The Ochre or Purple Star, Pisaster ochraceus, is by far the most common sea star in British Columbia and along much of northwest coast of North America. The species primarily eats mussels and barnacles, but has a broad range of other dietary items. Its role in the community dynamics of the shore is so influential that it has been termed a "keystone predator", that is, a species of high trophic status that through its feeding activities exerts a disproportionate influence on community structure. Much research has been done on the species' feeding ecology and also on its coloration. In fact, the functional morphology of its predominant purple, brown, and orange colours has engaged the interest and curiosity of researchers for decades. If you would like to read about current research on this subject, and to learn why marine biologists on the coast are desperate for someone with lots of time on their hands to come along and rear up several thousands of these sea stars under different conditions of diet, salinity, parenthood, and so on, then visit A SNAIL'S ODYSSEY.
Introduction Note Author: Tom Carefoot, Department of Zoology, University of British Columbia
Five or more arms. At least one adambulacral is fused into an adoral carina. The adambulacrals are wider than their length. Crossed and straight pedicellariae are present, the former usually in dense tufts around the spines. The aboral skeleton is meshlike. The tube feet are arranged in four rows.
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Pisaster ochraceus is our most common intertidal sea star, ranging in colour from purple to orange or yellow, with white spines and white to pale yellow tube feet. It has five stiff arms, a highly arched disc and a sunken mouth, and attaches strongly to the rocks at low tide. The arms are up to 25 cm long (an RBCM specimen) and the arm-to-disc ratio ranges from 2.7 to 4.1. The aboral surface has an extremely variable pattern of spines ranging from small numerous spines in a netlike pattern, to stout knobby spines in well-separated convex groups. The papulae, between aboral spines, give the sea star its basic colour. Among the papulae are furcate, crossed and smalllanceolate pedicellariae that can only be seen when magnified, and occasionally several larger straight toothed pedicellariae. The furcate pedicellariae are characteristic of this species. On lower side of the arm, superomarginals form a regular row with an intermarginal channel just below. Between the intermarginal channel and the adambulacrals are five to eight rows of similar spines. The first two rows are inferomarginals, the next three to six are oral intermediates. Each spine has a tuft of pedicellariae on the distal side. The adambulacrals have one slender spine per plate and smallianceolate pedicellariae attached to the plate but not the spines. Ten adambulacrals fuse together as an adoral carina. The mouth plates are sunken and difficult to see, but each plate has two stout marginal spines.
Pisaster ochraceus might be confused with Evasterias troschelii (the Mottled Star), but the latter has a mottled colour, a longer arm-to-disc ratio, a shallow mouth and, alternately, one and two spines on the adambulacrals rather than on
Pisaster ochraceus feeds primarily in summer, preferring mussels, barnacles, limpets and snails. About 30 prey items have been documented but diet depends on the availability of prey. In the presence of P. ochraceus large Turban Snails (Tegula funebralis) and the limpets Acmaea limatula and Acmaea scutum escape by moving up vertical surfaces to higher places on the shore. Margarites species, Calliostoma ligatum and Amphissa columbiana show a strong escape reaction. P. ochraceus eats mussels by inserting its stomach between the shells, secreting digestive enzymes and, at the same time, pulling the shells apart with its tube feet. In Puget Sound in winter, P. ochraceus migrates lower on the shore and stops feeding. Near Monterey, California, an average-sized sea star eats about 80 mussels per year and does not show a marked summer feeding or downward migration in winter. The clam Chama arcana is more abundant in patches of Strawberry Anemones (Corynactis californica) because P. ochraceus is deterred by the stinging cells of the anemone. If a number of P. ochraceus are placed on the shore in a group they tend to move away from each other as quickly as they can. This behaviour ensures that they are more evenly dispersed in the habitat and reduces competition for food. Their role as a keystone predator is discussed in papers by Paine (1984 and 1985).
The gonad starts to grow in January, in preparation for spawning from May to July, when it produces pale orange to salmon-pink eggs (diameter 150 to 175 micrometres). A 400-gram sea star can release 40 million eggs. The length of day and night (photoperiod) has been shown to control the onset of gamete production and gonad growth. A bipinnaria larva forms after about six days and can survive up to two months in the plankton. Towards the end of this planktonic phase, it develops into a brachiolaria larva. The brachiolaria has sticky arms and a sucker for attaching to the substrate, where it will metamorphose into a juvenile sea star. Larvae of this species use a chemical defence that causes filter feeders like the mussel Mytilus edulis and the sea squirt Styela gibbsii to reject a significant proportion of them. The larvae can tolerate a salinity as low as 20 parts per thousand, but many are abnormal and smaller. Young stars reach maturity at 5 years and live for 20 years or more. The larval nervous system is described in Burke 1983 and Lacalli et al. 1990. The scale worm Arctonöe fragilis is commensal on P. ochraceus. A parasitic ciliate Orchitophrya stellarum, endemic to the North Atlantic, was discovered in 1988 in the Purple Sea Star in British Columbia. It infests the male gonad causing castration and loss of sperm production. This same parasite was reported in Asterias amurensis in Japanese waters in 1990. The only predators known to prey on adult P. ochraceus are sea otters and sea gulls.
P. ochraceus is common and easy to collect and thus has been used for numerous general studies in physiology, embryology and pharmacology. Many of those papers are listed below.
Prince William Sound, Alaska, to Cedros Island, Baja California, from the intertidal zone to 97 metres deep. Abundant on rocky shores in middle to lower intertidal zone exposed to waves or currents. On more sheltered parts of the coast, such as mainland inlets, this species tends to be replaced by Evasterias troschelii.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2019. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
2021-04-18 10:50:55 AM]
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