Sea stars are frequently found inhabiting the diverse and productive kelp forest ecosystem. Despite their close proximity to these underwater algae, sea stars do not consume kelp. These marine invertebrates are active predators, relying on a carnivorous diet that positions them as a regulating force within their environment. Understanding their true diet and unique feeding process clarifies their indirect connection to the health of the kelp forests.
What Sea Stars Really Eat
Sea stars are opportunistic carnivores that primarily consume slow-moving or sessile prey found along the seafloor. Their diet consists largely of benthic invertebrates, including bivalves such as mussels and clams, barnacles, and snails. Some species, like the ochre sea star, are known for their heavy predation on mussel beds, which prevents these bivalves from dominating the intertidal zone. They are generalist predators, meaning they will also consume other small invertebrates, marine worms, and scavenge on dead or decaying organic matter. Several large species, such as the sunflower sea star, actively prey upon sea urchins.
The Unique Way Sea Stars Feed
The reason sea stars do not graze on kelp is rooted in their specialized anatomy. They lack the jaws or scraping mouthparts required to consume macroalgae. Instead, most sea stars employ a remarkable process called stomach eversion to digest their prey externally.
The sea star’s mouth is located centrally on its underside, allowing it to push out its entire cardiac stomach over its food. For shelled prey like mussels, the sea star uses the suction power of its tube feet to slowly pry the shell open.
Once the stomach is everted, powerful digestive enzymes are released directly onto the soft tissues, liquefying the prey. The sea star then absorbs the nutrient-rich fluid before retracting its stomach. This process is controlled by neurochemicals and is highly effective for consuming soft-bodied and shelled creatures but is entirely unsuitable for grazing on the rigid structure of kelp.
Sea Stars’ Indirect Role in Kelp Forest Health
While sea stars are not kelp eaters, their predatory behavior makes them an important, indirect guardian of the kelp forest ecosystem. The sunflower sea star plays the role of a keystone species by regulating populations of the primary kelp consumer, the sea urchin. By preying on urchins, these sea stars prevent the herbivores from overgrazing and destroying the kelp beds.
Studies show that a single sunflower sea star can consume nearly one sea urchin per day, a rate sufficient to keep the urchin population in check. When sea star populations suffer a significant decline, such as occurred during the Sea Star Wasting Syndrome event, the resulting loss of predation pressure leads to an explosion in sea urchin numbers.
These unchecked urchin populations can devour entire kelp forests, creating vast, barren areas known as “urchin barrens.” The presence of these predators not only reduces the number of urchins but also instills a “landscape of fear,” causing urchins to hide and limit their grazing activity.