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Species Database

Strongylocentrotus purpuratus - Purple sea urchin

Purple sea urchin image

Geographic range:

Alaska to Cedros Island, Mexico.

Key features:

Strong radial symmetry, vivid purple, spiny, oval, domed aboral and flattened oral sur-faces.

Similar species:

Strongylocentrotus franciscanus -- Red sea urchin


bay (rocky shore), exposed rocky shore, kelp forest, protected rocky shore

Primary common name:

Purple sea urchin

General grouping:

Sea stars, urchins, cucumbers, sand dollars, brittle stars

ITIS code:


Geographic Range

Range Description:

Purple sea urchins are found on the pacific coastline from Alaska to Cedros Island, Mexico. Above the low tide line, these urchins often live in rounded depressions in the rock which slowly erode with their teeth and spines. Common in lower intertidal zone on rocky shores and pilings, typically in areas of moderate to strong wave action.

Northern latitude extent:


Southern latitude extent:


Intertidal Height

Lowest intertidal height:

0 meters OR 2 feet

Highest intertidal height:

0 meters OR -2 feet

Intertidal height notes:

Purple urchins occur intertidally, both in large tidepools and deep in cracks and crevices. If the rock is sedimentary, they may also occur in worn-out depressions, much like a honeycomb. Constant motion of the calcareous spines and traction by tubefeet can wear down the soft stone and create a depression over multiple generations of occupants.

Subtidal Depth Range

Minimum depth:

0 meters OR 0 feet

Maximum depth:

160 meters OR 525 feet

Subtidal depth notes:

Low intertidal to 160 m (525 ft) depth. One reference sites a maximum depth of 65 m.


bay (rocky shore), exposed rocky shore, kelp forest, protected rocky shore

Habitat notes:

Purple sea urchins are primarily found in the low intertidal zone. Purple sea urchins thrive in areas with strong wave action and churning aerated water. Giant kelp forests provide a feast for S. purpuratus. Purple sea urchins can be found on the sea floor near the holdfast of the kelp.


Relative abundance:

Purple urchins show strong regional-scale patterns of abundance that were correlated to the distribution of their primary predator, the southern sea otter. Since the return of the voracious sea otter to the Monterey Bay sea urchins have become crevice dwellers that avoid exposed sites, obtaining sustenance not from living seaweeds but from detached kelp blades that drift within their grasp. Nevertheless, the urchins in the area are vulnerable, and evidence of purple sea urchins is found in sea otter scat, and the otters bones become stained purple.

Species Description

General description:

The purple sea urchin has a rounded body that consists of a radially symmetrical test, covered with large spines. The test itself varies from 50 mm in diameter up to 100 mm in diameter. The test is covered with spines, which are usually bright purple in adults. Urchins that are younger, have purple tinged spines that are primarily pale green in color. Also covering the test, are pedicellariae and tube feet. The oral side of an urchin is normally the side facing the substrate, and the aboral side of an urchin is normally the side of the urchin facing the observer. Female and male urchins are monomorphic; not physically different from one another.

Distinctive features:

Purple sea urchins often decorate themselves in a manner similar to decorator crabs. This is done to protect from UV light, dessication, and visual predators. The skeleton, called a test, consists of rows of radially arranged plates permanently joined to each other. Moveable purple spines, each with a concave base, fit on correspondingly convex bumps on each plate. Muscle fibers attached to each spine enable it to move in any direction. In sea urchins, the middle of the upper surface has a circular area, usually with scaly plates, bearing the anus. It is surrounded by 5 petal-shaped plates, each with a large pore, the opening of a sex duct. One of these plates is also full of small pores, and is the sieve plate. Tube feet on an urchin are arranged in 5 pairs of rows that extend longitudinally around the test. They are tipped with suckers, and are long enough to reach beyond the spines. The tube feet are used in respiratory exchange, locomotion, and decorating. Urchins also have stalked pinchers, all of which have 3 jaws, and some have poison glands. These structures are defensive, protecting against predators and discouraging larval animals from settling on the urchins.


Width: 50 to 100 mm (2 to 4 in.) Height: average ~ 44 mm (1 ĺ in.)

Natural History

General natural history:

Purple sea urchins have adapted the ability to burrow itself into substrate. In most cases that substrate is rock. Strongylocentrotus purpuratus uses five bony teeth in combination with its spines, to slowly scrape away at the substrate. This results in a depression in the substrate into which the urchin can settle with a firm grip. The hard surface of the rock or substrate that S. purpuratus scrapes, does wear out its spines. However, this does not create a problem since the spines are being continually renewed by growth. This feature is unique, and can sometimes prove deadly. When purple sea urchins are young, they may start to scrape into the substrate. As they grow, urchins may find themselves trapped for life. As the urchin grows, it gouges out a big enough cavity for its body to fit. Since the initial entrance hole was made when the urchin was much smaller, once grown it may be unable to escape from its self-created depression. S. purpatus requires well-oxygenated water, obtaining oxygen mainly through their tube feet, which are extended at least partway when under water. Symbionts of purple sea urchins include ciliated protozoans and the flatworm Syndisyrinx franciscanus in the gut, and externally the purple polychaete Flabelligera commensalis and the isopod Colidotea rostrata which live among its spines. Although the purple sea urchin has been important for biomedical research, its ageing process has not been studied in much detail. The species grows rather slowly, with large size reached after about 10 years. Based on those growth rates, it has been estimated that these animals may live for more than 50 years. Since evidence suggests that the red sea urchin is extremely long-lived with no detectable signs of ageing, the purple sea urchin may also be extremely long-lived and its maximum longevity may be considerably underestimated.


Primary predators of S. purpuratus include sea stars (Solaster stimpsoni, Pycnopodia helianthoides, and Astrometis sertulifera) as well as the sea otter. The purple sea urchin defends itself with its rather sharp spines.


S. purpuratus primarily feeds on algae. Urchins may remain in crevices to avoid predators, and wait for drift kelp to come within reach of their waving tube feet. S. purpuratus can also forage directly on living algae or scrape algae off the substrate. Although the algal diet of the purple sea urchin is diverse, the species prefers giant kelp Macrocystis pyrifera.

Feeding behavior


Feeding behavior notes:

The spines, pedicellariae, and tube feet that cover S. purpuratusare used to grab the food and aid it into the mouth. Its mouth is composed of a strong jaw piece called Aristotle's lantern. This remarkable structure consists of a set of skeletal rods and muscles arranged to open and close 5 teeth, like the jaws of a drill chuck. The lantern can be protruded out of and completely retracted back into the mouth. The five bony teeth are instrumental in scraping algae off the substrate. The area around the mouth is usually adorned with 10 frilly gills. Purple sea urchins feed on giant kelp. During feeding, urchins can destroy entire kelp forests. These kelp forests are commercially important for fisheries, and the blades of kelp can be harvested for algin.

January - March


Sexes are separate, although some hermaphrodites may be found. January, February, and March are the primary reproductive months for S. purpuratus. However it has been noted that ripe individuals can be found even through the month of July. Sea urchins breed once a year. Purple sea urchins reach sexual maturity at the age of about two years, at that time they are about 25mm or greater in diameter. Once they have reached sexual maturity, females and males release gametes into the ocean, where fertili-zation takes place. The fertilized egg later settles, and begins growing into an adult. Upon fertilization and settling onto a substrate, the urchin starts to develop. The shell or test develops quickly to protect the developing urchin. Plates of the test begin to form indivi-dually and then grow tighter together to form the test.

Monterey Bay National Marine Sanctuary:

em>Strongylocentrotus purpuratus is used for many seafood recipes. Sea urchin is commonly used to make sushi. It is also considered a delicacy in many countries, including Japan. The main urchin harvesting company in California sends 75% of their harvest to Japan. The market value for sea urchins in Japan ranges from $2.20 to $43.00 per tray. In 1994, Japan imported over 6130 metric tons of sea urchins, totaling a value of 251 million dollars. Sea urchin harvesting is one of the highest valued fisheries in California, bringing about 80 million dollars in export value per year. Strongylocentrotus purpuratus, currently has no special status listing. However, harvesting of sea urchins poses concerns for the welfare of the overall sea urchin population. Sea urchins are being exported to other countries such as Japan, in extremely high numbers, leading some to believe that the populations of sea urchins are dramatically declining. The California Department of Fish and Game is now trying to control harvesting of sea urchins, to insure urchin populations donít become endangered. There is now simply a limit to the number of permits available for fisheries. There are discussions over other conservation techniques as well, which have not been implemented yet.
Click on an image below to view a larger version in the SIMoN Photo Library. You will also be able to view important information on each photo such as photographer, date, caption and more.
Carlton, J.T. 2007.
The Light and Smith Manual, 4th edition
Intertidal Invertebrates from Central California to Oregon
University of California Press. 1001 p.

Lamb, A. and B. P. Hanby. 2005. Marine life of the Pacific Northwest. Harbour Publishing. 398 p.
Langstroth, L. and L. Langstroth. 2000. A Living Bay: The Underwater World of Monterey Bay. University of California Press, Berkeley and Los Angeles, CA. 287 p.
Meinkoth, N.A. 1998. National Audubon Society Field Guide to North American Seashore Creatures. A.A. Knopf, New York, NY. 813 p.
Morris, R.H., D.P Abbott, and E.C. Haderlie. 1980. Intertidal Invertebrates of California. Stanford University Press, Stanford, California. 690 p.
Ricketts, E. F., J. Calvin, and J.W. Hedgpeth. 1985. Between Pacific tides. Stanford University Press, Stanford, CA. 652 p.
Animal Diversity Web
Accessed 8/19/09 for Giant green anemone
Accesed 3/31/09 for Pycnopodia helianthoides
Accessed 8/1/09 for purple sea urchin
Accessed 11/11/09 for American Avocet
Accessed 12/15/2009 for Black-crowned Night Heron
Accessed 07/07/2009 for Pied-billed Grebe