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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.


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© Rosario Beach Marine Laboratory

Source: Invertebrates of the Salish Sea

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