These fantastical structures are the microscopic crystals that make up a sea urchin’s tooth. Each shade of blue, aqua, green, and purple—superimposed with Photoshop on a scanning electron micrograph (SEM)—highlights an individual crystal of calcite, the abundant carbonate mineral found in limestone, marble, and shells. The curved surfaces of the crystals look nothing like normal calcite crystal faces. Instead of flat sides and sharp edges, the sea urchin produces “incredibly complex, intertwined” curved plates and fibers that interlock and fill space in the tooth as they grow. Though made of a substance normally as soft as chalk, the teeth are hard enough to grind rock, gnawing holes where the sea urchins take shelter from rough seas and predators.
Photo and caption credit: Pupa U. P. A. Gilbert and Christopher E. Killian; University of Wisconsin, Madison