Strongylocentrotus purpuratus

Geographic Range

Purple sea urchins are found on the pacific coastline from Alaska to Cedros Island, Mexico. (Olhausen and Russo, 1981)


Strongylocentrotus purpuratus is primarily found in the low intertidal zone. The purple sea urchin thrives amid strong wave action and areas with churning aerated water. The giant kelp forests provide a feast for S. purpuratus. Many sea urchins can be found on the ocean floor near the holdfast of the kelp. (Calvin et al., 1985; Olhausen and Russo, 1981)

Physical Description

Strongylocentrotus purpuratus has a round body that consists of a radially symmetrical test, or shell, covered with large spines. The test itself ranges from 50mm in diameter to an occasional 100mm in diameter. This test is covered with spines that are generally bright purple for adults. Younger urchins have purple tinged spines that are mostly pale green in color. Also covering the test or shell, are tube feet and pedicellariae. The oral side of the urchin, on which the mouth is located, is usually the side facing the substrate (down). The aboral side of the urchin is usually the side of the urchin facing the observer (up). The body of S. purpuratus is radially semetrical. Male and female urchins are monomorphic; they are not physically distinguishable from one another. (Abbott et al., 1980; Olhausen and Russo, 1981; Ebert and Russell, 1988)


January, February, and March are the primary reproductive months of S. purpuratus. It has been noted, however, that ripe individuals can be found even into the month of July. Purple sea urchins reach sexual maturity at the age of two years. At this time they are about 25mm in diameter or greater. Once sexually mature, females and males release their gametes into the ocean where fertilization occurs. The fertilized egg then settles and begins to grow into an adult. After the egg is fertilized and settles onto a substrate, the urchin begins to develop. The test develops quickly to protect the young urchin. The plates of the test begin to form individually and grow tighter together to form the test. As with most echinoderms, the sexes are usually separate. There is however an occasional hermaphrodite. (Abbott et al., 1980; Calvin et al., 1985; Mead and Denny, 1995)

  • Breeding interval
    Sea urchins breed yearly.
  • Breeding season
    January, February, and March are the primary reproductive months of S. purpuratus.
  • Average age at sexual or reproductive maturity (female)
    2 years
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    730 days
  • Average age at sexual or reproductive maturity (male)
    2 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    730 days


  • Average lifespan
    Status: captivity
    20 years


Strongylocentrotus purpuratus is both a social and independant organism. It does not tend to live in colonies or around other urchins, nor does it always live by itself. Urchins can be found in groups, particualrly when a good food source is near. Around the holdfasts of giant kelp beds, S. purpuratus can be found in very high numbers. On the other hand, during low tide, purple sea urchins can be found all by themselves. Strongylocentrotus purpuratus is generally a sedentary individual. However, the purple urchin does have the ability to move, and can do so by using its tube feet to push and pull itself along. This action, in coordination with the movement of the spines can be used to move the urchin. This movement is usually very slow.

Primary predators of S. purpuratus include sea stars(Solaster stimpsoni, Pycnopodia helianthoides, and Astrometis sertulifera) as well as the sea otter. Strongylocentrotus purpuratus defends itself with its rather sharp spines. Also, the pedicellariae can grap or pinch a predator in order to ward them off. In addition to fighting off predators, pedicellariae also keep barnacles, sponges, and other animals from growing on the aboral surface of the urchin. (Abbott et al., 1980; Clavin et al., 1985; Olhausen and Russo, 1981)

Food Habits

As a sedentary invertebrate, S. purpuratus primarily feeds on algae. Bits of algae are a common food that urchins snag out of the water. The tube feet, spines, and pedicellariae that cover S. purpuratus are used to grab the food and aid it into the mouth. In addition to grabbing food out of the water, S. purpuratus scrapes algae off the rocks or substrate. It's mouth consists of a strong jaw piece called Aristotle's lantern. The mouthpiece itself has five bony teeth that are instrumental in scraping the algae off the substrate. While any algae will satisfy the appetite of the purple sea urchin, this species prefers the giant kelp Macrocystis pyrifera. (Calvin et al., 1985)

Economic Importance for Humans: Positive

Strongylocentrotus purpuratus is actually used in many seafood recipes. Sea urchin is common in sushi. It is also considered a delicacy in some countries, especially Japan. The primary urchin harvesting company in California sends 75% of the harvest to Japan.The market value for urchins in Japan ranges from $2.20 per tray to $43.00 per tray. In 1994, Japan imported 6, 130 metric tons of sea urchins at a total value of 251 million dollars. Sea urchin harvesting has become one of the highest valued fisheries in California, bringing 80 million dollars in export value per year. (Calvin et al., 1985; Sea Urchin Harvesters Association, 2000)

Economic Importance for Humans: Negative

A negative effect directly on humans is not evident with S. purpuratus. It does, however, have an adverse effect in an indirect way. Purple sea urchins feed on the giant kelp, as mentioned previously. In their feeding, they can destroy entire forests of kelp. These kelp forests are commercially important for fisheries. They are even more important in that the blades of the kelp can be harvested for algin. Algin is a product that is used in the manufacturing of plastics and paints. It is also used as a thickening agent in foods such as gravy and pudding. Another use for algin is in making fibers that are instrumental in the manufacturing of fire resistant clothes. Without the kelp, algin could not be harvested. Strongylocentrotus purpuratus aides in the demise of the kelp forests that provide us with so many different products. (Readdie, 1998)

Conservation Status

There is no special status listed for Strongylocentrotus purpuratus. However, the harvest of sea urchins poses some concern for the wellfare of the sea urchin population. As mentioned previously, sea urchins are being exported to Japan and other countries in astounding numbers. This leads some to believe that the populations of sea urchins are drastically declining. The California Department of Fish and Game is trying to control the harvesting of the sea urchin now, to insure that urchin populations do not become endangered. For now, they are simply limiting the number of permits available to fisheries. They are discussing other conservation techniques as well, that have not yet been implimented. (Sea Urchin Harvesters Association, 2000)

Other Comments

Strongylocentrotus purpuratus has adapted the ability to burrow itself into the substrate. In many cases this substrate is rock. Strongylocentrotus purpuratus uses its five bony teeth in concert with its spines to slowly gauge and scrape away at the substrate. The result is a depression in the substrate into which the test of the urchin can settle with a firm hold. The hard surface of the rock or substrate S. purpuratus is scraping does wear out the spines. This does not create a problem since the spines are continually being renewed by growth. This is a unique feature that can sometimes prove deadly. When S. purpuratus is young, it may begin to scrape into the substrate. As it grows, the urchin may find that it has trapped itself for life. As the urchin grew, it gouged out a big enough cavity for its body. However, the initial entrance hole was made when the urchin was much smaller and once grown, it may not be able to retreat from its self-made depression. (Calvin et al., 1985; Olhausen and Russo, 1981; Abbott et al., 1980)


Alisa Worley (author), Western Oregon University, Karen Haberman (editor), Western Oregon University.


Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


the nearshore aquatic habitats near a coast, or shoreline.


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

seasonal breeding

breeding is confined to a particular season


Abbott, D., E. Haderline, R. Morris. 1980. Intertidal Invertebrates of California. Stanford, CA: Stanford University Press.

Calvin, J., J. Hedgepeth, E. Ricketts. 1985. Between Pacific Tides: Fifth edition. Stanford University: Stanford University Press.

Ebert, T., M. Russell. March 1988. Latitudinal variation in size structure of the west coast purple sea urchin: a correlation with headlands. Limnology and Oceanography, 33: 286-294.

Mead, K., M. Denny,. Feb/Mar 1995. The effects of hydrodynamic shear stress on fertilization and early development of the purple sea urchin Strongylocentrotus purpuratus. Biological Bulletin, 188: 46-56.

Olhausen, P., R. Russo. 1981. Pacific Intertidal Life. Rochester, NY: Nature Study Guild.

Readdie, M. 1998. "UCSC Marine Botany, University California Santa Cruz" (On-line). Accessed October 30, 2000 at

Sea Urchin Harvesters Association -California, April 29, 2000. "SUHAC Homepage" (On-line). Accessed November 26, 2000 at