata, which also includes sea stars and sea cucumbers. These globe-shaped or flattened animals inhabit the ocean floor across all the world’s oceans, from shallow tide pools to depths of up to 5,000 meters. Sea urchins are slow-moving herbivores that play a significant part in the ecology of their marine environments. There are approximately 950 living species, typically measuring between 3 and 10 centimeters across the main body.
The Defining Physical Characteristics
The most prominent feature of the sea urchin is its array of movable spines, which serve both as a defense mechanism and an aid for locomotion. These spines are attached to the body by a ball-and-socket joint, which allows them to pivot in many directions. Beneath the spines lies the rigid, protective internal shell, which biologists call the “test.”
This test is composed of fused plates of calcium carbonate, forming a hard endoskeleton that encases the internal organs. The surface of the test is covered in small, rounded knobs called tubercles, and it is to these tubercles that the spines are articulated. The test also features five bands of minute pores that run from the top to the bottom of the animal.
Through these pores extend the hundreds of flexible, slender, and often sucker-tipped tube feet. These tube feet operate using a water vascular system, which employs hydraulic pressure to extend and retract the feet for movement and to help gather food. While the spines can assist by pushing the body along or lifting it off the substrate, the tube feet are the main ambulatory structures, allowing the urchin to crawl slowly across the seabed.
Classification and Internal Structure
Sea urchins are classified in the class Echinoidea within the phylum Echinodermata. A defining characteristic of all adult echinoderms, including sea urchins, is their pentaradial symmetry, meaning their body parts are arranged in fives around a central axis. While this five-part symmetry is not always immediately obvious in a living urchin, it is clearly visible in the structure of the dried test.
Internally, one of the most remarkable features is the complex chewing apparatus known as Aristotle’s lantern. This structure is a highly intricate system of muscles and five continuously growing, self-sharpening calcareous teeth. The five teeth are arranged in a circle and are typically extruded through the mouth, located on the underside, or oral surface, of the urchin.
Aristotle’s lantern is used for scraping algae and other food from hard surfaces like rocks. This specialized jaw allows some sea urchin species to be so efficient at grazing that they can even excavate small depressions in rock or coral to create stable hiding places.
Ecological Role and Habitat
Sea urchins are found in marine benthic environments worldwide, from the chaotic intertidal zone to the calm of the deep ocean floor. They prefer to settle on rocky substrates and use their tube feet and spines to navigate and anchor themselves. Their habitat range is vast, covering all climates from tropical regions to polar seas.
The sea urchin functions in the marine ecosystem as a grazer, or herbivore, as it feeds mainly on algae and other plant material. This grazing activity has a profound effect on the structure of shallow-water communities, particularly in kelp forests. Sea urchins are often considered a keystone species because of their ability to shape the entire ecosystem through their feeding habits.
When sea urchin populations are left unchecked by natural predators, their intense feeding can decimate vast stretches of kelp forest. This overgrazing transforms the lush, biodiverse kelp beds into expansive, desolate areas known as “urchin barrens,” which are dominated by crustose coralline algae and have significantly reduced biodiversity.