When I hear the words “sea urchin,” spines come to mind, but that’s from personal experience (and a few remnants still embedded in my knee from 20 years ago). It’s time to move on from those previous run-ins and spotlight urchin teeth, which are also worth screaming about. Sea urchins not only use their teeth to scrape algae off rocks but they actually bore into rock, carving out nooks for sanctuary from predators and water forces. Physically, the mouth is centrally located in spherically shaped urchins, where it is surrounded by lips of softer tissue to which (typically) five bony plates less than an inch long are embedded. The hard pieces, made of calcium carbonate, are the teeth. It’s well known that despite literally chewing rock, urchin teeth never wear down to nubs or become dull. Grinding rock is pretty impressive, but to do so and maintain sharp teeth is an even cooler party trick. Yet, the “Ripley’s Believe-It-or-Not!” moment is knowing that both rock and urchin teeth are composed of an identical form of calcium carbonate known as calcite. Common sense tells us that to successfully slice or bore into something, it is prudent to choose a tool that’s harder than the material on which you want to work. For sea urchins to do what they do, somehow their teeth must be stronger than rock. But how? It has been recently revealed that the tooth’s calcite structure is uniquely designed in ways that make it stronger than rock. As water can take on different properties like ice, liquid, and steam, other compounds as well can share the same composition but have a different execution. Urchin teeth are made of two kinds of calcite crystals, fibers and curved plates, which are arranged crosswise to each other like a mosaic and cemented together by tiny particles also made of calcite. Between the crystals are shallow layers of a weaker organic material (a fraction the thickness of a human hair), and it is here where breakage occurs. When subjected to stress, urchin teeth don’t break off in chunks like with mammal teeth and, well, most things that break. Scientists think the weak layers aren’t a genetic mistake but have evolved to let parts of the tooth pull away, not unlike perforated paper where you “tear on the dotted line.” By regularly shedding tads of a worn tooth at its weak links, the central part of the tooth, called “the stone” because it is the strongest and sharpest part, is always exposed. Hmm. What a novel idea — synthesizing material with inherent fault lines so damaged teeth parts fall away to expose the razor-sharp pearly white layer underneath. Taking advantage of its 200-million-year history, sea urchins have evolved the ideal self-sharpening mechanism. Meanwhile, I’ve been operating with dull kitchen knives for years instead of enjoying tools that sharpen themselves with use. Why not copycat the urchin and make fracture-resistant, dull-resistant materials for me, and others of course? In theory, this could happen, and scientists are enthusiastic that discovering the self-sharpening principle may lead to new technology. But first, researchers must decipher the exact composition of the weak layers and determine the individual functions of the calcite plates and fibers. While I wait for scientists to puzzle together these unknowns, the mystery of urchin teeth has led me to a discovery of my own: On rare occasions, it’s a good thing when style (plates and fibers) trumps substance (calcite). — Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. She can be reached at [email protected].