Purple sea urchin (Strongylocentrotus purpuratus)

Purple sea urchin
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Purple sea urchin fact file

Purple sea urchin description

GenusStrongylocentrotus (1)

The purple sea urchin is an aptly named species, easily identified by the strikingly vivid, purple spines that project from its rounded body. These conspicuous spines are attached to a hard outer body wall, known as the ‘test’ (3). Also attached to the test are suckered, tentacle-like appendages, known as ‘tube feet’, which are used in locomotion, feeding and respiration (2) (3) (4). The purple sea urchin exhibits a phenomenon called ‘radial symmetry’, where the body of the sea urchin is arranged around the central axis of the mouth (2) (3) (5). As with other sea urchin species, the mouth is located underneath the body, close to the substrate. In the purple sea urchin, the mouth is part of an unusual feeding structure, commonly called ‘Aristotle’s lantern’, in which five toothed, calcareous plates are arranged to make up a powerful jaw that is well adapted to scraping at the substrate (2) (4) (5).

Width: 5 - 10 cm (2)
Height: 4.4 cm (2)

Purple sea urchin biology

The characteristic spines of the purple sea urchin have a number of roles in feeding, locomotion, defence against predators and protection from damage during bad storms (5) (8). The spines are also used by the purple sea urchin to dig cavities in the substrate, although not all individuals demonstrate this behaviour. Using its Aristotle’s lantern, the purple sea urchin is capable of biting pieces out of the rock, and the spines slowly erode away at the surface, creating a small pit in which the urchin may remain for long periods of time (2) (7).

Feeding mainly on algae, such as the giant kelp Macrocystis pyrifera (3) (4) (5) (6), the purple sea urchin spends much of its time stationary, feeding on kelp strands that drift past. However, kelp forests are very sensitive, and are often decimated by storms, or by warm water temperatures and nutrient reductions associated with El Niño events (9) (10). During these periods, shortages of drifting kelp may cause the purple sea urchin to switch its foraging behaviour, and it will actively feed on young kelp plants. This can prevent the kelp forest from re-establishing, and the area may become a sea urchin ‘barren’, with very high densities of urchins and very little algae (4) (6) (10). Predators of the purple sea urchin, including the sea otter, spiny lobster, sea star and the California sheephead, all play an important role in limiting the abundance of the purple sea urchin in areas prone to becoming sea urchin barrens (4) (10).

The purple sea urchin spawns seasonally, usually during winter and early spring. The larvae are planktonic, drifting along with the current during the first 6 to 12 weeks of development. After reaching a certain stage in maturity, the larvae of the purple sea urchin will settle on the sea bed and progress to the juvenile life-stage (4) (5) (11). Once settled, usually in late spring or early summer, the juvenile will begin to grow rapidly (2), although the rate of growth is highly variable and dependent on food availability. After one year, the young purple sea urchin tends to reach between one to three centimetres in size (4).


Purple sea urchin range

The purple sea urchin is found in the Pacific Ocean along the coast of North America, ranging from Alaska to Cedros Island, Mexico (2) (5) (6).


Purple sea urchin habitat

The purple sea urchin inhabits exposed rocky shores in the intertidal zone, usually along open coastlines in shallow waters and tide pools, down to depths of 160 metres (2) (5) (7). It is found on the sea bed, typically occupying areas that experience strong wave action (2). In places where the rock is suitably soft, the purple sea urchin may hollow out and inhabit small pits, or cavities (7).


Purple sea urchin status

This species has yet to be classified by the IUCN.


Purple sea urchin threats

Ocean acidification is emerging as one of the greatest threats to marine ecosystems, and studies have demonstrated that the increasing acidity of seawater is likely to have a detrimental effect on the growth and development of many marine species, including the purple sea urchin (12) (13).

The purple sea urchin is harvested for export to Japan, where it is considered a delicacy. Although not as commercially valuable as its larger relative, the red sea urchin (Strongylocentrotus franciscanus), it is still taken by fisheries along the Pacific coast, which may have a negative impact on the population if the resource becomes overexploited (2) (4).


Purple sea urchin conservation

There are currently no specific conservation actions in place for the purple sea urchin. The California Department of Fish and Game have limited the number of permits available for urchin fisheries, and, with the exception of minimum size limits and closed fishing periods, the sea urchin fishery regulations that were put in place for the closely related red sea urchin also apply to the harvest of purple sea urchins (2) (4).


Find out more

For more information on the purple sea urchin, see:



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Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.
Aristotle’s Lantern
The jaw structure of most sea urchins, made up of five, toothed, calcareous plates.
Composed of, or containing, calcium carbonate.
El Niño
A natural phenomenon that happens every 4 to 12 years, and lasts for several months, when upwelling of cold, nutrient-rich water does not occur. This causes the warming of ocean surface water off the western coast of South America and causes die-offs of plankton and fish. It also affects Pacific jet stream winds, altering storm tracks and creating unusual weather patterns in various parts of the world.
Intertidal zone
The region between the high tide mark and low tide mark.
Stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
Aquatic organisms, usually tiny, that drift passively with water movements; may be phytoplankton (plants), zooplankton (animals), or other organisms such as bacteria.
The production or depositing of large quantities of eggs in water.


  1. ITIS Report (July, 2010)
  2. Sanctuary Integrated Monitoring Network (SIMoN) (July, 2010)
  3. Campbell, A. and Dawes, J. (2004) Encyclopedia of Underwater Life. Oxford University Press, Oxford.
  4. California Department of Fish and Game (July, 2010)
  5. DigiMorph (July, 2010)
  6. Ebert, T.A. (1968) Growth rates of the sea urchin Strongylocentrotus purpuratus related to food availability and spine abrasion. Ecology, 49(6): 1075-1091.
  7. Grupe, B.M. (1996) Purple Sea Urchins (Strongylocentrotus purpuratus) In and Out of Pits: The Effects of Microhabitat on Population Structure, Morphology, Growth, and Mortality. Master of Science Thesis, Graduate School of the University of Oregon, USA.
  8. Edwards, B.P., and Ebert, T.A. (1991) Plastic responses to limited food availability and spine damage in the sea urchin Strongylocentrotus purpuratus. Journal of Experimental Marine Biology and Ecology, 145: 205-220.
  9. Tegner, M.J. and Dayton, P.K. (1991) Sea urchins, El Ninos, and the long term stability of Southern California kelp forest communities. Marine Ecology Progress Series, 77: 49-63.
  10. Pearse, J.S. (2006) Ecological role of purple sea urchins. Science, 314: 940-941.
  11. Ebert, T.A. and Russell, M.P. (1988) Latitudinal variation in size structure of the west coast purple sea urchin: A correlation with headlands. Limnology and Oceanography, 33(2): 286-294.
  12. O’Donnell, M.J., Hammond, L.M. and Hofmann, G.E. (2009) Predicted impact of ocean acidification on a marine invertebrate: elevated CO2alters response to thermal stress in sea urchin larvae. Marine Biology, 156: 439-446.
  13. Hoffmann Laboratory (July, 2010)

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Purple sea urchin  
Purple sea urchin

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