The purple sea urchin, Strongylocentrotus purpuratus, is a small, spiny invertebrate that plays a large role in the nearshore Pacific environment. This organism is a prolific herbivore, using its five-toothed mouth, known as Aristotle’s lantern, to scrape and consume macroalgae like giant kelp. When their populations are unchecked, these grazers reach high densities, transforming vibrant underwater ecosystems. The presence or absence of the urchin’s natural enemies determines the health of the entire kelp forest.
The Primary Predators
The purple sea urchin has several natural predators that employ different tactics to breach its spiny defenses. The southern sea otter (Enhydra lutris) is the most well-known predator, utilizing its intelligence and powerful bite to crush the urchin’s hard shell, or test. Otters often use a rock as an anvil to smash the urchin open, consuming dozens daily.
The California sheephead (Semicossypus pulcher), a large, colorful fish, is another significant predator in Southern California waters. This fish possesses powerful jaws and teeth capable of cracking the urchin’s shell to access the soft interior tissue. Only larger sheephead, typically exceeding one foot in length, successfully prey on the biggest urchins.
Smaller invertebrates also contribute to control, particularly the California spiny lobster (Panulirus interruptus), which prefers purple sea urchins over other species. The sunflower sea star (Pycnopodia helianthoides), one of the largest and fastest sea stars, was historically a substantial predator along the Pacific coast. Using its 24 arms and thousands of tube feet, the sea star can envelop and consume the urchin whole.
The Urchin Barren Phenomenon
When ecological pressure from predators is released, the purple sea urchin population rapidly expands, leading to an “urchin barren.” This is a degraded marine ecosystem characterized by an overabundance of sea urchins and a near-total absence of kelp. In these areas, urchin densities can reach 50 or more individuals per square meter, a massive increase from healthy kelp forests.
The purple sea urchins actively graze on kelp holdfasts and stalks, effectively clear-cutting the underwater forest. Once destroyed, the urchins persist in a starved state, sometimes for years or decades. They survive by consuming nutrient-poor encrusting algae and shrinking their body size to reduce metabolic needs, becoming “zombie urchins” that wait for new kelp growth. This state replaces the kelp forest, which provides structure, food, and shelter, with a relatively barren seafloor.
How Predators Maintain Ecosystem Balance
The control exerted by these predators is an example of top-down forcing, where the top of the food chain regulates the lower levels. The sea otter is recognized as a keystone species because its effect on the ecosystem is disproportionately large compared to its abundance. The otter’s appetite suppresses the urchin population, allowing the kelp to flourish, which creates a trophic cascade. This cascade increases biodiversity, as healthy kelp forests support a wide array of marine life that rely on the habitat for sustenance and refuge.
Flourishing kelp forests also have a positive impact on the global climate through carbon sequestration. Kelp plants guarded by sea otters absorb up to 12 times more atmospheric carbon dioxide than unguarded forests overrun by urchins.
The presence of predators also influences urchin behavior, which is another mechanism of ecosystem control. For example, the waterborne chemical cue from a spiny lobster can cause purple sea urchins to reduce their foraging and grazing rates by over 40 percent. This non-consumptive effect encourages urchins to remain hidden in crevices rather than actively grazing, protecting the kelp even when the predator is not hunting.
Current Threats to Predator Populations
The balance maintained by these predators is threatened by environmental stressors and human activities, leading to the resurgence of urchin barrens. A major factor has been the widespread outbreak of sea star wasting disease (SSWD), which began in 2013 and decimated populations of the sunflower sea star. The decline of this specific urchin predator removed a significant control mechanism, contributing to the explosion of purple sea urchin numbers.
Fishing pressure also impacts the system, particularly concerning the California sheephead. Because this species is protogynous (the largest individuals are typically males), commercial and recreational fishing disproportionately removes the biggest, most reproductively valuable fish. The removal of these large males skews the sex ratio and reduces the population’s reproductive output, limiting the number of large predators available to consume the largest urchins.
Sea otters face vulnerabilities, including susceptibility to oil spills, which destroy the insulating properties of their fur and lead to hypothermia. They are also threatened by diseases and pathogens, such as Toxoplasma gondii, carried in freshwater runoff from land-based sources.