The purple sea star, Pisaster ochraceus, is a prominent marine invertebrate found along the Pacific coast. Recognized by its often vibrant purple, orange, or brown coloration and five stout arms, this species can reach a diameter of up to 40 centimeters. It plays a significant role in its ecosystem as a predator, influencing the diversity and structure of intertidal communities. This sea star is considered a keystone species, having a disproportionately large impact on its environment.
Geographical Distribution
The purple sea star inhabits an extensive range along the Pacific coast of North America. Its distribution stretches from Prince William Sound in Alaska in the north, southward to Baja California, Mexico. Within this area, Pisaster ochraceus typically resides in the low to middle intertidal zones, where it is frequently exposed during low tides. It can also be found in subtidal environments, extending to depths of approximately 90 meters (300 feet). These sea stars are most commonly observed on wave-washed rocky shores, where they can attach firmly using their tube feet.
Environmental Influences
Its range is shaped by environmental conditions conducive to its survival and reproduction. It thrives in cool, oxygenated waters typical of the Pacific coast. While able to tolerate some changes, the species can be susceptible to high water temperatures and low oxygen levels, which generally keep it out of shallow, warm bays and high tide pools. Pisaster ochraceus can tolerate significant loss of body fluids, yet remains sensitive to large osmotic changes.
Salinity levels also influence its distribution and performance; populations can acclimate to varying salinities, with feeding rates on mussels depending on salinity. The sea star’s habitat preference for rocky intertidal and subtidal zones is directly linked to its feeding strategy. It preys primarily on sessile invertebrates such as mussels and barnacles, but also consumes snails, limpets, and chitons. The availability of these prey species, which themselves require rocky substrates, significantly dictates where the purple sea star can establish and maintain populations.
Current Challenges to Its Range
The purple sea star’s range has faced considerable challenges in recent years, primarily due to Sea Star Wasting Disease (SSWD). This disease has caused severe mortality in Pisaster ochraceus populations across its entire geographical range, from Alaska to Baja California. SSWD manifests with symptoms such as twisting arms, deflation, lesions, and eventual body disintegration, often leading to death within days. This widespread outbreak resulted in unprecedented declines, with some adult populations experiencing an 81% reduction between 2012 and 2015.
The impact of SSWD was not uniform across the affected area, with southern regions generally experiencing more severe population crashes than northern ones. While sea star populations have shown some signs of recovery in certain areas due to increased recruitment of juveniles, overall numbers in many locations, particularly in Alaska and Washington, remain lower than pre-SSWD levels. The long-term effects of this disease on the sea star’s range and its ecological role are still under investigation, as the disease continues to be observed in varying levels of impact.