The ochre sea star, Pisaster ochraceus, is a prominent marine invertebrate recognized for its vibrant appearance. These sea stars typically have five stout arms radiating from a central disk, measuring 10 to 25 centimeters across. Their coloration varies widely, encompassing shades of purple, orange, yellow, reddish, and brown, with a rough surface covered by small spines. This animal is iconic along the Pacific coast of North America.
Coastal Distribution
The ochre sea star is widely distributed along the Pacific coast of North America, extending from Alaska in the north down to Baja California, Mexico. While their presence spans thousands of kilometers of coastline, their density can vary significantly from one region to another.
Intertidal Zone Home
Ochre sea stars primarily inhabit the rocky intertidal zone, the area along coastlines that is alternately submerged and exposed by the tides. This habitat is characterized by the presence of numerous rocks, crevices, and tide pools. Sea stars cling firmly to rocky substrates using specialized tube feet, which helps them withstand crashing waves and tidal changes. They can also be found in deeper, subtidal waters, sometimes as far down as 90 meters.
Life in the Zone
The ochre sea star thrives in the intertidal zone due to its ecological role and adaptations. It is a predator, primarily feeding on mussels, barnacles, and limpets abundant in this environment. Its feeding behavior, which involves prying open shells with tube feet and everting its stomach to digest prey, helps control mussel populations. This species is a keystone species; its predatory actions prevent mussels from dominating the rocky substrate, allowing other organisms to coexist. The sea star also tolerates significant air exposure during low tide and alters its body shape to reduce dislodgement by waves.
Factors Affecting Its Presence
Environmental and biological factors influence ochre sea star populations and their health. Natural factors such as water temperature, salinity, and wave exposure play a role in their distribution and survival. The availability of their prey species, like mussels, also directly impacts their local abundance.
A recent factor affecting ochre sea star populations has been Sea Star Wasting Disease (SSWD), which caused widespread mortality events starting in 2013. This disease leads to lesions, tissue decay, and fragmentation, resulting in high mortality rates, sometimes reaching 99% in certain areas. The disease has impacted populations across their entire range from Alaska to Baja California. Beyond disease, human activities like collecting sea stars and disturbing their habitats can also affect their presence.