Strongylocentrotus purpuratus, or the purple sea urchin, is one of the sharp-spine sea urchin species. The spines are used as a means of defense against would-be predators. This urchin is deep purple in color, and grows to a diameter of about 4 inches. The are found along the Pacific coast of the United States and Canada where they cling to depressions in the rocks during low tide.