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California mussel and purple sea urchins in the lower intertidal zone of Santa Cruz island, California. Purple urchins usually feed on plant matter, such as kelp, but, in tough times when algae is depleted, can also use their heavy-duty teeth to grind through barnacle and mussel shells. With their ability to withstand a wide range of ocean temperatures. purple sea urchins may well be among the ocean’s ultimate survivors. Areas dominated by purple urchins are increasing throughout the Channel Islands.