|Sea urchin mouth, with its five teeth.|
If you're a sea urchin, you can even chew into rock with them.
|Small green sea urchin, Strongylocentrotus droebachiensis, with its spiky shadow.|
The teeth are the bottom (because the mouth faces down) part of the mouth structure called Aristotle's Lantern, supposedly because when Aristotle drew one, he thought it looked like a lamp. They are made, like the test and mollusc shells, of calcium carbonate, CaCO3. We see CaCO3 every morning as we make breakfast; egg shells are made of calcium carbonate. So is chalk. And the antacid we take after too big a meal.
So how does a sea urchin bore a hole in rock with chalk?
A 2016 study by the Pacific Northwest National Laboratory shows how it's done, although they don't mention sea urchins specifically.
Calcium carbonate is one of the most important materials on earth, crystallizing into chalk, shells, and rocks. Animals from mollusks to people use calcium carbonate to make biominerals such as pearls, seashells, exoskeletons, or the tiny organs in ears that maintain balance. These biominerals include proteins or other organic matter in the crystalline matrix to convert the weak calcium carbonate to hard, durable materials.
If a tooth does chip, it repairs itself, filling in the gap with a freshly built CaCO3 and protein structure. No need for dentists!
The sea urchin's main diet is a lot softer than rock, of course; it eats mostly algae. A swarm of urchins can demolish a kelp forest. Chewing rock is construction work, making safe holes to sleep in.