Coastal shorelines are home to diverse marine life, including starfish and mussels, which often coexist on rocky surfaces. These organisms are integral to the intricate web of interactions that define intertidal ecosystems. Understanding their relationships provides insight into the delicate ecological balance present in these areas.
Starfish as Shoreline Predators
Certain starfish species, particularly the ochre sea star (Pisaster ochraceus), are significant predators within shoreline communities. These sea stars primarily consume mussels such as Mytilus californianus and Mytilus trossulus, though they can also prey on barnacles, snails, limpets, and chitons when mussels are scarce. An adult Pisaster ochraceus can consume a substantial number of California mussels in a year.
The feeding process of a starfish is distinctive. A sea star uses its powerful tube feet to pull apart the shells of its prey, applying significant pressure, sometimes up to 12 pounds, to create a small gap. Once a minimal opening is achieved, the starfish everts its stomach into the mussel’s shell. Digestive enzymes are then secreted to break down the mussel’s soft tissues externally, before the liquefied meal is absorbed and the stomach retracted.
Immediate Changes in Mussel Populations
When starfish are removed from a shoreline, mussel populations experience immediate changes. Without predation pressure, mussels grow rapidly and expand their distribution, quickly colonizing available space on rocks and within tide pools. Studies show mussel cover can increase significantly, sometimes from around 5% to over 18% within three years, following a decline in sea star numbers.
This rapid proliferation of mussels, particularly Mytilus californianus, outcompetes other sessile organisms for attachment sites. Mussels effectively secure space by forming dense beds and attaching firmly to substrates using byssal threads. The physical expansion of these mussel beds displaces species that require bare rock for survival, transforming the intertidal landscape into a uniform, mussel-dominated environment.
Ripple Effects Across the Ecosystem
The unchecked proliferation of mussels following starfish removal triggers a cascade of effects throughout the ecosystem. As mussels monopolize space and resources, they displace other species that typically inhabit the rocky intertidal zone. Organisms such as barnacles, limpets, and various species of algae are outcompeted, and their populations decline or disappear from dense mussel beds.
This reduction in available space and resources leads to a decrease in overall biodiversity, transforming a once diverse community into a monoculture-like environment. The loss of these species affects the broader food web, as many marine animals rely on prey or habitats no longer available. The absence of starfish impacts not just their immediate prey, but the entire community structure.
The Ecological Importance of Starfish
Starfish, particularly the ochre sea star, are considered keystone species in their intertidal habitats. A keystone species is an organism whose presence has a disproportionately large effect on its environment relative to its abundance or biomass. Despite not being the most abundant species, their predatory role significantly shapes the community structure and maintains biodiversity.
Starfish predation prevents a single species, like mussels, from dominating available space. By controlling mussel populations, starfish create and maintain open areas on rocky shores, allowing other species to settle and thrive. This fosters a more diverse and resilient ecosystem.