Pisaster ochraceus (Brandt, 1835)  Common Names: Ochre sea star, Purple sea star
Synonyms: None
Phylum Echinodermata  Class Asteroidea   Order Forcipulatida    Family Asteriidae
	Individual found at Swirl Rocks, WA in the mid-intertidal zone.  Top view, approximately 40 cm in diameter.
	Photo by: Melissa McFadden, June 2002.

How to Distinguish from Similar Species:  Troschel’s Sea Star (Evasterias troschelii) may be confused with P. ochraceus at times. E. troschelii is distinguished from P. ochraceus by the smaller disk size and longer, tapering rays which are often thickest a short distance out from the base rather than at the base as in P. ochraceus; clusters of pedicellariae among the spines that border the ambulacral grooves, and the absence of a stellate pattern of spines on the aboral surface of the disk.  There are two other, mostly subtidal, local species of Pisaster (Pisaster giganteus and Pisaster brevispinus) but they have different aboral spines and coloration which allows one to distinguish between the species.

Geographical Range:  P. ochraceus occurs from Prince William Sound in Alaska to Point Sal in Santa Barbara Co., California.

Depth Range:  P. ochraceus occurs in the low and middle intertidal zones, and sometimes in the subtidal zone (to 88 m).

Habitat:  This organism occurs on wave-washed rocky shores.  The juveniles are often found in crevices and under rocks.

Biology/Natural History:  This species of seastar is often considered a keystone species in many intertidal regions. P. ochraceus feeds mainly on mussels (especially Mytilus californianus and Mytilus trossulus) or will also feed on barnacles, snails, limpets, and chitons when mussels are absent. P. ochraceus will insert its stomach into snail shells or slits as narrow as 0.1 mm between the shells of bivalves.  Numerous species of mollusks have avoidance responses to the Ochre Sea Star, often involving moving away.  Adult ochre seastars have few predators, but may be eaten by sea otters and sea gulls. P. ochraceus is more tolerant to air exposure than others in the Pisaster genus and regularly withstands up to 8 hours exposure during low tides.  It is apparently unharmed by up to 50 hours of exposure in laboratory setting; but they have an inability to tolerate high water temperatures and low oxygen levels, keeping them out of shallow bays and high tidepools (See Pincebourde et al., 2008).  Sexual reproduction occurs in the late spring or in the early summer.  When ready to reproduce, mature gonads may account for up to 40 percent of the animal's weight.  Spawning occurs in the Puget Sound around May to July.  Fertilization occurs in the sea and development results in free-swimming, plankton-feeding larvae.  Embryonic development and larval feeding have been studied in detail, however little is known of juvenile life following settlement and metamorphosis. P. ochraceus has been the focus of many major studies including tests on their digestive gland tissue (which is similar to cells in the mammalian pancreas and secretes materials similar to insulin).

Pisaster ochraceous is less water permeable than some other intertidal species such as Pycnopodia helianthoides.  It makes extensive use of water intake through its madreporite to maintain internal fluid balance (Ferguson, 1994).  The species is still highly susceptible to osmotic changes, however.  Held and Harley (2009) studied populations from high and low salinity sites.  Individuals from both populations were almost complete osmoconformers over the range of 15 to 30 psu.  In both populations activity (as measured by the righting response) was lowest at the lowest salinity (15 psu), and the population which had been living at lower salinity did not have any better righting response than did the one living at high salinity.  The population living at high salinity, however, did experience a higher mortality after exposure to 15 psu than did the other population.  Feeding rates on mussels also varied with salinity, but the maximum feeding rate in the population living at low salinity was at a lower salinity than that of the population which lived at a higher salinity.  

References:
Dichotomous Keys:

  Kozloff, 1987.

General References:
Kozloff, 1993.
  Morris, Abbott, and Haderlie, 1992.
 

Scientific Articles:

Ferguson, John C., 1994.  Madreporite inflow of seawater to maintain body fluids in five species of starfish.  pp. 285-289 in Bruno David, Alain Guille, Jean-Pierre Feral, and Michel Roux (eds).  Echinoderms through time.  Balkema, Rotterdam.

Held, Mirjam B.E. and Christopher D.G. Harley, 2009.  Responses to low salinity by the sea star Pisaster ochraceus from high- and low-salinity populations.  Invertebrate Biology 128:4 381-390

Perumal, Viren, 2006.  Responses to salinity of color polymorphs in two populations of the seastar Pisaster ochraceus.  M.S. thesis, Loma Linda University Department of Earth and Biological Sciences.

Pincebourde, Sylvain, Eric Sanford, and Brian Helmuth, 2008.  Body temperature during low tide alters the feeding performance of a top intertidal predator.  Limnology and Oceanography 53(4): 1562-1573

Pisaster ochraceus is a major predator of the mussel Mytilus californianus in the intertidal zone.  Photo by Dave Cowles, Goodman Creek, WA, July 2002

The seastars have surrounded this patch of Mytilus californianus on this nearly horizontal rock surface at Beach #4 and appear to be cleaning the rock entirely of mussels from the edges inward.  Photo by Dave Cowles, July 2012

This individual is eating a mussel.  Photo by Dave Cowles, July 2006

The images above and below show some of the variations in the reticulate pattern of ossicles on the aboral surface.  Photos by Dave Cowles at Dana Point, CA March 2005

This is the underside (oral side) of the individual above, showing the ossicles, ambulacral grooves, and tube feet.  The stomach is not everted.
Photo by Dave Cowles of an intertidal specimen from Sares Head, July 2001

2006:  Viren Perumal is studying metabolism of the various color phases of Pisaster ochraceus as a function of salinity for his MS research.  We have noted that a number of the seastars brought into the lab during late June and July have shed gametes.  They always seem to shed in the evening or at night.  Click here to see a movie of Pisaster ochraceous shedding eggs, and here to see one shedding sperm.

This closeup of the aboral surface shows the extended papulae or coelomic pouches, used for respiration and waste excretion, which are
often extended when seastars such as P. ochraceus are underwater.  The papulae give the seastar's surface a soft, fuzzy apperance.
Photo July 2007 by Kirt Onthank.

This individual on the north side of a large, fairly isolated rock at Shi Shi beach is dying (see how the central disk and inner ray tissue are white and sloughing off and one ray has fallen entirely off).  I could find no mussels and hardly any large barnacles on this rock so perhaps the seastar has starved to death.  Seastars on other, nearby rocks which had mussels were in good condition.  Photo by Dave Cowles, July 2008

These images, taken of live seastars naturally encountered at Beach #4 in the positions in which they were found, show different stages of regeneration.  Normally few individuals in a population are found regenerating so I do not know what phenomenon had damaged all these seastars within a relatively small area.  Perhaps sea otters have moved into the area.  Photos by Dave Cowles, July 2012