Sea urchins are marine invertebrates belonging to the phylum Echinodermata, which also includes sea stars, sea cucumbers, and sand dollars. They inhabit ocean floors worldwide, from tropical reefs to polar waters. Defined by their unique radial symmetry and a body covering of movable spines, sea urchins navigate and survive in diverse benthic environments. They play an active part in many marine ecosystems, functioning primarily as grazers on the seafloor.
Anatomy and Distinctive Features
The most defining physical characteristic of a sea urchin is its rigid internal shell, known as the test. This spherical or slightly flattened endoskeleton is made of interlocking plates of calcium carbonate, covered by a thin layer of skin. The test surface is covered with small knobs called tubercles, to which the spines are attached by flexible ball-and-socket joints.
The highly mobile spines serve multiple purposes, including defense against predators and locomotion across the substrate. Interspersed among the spines are numerous flexible, slender appendages called tube feet, which protrude through tiny pores in the test plates. These tube feet are hydraulically operated by an internal water vascular system, allowing the urchin to move slowly and grasp food particles.
The mouth is located on the underside of the body and contains a complex, five-part jaw structure unique to urchins called Aristotle’s lantern. This specialized apparatus consists of five calcified teeth and associated muscles. These teeth are used to scrape algae and other organic matter from rocks and hard surfaces. The robust nature of the teeth allows some species to bore into soft rock, creating protective depressions for shelter.
Ecological Role and Habitat
Sea urchins are found in every ocean, inhabiting a range of benthic environments including rocky shores, seagrass beds, and coral reefs. Their primary ecological function is that of a herbivore and grazer, consuming large quantities of algae and detritus. They are particularly abundant in temperate zones, where they feed heavily on the massive brown algae that form kelp forests.
Their grazing activity is a significant factor in maintaining the balance of these marine habitats. When sea urchin populations are kept in check by natural predators, such as sea otters or large sea stars, they graze the kelp that grows on the seafloor, ensuring a healthy turnover in the ecosystem. However, a decline in these predators can lead to a phenomenon known as a trophic cascade, causing an unchecked population explosion of urchins.
Excessive grazing by dense urchin populations can decimate a kelp forest by eating the holdfasts that anchor the kelp. This destructive feeding leads to the formation of “urchin barrens,” vast areas of the ocean floor characterized by a lack of kelp and low marine life diversity. Urchin barrens represent an alternative stable state that supports a higher density of urchins but significantly lower biodiversity compared to a healthy kelp forest.
Interaction with Humans
Sea urchins interact with human societies both as a valued food source and as a marine hazard. The gonads, or roe, are a highly prized delicacy known as uni in Japanese cuisine, valued for their creamy texture and rich, briny flavor. Japan is the world’s largest consumer of uni, driving a global industry involving harvesters in countries like the United States, Chile, Canada, and South Korea. Consumption is also notable in several European Mediterranean nations, including Italy, France, and Spain.
Unintentional contact with sea urchins presents a common hazard for swimmers and divers, typically resulting in painful puncture wounds. The sharp, brittle spines can easily penetrate skin and break off, leaving fragments embedded in the tissue. Some species possess pedicellariae—small, claw-like appendages between the spines—that can deliver venom, intensifying the immediate, burning pain and potentially causing local muscle ache.
First aid for a spine puncture injury involves immersing the affected area in water as hot as can be tolerated for up to 90 minutes to alleviate pain and neutralize heat-sensitive toxins. Embedded spines should be removed carefully with tweezers if visible. Seeking medical attention is advised for deeply lodged spines or any signs of systemic symptoms like nausea or paralysis. Wearing protective footwear in rocky or reef areas is the most effective prevention strategy.