Pacific Rocky Intertidal Monitoring: Trends and Synthesis
Pisaster ochraceus (Brandt 1835)
Phylum Echinodermata, class Asteroidea, order Forcipulatida
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Highly variable in color; most commonly purple, but can also be orange, orange-ochre, yellow, reddish, or shades of brown. A “brilliant purple” morph is common in the inland waters of Washington and British Columbia. Average arm radius in CA/OR is around 9 cm (Harley et al. 2006, Raimondi et al. 2012) but can reach 3x this size. Individuals usually have 5 arms but this can vary from 4 to 7. Aboral surfaces have many small white spines arranged in detached groups or in a reticulate pattern, generally forming a star-shaped design on central part of disk (Morris et al. 1980). Tube feet on the undersides of arms have suckers that allow them to remain attached to rock in high wave energy shores.
Habitat and Geographic Range
Common in the middle to low intertidal zones on wave-swept rocky shores. Also found subtidally on rocks to 90 m. Juveniles are cryptic and are often found in crevices, under rocks and within mussel beds. Prince William Sound (Alaska) to Baja California, Mexico (Lamb & Hanby 2005)
Pisaster giganteus has fewer, bigger, and longer aboral spines surrounded by blue rings than P. ochraceus and these spines are more uniformly spaced and never form a star-shaped pattern. Pisaster brevispinus is pink with small, white spines and is a low intertidal to subtidal species (Morris et al. 1980). Evasterias troschelii has longer, more slender arms than P. ochraceus, and spines on the central part of the disk do not form a star-shaped pattern.
Pisaster ochraceus sea stars have long been referred to as keystone species in the rocky intertidal (Paine 1966, Menge 2004) and, while they are known to have a wide diet (including barnacles, snails, limpets, and chitons), mussels are their primary prey items on the open coast (Morris et al. 1980, Harley et al 2006). In the protected inland waters of Washington and British Columbia, mussels are often rare and Pisaster feeds primarily on barnacles and whelks (Harley et al. 2006). Using their tube feet to pull the valves apart,Pisaster are able to evert their stomachs and insert them between the valves of a mussel (Morris et al. 1980). Interactions between ochre stars and their prey have been well researched, especially the role of P.ochraceus in determining the lower limit of northern mussel beds (Paine 1966, 1974; Dayton 1971). Motile prey have been shown to exhibit escape responses to the chemical presence of Pisaster (Morris et al. 1980). A study examining the effect of low tide body temperature of P. ochracheus on feeding rates showed that aerial body temperatures experienced by P. ochraceus can have profound effects on predation rates (Pincebourde et al. 2008).
Ochre sea stars stand out in the intertidal due to their vibrantly contrasting color differences, ranging from bright orange to purple. Data from long term monitoring has shown a consistent color frequency of approximately 20% orange stars across a large geographic range of exposed coast (Raimondi et al. 2007). The underlying cause of color polymorphism in P. ochraceus is not fully understood, but it has been suggested that diet may play a key role (Harley et al. 2006).
Pisaster ochraceus is a broadcast spawner, with fertilization occurring in the water and development resulting in a free-swimming, feeding larva (Morris et al. 1980). These sea stars are able to regenerate arms that are lost and are thought to live up to 20 years (Morris et al. 1980). Ochre stars have few predators, but seagulls and sea otters occasionally eat them, and they are often collected by curious tidepool visitors due to their striking colors. Throughout southern California, severe declines of P. ochraceus (and other seastar) populations have been documented in association with warm-water periods since 1978, with greatest losses during El Niño events such as occurred in 1982-1984 and 1997-1998 (Eckert et al. 2000). The causative agent for this seastar “wasting disease” has not been confirmed, but may be a Vibrio bacterium (Eckert et al. 2000). Population recovery, apparently due to cooler-water conditions and large recruitment events, has been documented in many, but not all areas (Blanchette et al. 2006, Raimondi et al. 2012). P. ochraceus wasting disease has recently been recorded as far north as British Columbia, also associated with high water temperatures (Bates et al. 2009). Sensitivity to oil spills is not well known, but Chan (1973) saw no obvious effects from a San Francisco oil spill.