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Purple sea urchin (Strongylostrotus purpuratus) decorated/camouflaged with Halimeda (calcareous) algae. Fitzgerald State Park, California.

Both organisms have skeletons made of calcium carbonate. The urchin's skeleton is made of a slightly more robust crystalline form, calcite, while the algae's skeleton is mostly aragonite.

Blogged about at Climate Change Blog.

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Taken on July 4, 2004