The Ochre Starfish (Pisaster ochraceus) is a large sea star found along the western coast of North America, recognized for its vibrant colors and significant ecological influence in the intertidal zone. This species is often described as a keystone species. The presence or removal of the ochre starfish has a disproportionate effect on the diversity and structure of the surrounding marine community. Its wide range and specific habitat preferences define where this important predator can be observed in the Pacific Ocean’s nearshore environments.
The Extensive Geographic Range
The ochre starfish exhibits an extensive and continuous distribution along the Pacific coastline of North America. Its northern limit reaches the cold waters of Prince William Sound in Alaska. From this northern point, the sea star’s range extends southward, following the temperate and cooler currents of the northeastern Pacific.
The southernmost extent of its distribution reaches down to Baja California, Mexico, with some sources citing Isla Cedros or Point Sal in Santa Barbara County, California, for the primary subspecies, P. ochraceus. This span covers thousands of miles of shoreline. The limiting factor for the southern range boundary is generally the increase in water temperature, as the ochre starfish is most abundant in the Pacific Northwest regions where cooler temperatures are consistently found.
Specific Intertidal Habitat Requirements
The preferred habitat of the ochre starfish is the rocky intertidal zone, which includes areas exposed to the air during low tide. It is found abundantly on wave-washed rocky shores, jetties, pilings, and within tide pools. This species is typically concentrated in the middle to low intertidal zones, though it can also be found in subtidal areas on rocks down to depths of about 90 meters (300 feet).
The necessity of a hard, rocky substrate is paramount for the ochre starfish, as its survival depends on its ability to anchor securely against powerful wave action. The sea star uses the thousands of suckers on its tube feet to cling tightly and resist being dislodged by strong surges. During low tide, the starfish must tolerate harsh environmental stresses, including desiccation and significant fluctuations in temperature. It can resist desiccation by tolerating a loss of up to 30% of its body weight in body fluids. To survive periods of exposure, individuals often aggregate in dense clusters under rocky ledges or in crevices to reduce moisture loss and limit sun exposure.
Identifying the Ochre Starfish
Recognizing the ochre starfish in its natural environment is straightforward due to its size and striking coloration. Most individuals possess the classic five-arm structure radiating from a central disk, although some may have four or as many as seven arms. The arms are stout and can measure from 10 to 25 centimeters (4 to 10 inches) in diameter, making it one of the largest sea stars in the intertidal zone.
The species displays a wide range of colors, which can vary from vivid orange, yellow, and reddish-brown to deep purple. The common name is derived from the ochre, or burnt orange, color found in many individuals. The upper surface of the body is rough and covered with numerous short, white, calcium carbonate components called ossicles, which appear as tiny spines or bumps.
Ecological Role and Diet
The ochre starfish’s presence in the intertidal zone is a defining factor in the community structure due to its function as a keystone predator. This means its impact on the ecosystem is disproportionately large compared to its overall numbers. Its predation habits prevent a single species from dominating the available space on the rocks.
The primary food source of the adult ochre starfish is the California mussel (Mytilus californianus), a highly competitive organism that can quickly monopolize rock surfaces. The sea star also feeds on other invertebrates such as barnacles, limpets, and snails. It captures prey by attaching its tube feet to the shell and exerting a steady pulling force to pry the valves open.
Once a small gap is created, the ochre starfish everts its stomach out through its mouth and inserts it into the shell opening. Digestive enzymes are then released to liquefy the mussel’s tissue, which the sea star absorbs. By preying on the mussels, the ochre starfish creates open patches on the rock, allowing less competitive species of algae and other organisms to colonize and thrive, thereby maintaining high biodiversity.