The purple sea star, Pisaster ochraceus, is a marine invertebrate. It exhibits various colors, most commonly purple, but also orange, yellow, reddish, or brown. It typically has five stout arms, though some have four to seven. They are common in intertidal zones, attaching firmly to rocky surfaces.
Geographic Distribution
The purple sea star’s main range spans the Eastern Pacific Ocean along the North American west coast, from Prince William Sound in Alaska, southwards to Baja California, Mexico. Within this area, Pisaster ochraceus primarily inhabits intertidal zones, particularly wave-swept rocky shores and mussel beds.
They are found in low and middle intertidal areas, exposed during low tides. Their depth range also extends into subtidal regions, found on rocky bottoms down to approximately 88 to 97 meters (300 to 320 feet). Younger sea stars often seek shelter within crevices and underneath rocks.
Habitat and Environmental Influences
Their preferred habitat includes wave-washed rocky shores, tide pools, and mussel beds, where they attach firmly. The specialized tube feet on their undersides, equipped with suckers, enable them to cling powerfully to rocky substrates, withstanding heavy wave action.
This species tolerates air exposure, an adaptation for intertidal life. They can endure up to eight hours of exposure during low tides and have been observed to survive up to 50 hours in laboratory settings. This resilience is partly due to their ability to tolerate a loss of up to 30 percent of their body weight in fluids. However, purple sea stars are generally sensitive to temperature fluctuations; they are a cold to temperate water species and do not thrive in hot, shallow waters or tolerate high water temperatures. They inhabit areas with salinities of 30 psu or higher and show limited tolerance to significantly reduced salinities.
Role in Coastal Ecosystems
The purple sea star is a keystone species in coastal ecosystems, a concept first identified by ecologist Robert Paine in the 1960s. This designation reflects its disproportionately large influence on its environment relative to its abundance. The sea star primarily preys on California mussels (Mytilus californianus), which are dominant competitors for space in intertidal zones.
By consuming these mussels, the purple sea star prevents them from monopolizing rocky surfaces, thereby controlling their populations. This activity frees up space, allowing other species such as barnacles, limpets, and other invertebrates to establish and flourish. Research has shown that when purple sea stars are removed from an area, mussel populations expand dramatically, leading to a significant reduction in the overall biodiversity of the intertidal community. The impact of diseases, such as sea star wasting syndrome, on purple sea star populations can result in mussel population booms and subsequent ecosystem changes.