Sea urchins are widespread marine invertebrates, easily recognized by their spherical, spiny shells. These echinoderms crawl across the seafloor and possess a highly specialized mouth structure that allows them to feed effectively on hard surfaces. Yes, sea urchins do possess teeth, but they are dramatically different from the dental structures found in mammals or fish. These teeth are integrated into a powerful feeding apparatus adapted for their abrasive diet.
Yes, They Have Teeth
The teeth are part of a larger, intricate skeletal-muscular system known formally as Aristotle’s Lantern. This complex structure is located on the ventral, or underside, of the urchin’s body, positioned centrally within the shell. It consists of a framework of 40 or more small calcium plates and five main jaws, each holding a single tooth.
The teeth are composed of magnesium-bearing calcite, a form of calcium carbonate reinforced with magnesium. This composition makes the material significantly harder and tougher than regular calcite, allowing the teeth to withstand the constant wear of scraping against rock and coral. The magnesium is especially concentrated at the grinding tip of each tooth, enhancing its effectiveness.
How Aristotle’s Lantern Functions
The five teeth are arranged in a circular formation, working together like a mechanical claw or chisel to process food. An intricate system of retractor and protractor muscles controls the movement of the jaws. This allows the teeth to be pushed outward, or protruded, to scrape and grasp the substrate, or withdrawn into the body when the urchin is not feeding.
This feeding apparatus is capable of scraping algae, chewing tougher materials, and even boring into solid rock to create protective depressions. To counteract the abrasion from constant use, the teeth exhibit a unique self-sharpening mechanism. New tooth material is continuously grown from the base, replacing the worn tips at a rate of approximately one to two millimeters per week. The outer layer is designed for controlled chipping, ensuring the tip remains sharp by selectively removing material from the cutting edge.
What Sea Urchins Eat
The continuously regenerating dental apparatus is necessary because the sea urchin’s diet consists mainly of tough, abrasive material. Their primary food sources include various types of algae, kelp, and organic detritus that settles on the ocean floor. They are efficient grazers, using their teeth to scrape these organisms directly off rocky surfaces.
In a healthy marine environment, sea urchins often consume kelp detritus, which are fragments that have naturally detached from the main plant. When preferred food is scarce, however, they will actively graze on living kelp plants, cutting through the structure, including the holdfast that anchors the plant to the seabed. Unchecked populations can be destructive to marine habitats, leading to the creation of “urchin barrens”—vast areas of the seafloor stripped bare of kelp and other large algae.