Sea urchins are marine invertebrates that inhabit oceans across the globe, from shallow coastal areas to the deep sea. These spiny, globe-shaped creatures possess a unique appearance that makes them instantly recognizable in various aquatic environments. Sea urchins play diverse and important roles within marine ecosystems, making them a subject of interest for understanding ocean health.
Defining Sea Urchins
Sea urchins are classified as echinoderms, a group that also includes starfish and sand dollars. Their name, “urchin,” originates from an Old English word for hedgehog, reflecting their spiny exterior. These animals typically range from 3 to 10 centimeters (1 to 4 inches) in diameter, though some species can reach up to 36 centimeters (14 inches). Most sea urchins exhibit a five-fold radial symmetry, which is evident in their internal structure, though less obvious in the living animal.
A sea urchin’s body is encased in a rigid internal skeleton known as a “test,” composed of fused plates made of calcium carbonate. Covering the test are numerous movable spines, which attach to the test plates via ball-and-socket joints. These spines, which can be hollow and vary in length and sharpness depending on the species, are used for defense, locomotion, and sometimes for trapping food.
Interspersed among the spines are hundreds of small, flexible projections called tube feet, which are part of the sea urchin’s water vascular system. They enable the sea urchin to move slowly across surfaces, cling to rocks, and manipulate food particles. Some species’ tube feet also function as photosensory organs, detecting changes in light.
The mouth of a sea urchin is located on its underside and features a complex chewing apparatus called Aristotle’s lantern. This structure consists of five calcium carbonate teeth that can be extended to scrape algae and other food from hard surfaces. Sea urchins are primarily herbivores, grazing on algae and kelp, but they are also omnivores, consuming detritus, small invertebrates, and other organic matter. They inhabit all oceans, from the intertidal zone to depths of 5,000 meters (16,000 feet), commonly found on rocky shores and coral reefs.
Their Place in the Marine World
Sea urchins are significant grazers within marine ecosystems, particularly in kelp forests and coral reefs. By consuming algae, they help prevent overgrowth that could otherwise smother corals or dominate kelp beds. When their populations are left unchecked, often due to a decline in natural predators like sea otters or certain fish, sea urchins can overgraze kelp forests, transforming them into “urchin barrens”—areas largely devoid of kelp. This shift reduces biodiversity and impacts species that rely on kelp for habitat and food.
Sea urchins also interact with humans. Their gonads, known as “uni” in Japanese cuisine, are considered a delicacy in many cultures and are eaten raw or incorporated into dishes. The taste of uni can vary based on the urchin’s diet and species. Historically, sea urchins have been part of the human diet for millennia.
Despite their culinary appeal, sea urchins can pose risks due to their sharp spines. Sea urchin populations face threats such as diseases, with a deadly pathogen recently causing widespread die-offs in some regions, including the Red Sea. Overfishing of their predators and ocean warming also contribute to population imbalances.