Green sea urchins (Strongylocentrotus droebachiensis) are common marine invertebrates inhabiting cold coastal waters across the Northern Hemisphere. They measure around 5 centimeters (2 inches) in diameter and reside in rocky intertidal zones, extending to depths of up to 1,150 meters (3,770 feet). As primary herbivores, green sea urchins graze on kelp and other marine algae, using a specialized mouthpart called Aristotle’s lantern to scrape and tear their food.
Primary Predators of Green Sea Urchins
Green sea urchins are preyed upon by a diverse array of marine animals, each employing different methods to overcome their spiny defenses. Sea otters are significant predators, utilizing rocks as tools to crack open the urchin’s hard shell. These marine mammals selectively feed on healthier urchins found within kelp forests, influencing local urchin populations.
Various fish species consume green sea urchins. Wolf eels swallow urchins whole, while cod, halibut, and several wrasses, including the California sheephead, also prey on them. Triggerfish use jets of water to dislodge urchins before consuming them.
Crabs, such as the red rock crab, hermit crabs, and pygmy rock crabs, use their powerful claws to crush the urchin’s shell or dismember them. Mud and sand crabs also eat sea urchins.
Starfish, including the sunflower sea star (Pycnopodia helianthoides) and other predatory species like the ochre star, prey on sea urchins. Starfish use their tube feet to pry open the urchin’s shell or evert their stomach to digest the urchin’s tissues. However, sunflower sea star populations have faced significant declines due to disease, impacting their role as a major urchin predator.
Diving birds, such as gulls and ducks, feed on green sea urchins, particularly in shallower waters. Gulls pick up urchins from tide pools and drop them onto rocks to break their shells before consuming the soft interior. Beyond natural predators, humans harvest green sea urchins for their gonads, known as “uni,” a delicacy in many cultures. This harvesting is primarily conducted by divers who hand-pick the urchins from the seafloor.
The Importance of Predation in Marine Ecosystems
The predation of green sea urchins plays a significant role in maintaining the health and balance of marine ecosystems, particularly kelp forests. Without sufficient predators, urchin populations can expand unchecked, leading to overgrazing of kelp beds. This excessive grazing can transform vibrant kelp forests into “urchin barrens,” areas of the seafloor largely devoid of kelp and other macroalgae.
The presence of predators helps regulate urchin numbers, allowing kelp forests to thrive. This dynamic illustrates a trophic cascade, where effects from higher levels of the food web, such as predators, influence lower trophic levels like herbivores and primary producers. For example, sea otters are considered a keystone species because their predation on urchins indirectly supports kelp forest growth.
Healthy kelp forests, supported by balanced predation, provide complex three-dimensional habitats that foster high biodiversity. These underwater forests offer food and shelter for a wide array of marine species, including various fish, invertebrates, and marine mammals. The consumption of green sea urchins by their predators supports the overall stability and diversity of coastal marine environments.