Purple Urchin

Purple Urchin(Strongylocentrotus purpuratus)

Contributed by TPERP Sara Chai

Purple Sea Urchin – J&W Tam
T. Woods

Location: They are found in the lower intertidal to approximately 160 meters in depth in the Pacific Ocean. They can be found off the western coast of North America, as far north as Vancouver Island, British Colombia and as far south as Isla Cedra in Baja California.

Diet: Purple sea urchins eat algae as well as decayed matter. When there are numerous urchins, they can completely devastate a kelp forest.  This is known as an urchin barren.

Predators: (who eats them): sea otters, California sheephead (a fish), sea stars, anemones, and some crabs. Lobsters also prey on purple urchins.

Adaptations: Urchins decorate themselves with algae and shell bits held on with tube feet to prevent getting dried out. Other hypotheses for doing this include protection from the sun’s ultraviolet rays and trying to avoid being eaten. The purple urchin can regenerate some of its parts, but if it has a severe injury, it cannot fix it and will die.

Reproduction: Most urchins are either male or female, but some urchins are hermaphrodites. Most fertilization occurs during winter, between January and March. The fertilized eggs feed on plankton until they eventually change into a tiny bottom dwelling urchin.

Life For A Purple Sea Urchin: A sea urchin often shelters itself in holes in rocks. The urchin can form its shelter by using its spines and gnawing at the rock with its teeth. They occupy pits in softer rock in more northern parts of their range, but not so much at Cabrillo. The sea urchin moves along the ocean floor and in the rocky intertidal using its spines and tube feet.

The purple sea urchin can usually defend itself against sea stars. The urchin lets some sea star arms get really close by moving its spines. Then, when the star tries to grab the urchin, the urchin’s pincers bite the star’s tube feet. However, this tactic doesn’t work for sunflower stars. The urchin tries to retreat away from them but if the urchin isn’t fast enough, it is swallowed whole.

Interesting Facts: 

The inner round shell of a sea urchin (its body) is called a “test.” The test is covered with pincers, tube feet, and the purple spines.

It is smaller than it’s relative, the red sea urchin.

If a sea otter eats a purple sea urchin, its bones turn purple.

A purple sea urchin’s mouth is on its bottom.

Purple sea urchins are one of the first animals to show distress in water that’s polluted.

Urchins are said to have more genetic variability than other species, including people.

Urchins can live up to thirty years.

Phylum: Echinodermata
Class: Echinoidea
Order: Echinoida
Family: Strongylocentrotidae
Scientific Name: Strongylocentrotus purpuratus
Common Name(s): purple urchin


1) Strongylocentrotus purpuratus (Stimpson, 1857) –

2) Monterey Bay Aquarium – Purple sea urchin (on exhibit) –

3) Purple sea urchin vidoes, photos and facts –

4)Purple Sea Urchin on Enchanted Learning  http://www.enchantedlearning.com/subjects/invertebrates/echinoderm/Purpleseaurchin.shtml
(This link has a neat diagram with labeled parts. If you could isolate the image, it’d be neat to have).

5) Purple Sea Urchin- Think Quest –

6) “Purple Sea Urchins Evolve in a Flash to Survive Growing Acidity” – NBC News (4-9-2013)

7) Life Between the Tides – J.L. Brandon and F.K. Rokop, 1985, p. 180-181.

Last revised 23-Jan-20