The secret of the hardest teeth on earth

They have the hardest teeth on earth: chitons. These mollusks—also called chitons—use their magnetic, iron-containing teeth to scrape algae off rocks, just like their distant relative the limpet. In Science , Japanese biologists now describe how the magnetite gets into those chiton teeth: thanks to a protein that ensures the right dosage and timing.

Sea snails that live on the rocks in intertidal zones are jacks of all trades. Their sturdy shell keeps starfish and other predators at bay, their "foot" can withstand strong waves, and with their radula—a tongue with rows of teeth—they graze the rocks. Precisely because of their extreme habitat, they have some adaptations in their appearance compared to other snails: their shell is flat and fits seamlessly with the substrate, preventing any soft tissue from protruding. And then there are those extra-hard teeth, which can wear down but are then immediately replaced by new ones.

In the common limpet (Patella vulgata) , a snail with a round, pointed "rice-cap shell" that is not directly related to the chitons, the teeth are made of the iron-containing mineral goethite. Previous research has shown that goethite teeth can withstand tensile forces of up to 6.5 gigapascals. For comparison, that's 2 gigapascals more than spider silk, that other super-strong biological material. Even as they wear down, the limpet teeth remain sharp. This is due to their structure: the goethite crystals are arranged on top of each other in a kind of roof-tile formation.

Right time, right place

The current Science article features the over-thirty-centimeter-long giant beetle snail Cryptochiton stelleri , with teeth made of the equally strong magnetite. The researchers discovered that a special protein, RTMP1, ensures the correct dosage of iron oxide in the early stages of tooth development. At that point, the teeth are merely a "transparent matrix" of chitin and proteins, the authors write. Subsequently, the not-yet-strong reddish-brown iron oxide takes its place, and eventually, hard black magnetite. RTMP1 binds to iron ions in this process, ensuring that mineralization occurs at the right time and in the right place. This allows the giant beetle snail to perform repetitive rock-scraping movements to its heart's content with its radula.

Sometimes these are so far-reaching that they even influence geology: for example, a 2022 article in the Journal of Composite Materials includes a fascinating image of mushroom-shaped rocks created by the scraping of chitons.

Incidentally, limpets also exhibit a phenomenon called mushrooming , although this refers to something entirely different: when an enemy—such as a starfish—approaches, the snail pushes its shell slightly upward, giving it a mushroom-like appearance. When the starfish moves one of its arms under the shell, the limpet snaps back down rapidly, clamping the arm.