Sea urchins are marine invertebrates, recognized by their globular, spiny appearance. They belong to the class Echinoidea within the phylum Echinodermata, a group that also includes starfish and sea cucumbers. Sea urchins play an important role in marine ecosystems. They are found in all oceans and climates, from tropical waters to polar regions.
Physical Characteristics
The body of a sea urchin is encased in a hard, spherical shell known as a test, 3 to 10 centimeters across. This test is made of fused calcium carbonate plates. Extending from this test are movable spines, which serve both for protection against predators and for locomotion. The length and thickness of these spines can vary between species.
In addition to spines, sea urchins possess tube feet. These tube feet are part of a hydraulic water vascular system, allowing them to extend and retract. Small, pincer-like organs called pedicellariae are also found at the base of the spines, used for self-defense and removing debris. On the underside is its mouth, which contains a complex chewing apparatus known as Aristotle’s lantern. This structure consists of five hard, self-sharpening teeth made of calcium.
Habitat and Behavior
Sea urchins inhabit a wide range of marine environments, from shallow intertidal zones to depths of 5,000 meters. While found in all oceans, including polar and tropical regions, most species thrive on temperate and tropical coasts where photosynthetic food sources are abundant. They prefer rocky bottoms and coral reefs.
Sea urchins move slowly across the seafloor using their tube feet as their primary means of locomotion. The tube feet extend and retract through a hydraulic process, allowing the urchin to pull itself along or adhere to surfaces. They can also use their spines to push themselves.
When feeding, sea urchins use Aristotle’s lantern to scrape algae off rocks and other surfaces. While algae form their main diet, they are omnivores, also consuming organisms like sponges, mussels, and brittle stars.
Ecological Significance and Human Interaction
Sea urchins play a role in marine ecosystems, particularly in kelp forests, where they act as grazers. When their populations are not controlled by predators, such as sea otters, sea urchins can overgraze kelp forests, leading to “urchin barrens” where kelp is severely diminished. This can alter the ecosystem’s structure and biodiversity. However, sea urchins also contribute by breaking down tough kelp into smaller pieces, providing food for other detritivores on the seafloor.
Interaction with humans can occur accidentally, such as stepping on them. While the spines can inflict painful puncture wounds, serious harm or fatalities are rare. The spines can break off in the skin, causing discomfort, swelling, and potential infection. Beyond accidental encounters, sea urchins are a culinary delicacy in many cultures. The edible part, known as “uni” in Japanese cuisine, is the animal’s gonads, prized for its creamy, briny flavor, often served raw or in sauces.