Ochre Sea Star

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

This species of seastar has a radius of up to about 25 cm with stout rays that taper towards the end. There are usually five rays, but occasionally as few as 4 or rarely as many as 7. The color ranges from pale orange to dark brown or deep purple. The aboral surface contains many small spines that are arranged in a netlike or pentagonal pattern on the central disk.

How to Distinguish from Similar Species: Troschel’s Sea Star 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 but they have different aboral spines and coloration which allows one to distinguish between the species.

Biology/Natural History: This species of seastar is often considered a keystone species in many intertidal regions. P. ochraceus feeds mainly on mussels 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. 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. 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. The species is still highly susceptible to osmotic changes, however. Held and Harley 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 was lowest at the lowest salinity, 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.

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.