The purple sea star, Pisaster ochraceus, is a common inhabitant of the rocky intertidal zones along the Pacific coast of North America. Also known as the ochre sea star, it displays a distinctive five-rayed body. Its rough surface is covered in small spines, allowing it to firmly attach to rocks in wave-swept areas.
Main Food Sources
Adult purple sea stars are opportunistic carnivores, feeding on sessile or slow-moving invertebrates. Their diet largely consists of bivalves, particularly mussels such as the California mussel (Mytilus californianus) and Mytilus trossulus. Barnacles are another frequent prey item for these sea stars. Beyond mussels and barnacles, they also consume other organisms like chitons, limpets, and snails, including Tegula snails.
The specific prey consumed can vary depending on what is most available in their local environment. For instance, in areas where mussels are scarce, sea stars may rely more heavily on barnacles and whelks. A single purple sea star can consume up to 80 California mussels in a year.
Unique Eating Method
The purple sea star employs a unique method for consuming its prey. The sea star uses its powerful tube feet, which can exert significant force, to pull open the shells of bivalves, sometimes applying up to 12 pounds of pressure. Once a small gap is created, or if the prey is already exposed, the sea star everts its cardiac stomach out through its mouth, which is located on its underside.
This everted stomach then envelops the soft tissues of the prey, even capable of squeezing into openings as narrow as 0.1 millimeters. Digestive enzymes are released directly onto the prey, initiating external digestion. The partially digested, liquefied material is then absorbed back into the sea star’s body as the stomach retracts. This feeding strategy allows the purple sea star to consume prey larger than its small mouth opening would otherwise permit.
Ecological Significance
The feeding habits of the purple sea star affect the structure and biodiversity of its ecosystem. This species is recognized as a keystone species, meaning its presence has a disproportionately large impact on its environment relative to its abundance.
By preying on mussels, the purple sea star prevents these bivalves from monopolizing space on the rocky intertidal surfaces. Without this predation pressure, mussel populations can expand unchecked, outcompeting other species for space. The sea star’s consumption of mussels creates openings and frees up substrate, allowing a wider variety of other organisms, such as algae, barnacles, and other invertebrates, to settle and thrive. Research, including early studies by Robert Paine in the 1960s, demonstrated that removing purple sea stars led to a significant decline in species diversity in intertidal communities, highlighting their role in maintaining ecosystem balance.